Saturday, December 19, 2009

How to Make Your Equipment Look Like a Million Bucks!

There's a lot of surplus equipment out there that can be used as the basis for a homebrew project. Here's a way to make it look more professional.

For example, let's take this...


...and make it look like this:


It's actually very easy. Here are the steps I follow.

First, For the panel, I measure its dimensions and the locations of all holes I want to use. Then, using an electronic drawing program (I use Adobe Illustrator, but others I know use Autocad), I accurately place all hole locations as well as any labels I want to add on my panel drawing.


I then print out the panel drawing. If the panel is larger than, say, 8.5 x 11 inches, you'll need to create a "B" size sheet (11" x 17"). My printer does not print B-size sheets, so I instead print two "A" size sheets, trim their edges, align them (note the alignment marks in the overlap area), and then tape them together so that they form a larger sheet. If you don't have a light-table for accurately aligning them, a window works fine...


I then go to the local copier store (here it's the "Kinko's" chain) where I copy my "composite" drawing onto a B-size sheet (so that it's a single piece of paper, not two taped together).

(If you can squeeze in an extra panel, do it, just in case you "goof.")


I then laminate it (the local Kinko's has a laminator).


And I cut out the panel overlay (and any large holes) with an Xacto knife...


Finally, I glue my overlay to the original metal panel using "Adhesive 77" spray adhesive (manufactured by 3M) and cut out the remaining holes, using the holes in the original panel to guide my Xacto knife.

And voila!

[Many thanks to Dick, W1QG, who taught me this technique. His great looking panels on the equipment he built were an inspiration.]

Monday, December 14, 2009

Converting an HP Counter into a Nixie Tube Clock

Hmmm...I had an old HP 5233L nixie tube counter with 6 nixie tube digits gathering dust in the corner. What to do with it...?

Let's see, six digits...why not make it into a clock? After all, nixie tube clocks are pretty cool. And the counter came integrated with almost everything I needed: power supply and even a case with nicely milled holes. Mechanical work would be minimal, which defines my ideal project!

And as a nice bonus, there was also a back-panel BNC input for an external frequency reference, so I could use my GPS-locked frequency standard to keep the clock's time from "drifting" over time.

Conceptually, I figured it should look something like this (this includes a simple method for setting time):

Block Diagram


(Click on image to enlarge)


Pretty straight forward. But there was a small complication: I didn't have schematics for the 5233L.

This really didn't prove to be much of a problem. HP used the same nixie "single-digit" decimal counter plug-in module in a variety of its counter products. And I did have a manual for an HP 5232A. A quick glance revealed that it had the all-important schematic for the decimal counter module (HP part number 5212A-4A -- and although my counter used 5212L-4A modules, the 5212L-4A design seemed to be very close (if not identical) to that of the 5212A-4A)).

That manual, plus some scope probing of various signals within my 5233L counter, told me pretty much all I needed to know.

Some things I needed to do were:
  1. Change two of the counter modules to count from 0 to 5 (instead of 0 to 9), so that minutes and seconds would each count from 0 to 59, instead of 0 to 99.
  2. Add a reset circuit to the two "hours" digits such that, if the hour count increments to '13', it immediately resets the two hour digits.
  3. Modify the Hour LS (Least Significant) digit to reset to '1' instead of '0' (so that hours count from '1' to '12'.
  4. Disable the "Storage" feature of the Decimal Counter modules so that we can see the module counting.
So first, to count from 0 to 5, the 5212L-4A assembly(or 5212A-4a, or other variants) can be modified as follows:
  1. Remove R45, R50, and C10
  2. Change C11 from 200 to 470 pF
  3. Move R59 from CR12 to CR11
If you're trying to understand how the decimal counter circuit works from its schematic, note that it counts per the following pattern:
D C B A
0 0 0 0 (0)
0 0 0 1 (1)
0 0 1 0 (2)
0 0 1 1 (3)
0 1 1 0 (4)
0 1 1 1 (5)
1 1 0 0 (6)
1 1 0 1 (7)
1 1 1 0 (8)
1 1 1 1 (9)
So, to count from 0 to 5 instead of from 0 to 9, we can use the transition of B from 1 to 0 to set C to 0. And D must be always kept at 0.

(Important schematic note: Q1/Q2 control bit A, Q3/Q4 control bit B, Q5/Q6 control bit D (not C!), and Q7/Q8 control bit C (not D!)).


For item 2, the Hours Reset circuit needs a few more parts. I mounted them under the counter chassis.

(Click on image to enlarge)

For item 3 -- to modify the HOURS LS digit decimal counter module (HP Assembly 5212(x)-4A) to reset to a count of '1' instead of '0', simply modify that module so that R23 connects to the base of Q2 instead of to the base of Q1.

And finally, for item 4:

The "Transfer Line" signal to the counter modules controls the modules "storage" function (necessary for a counter to maintain a stable display while the modules are counting). But there's no reason to use a storage function when operating as a clock -- we can simply view the count while it's incrementing, and it makes the modification simpler

To disable the "Transfer Line", I cut the wire from the driver that drove pin 5 of the Decimal Counter card-edge connectors. This line will then float at about-12V, and the storage feature of the Decimal Counter modules will be disabled).

So...that's essentially it!

[I did add various switches and buttons to give me some functions that I wanted (such as turning off the display, but continue running the clock, to save on "wear and tear" of the nixies). But the modifications listed above are the main mods to the basic counter function.]

Here's the top view of the modified counter...

...with the modified decimal-counter modules:

And a view under the chassis. The HOURS reset circuit is wired between two of the edge connectors towards the right.


The counter, before I made an overlay for the front panel...


...and after:

(Martini time!)


From left to right, front panel controls are:
  1. Toggle Switch: Mode, RUN/SET-Time
  2. Push Button: Increment clock when in SET-Time mode
  3. Toggle Switch: FAST/SLOW increment speed when in SET-Time mode
  4. Rotary Switch, 3 Position. Selects Display Format: H:M:S, H:M, Display OFF (but clock still counting)
  5. Push Button, Reset SECONDS count
  6. Pot with Power-On switch. Only the power-on switch is used.

Other Notes:


1. HP Manuals are great. I've always found their "Principles of Operation" section to be an excellent description of how their equipment operates -- very useful and well worth checking out.

2. To use a 5212(x)-4A module in slot A19 of the 5233L counter, clip wire going to pin 8 of that slot's card-edge connector. (I no longer recall which module was originally in this slot).

3. You can use the "decimal point" neon lamps in the modules as separators to differentiate between hours, minutes, and seconds digits (refer to photo of my counter).

[Note: the 5212L-4A modules in the 5233L counter do not have neon lamp decimal points within the modules themselves (unlike the 5212A-4A module). Instead, the neon lamps are mounted on a separate small PCB that runs underneath the modules, next to the front panel.]

4. You could probably make this a 24-hour clock as follows:
  • Do not modify the Hours LS digit to reset to 1.
  • Change the Hours Reset Circuit to use the following 2 bits instead of the 3 bits shown in my schematic above:
  • Hours LS Digit pin 13
  • Hours MS Digit pin 9
(I haven't tried this, so take this recommendation with a grain of salt.)

5. Pin 7 is the Clock input pin for a 5212(x)-4A module. It is to this pin of the first module (the least-significant digit of the "seconds" modules) that you connect the 1 Hz time reference (and the faster clocks, when in SET-Time mode).

6. In an HP counter you should find a number of identical divide-by-10 modules that are used to divide down the time-base reference frequency (HP p/n 5212A-65C). It is on the card-edge connectors of these modules that you can find the 1Hz, 100Hz, and 1KHz signals (as well as others, should you need them).

Note 1: These boards have pin 7 as their input and pin 5 as their divide-by-10 outputs.

Note 2: Although my block-diagram above has 1 KHz as the "fast" set-time frequency, I might have actually 10 KHz.

7. After I'd mucked around with making an overlay and installing it on the front panel, the counter started "double-counting" (it would increment twice in one second -- on the rising and the falling edges of the 1 Hz clock). It turns out that CR9 on the first nixie tube module had opened up. HP's p/n for this part is 1910-0015, but unfortunately it didn't list the manufacturer's part number. I replaced it with a 1N4148 -- and although this new part is silicon, rather than germanium of the original diode, it fixed the problem.

8. A note regarding my "Reset Seconds" pushbutton. This button simply zeroes out the MS and LS seconds digit (and it also keeps the other digits from counting). But if one does this when the seconds count is greater than '19', the minutes count will increment by one. This isn't a big deal for me, and actually, it's kind of useful when setting time. Here's what I do:
  1. I'll run up the time to the current time (per WWV), making sure that there are at least 19 seconds on the clock.
  2. Then I'll flip the RUN switch to RUN and depress and hold the Reset Seconds button. This will set the clock to the next minute and zero the seconds.
  3. I then wait, with button depressed, until WWV hits the minute mark before releasing the button.
  4. The clock is now within a fraction of a second of WWV!

9. The +20VDC rail within my counter was way off (it read +31 VDC). Some probing revealed that a couple of transistors, as well as a zener diode, in the voltage regulator circuit had blown. The two transistors were both germanium PNP parts (one was an HP 1850-0062, which crosses to a 2N404A, and the other was an 1850-0105, for which I cannot find a cross reference). I replaced these both with 2N3905 transistors (PNP silicon). I replaced the blown zener with a 6.8 volt one, and, when finished, the voltage read +19.98 VDC. The voltages across the 2N3905 transistors are well within their range, and they aren't getting warm, so I believe everything should be copacetic.

Final note
...this posting was written not to give detailed instructions for modifying an HP counter to be a clock, but rather to generate ideas and inspiration. If you have an old HP counter kicking around somewhere, consider giving it a try!

- Jeff, K6JCA

Thursday, December 10, 2009

Improving AM Performance of the Heathkit MT-1 Cheyenne Transmitter

[Note (10 July 12):  The schematic shown below has an error in it.  The 1uF cap paralleled with the 25K pot added to the circuit should connect between pin 8 (cathode) of the 6DE7 and ground, not between pin 5 and ground.  (I would fix this drawing, but I no longer have the original).  - Jeff]

[Update (2 January 2010): New information on my Cheyenne can be found here]

I picked up this transmitter at a swapmeet earlier this year. Although the front panel was in nice condition, the topside of the chassis itself had oxidized quite a bit (as had the cabinet, which was in dire need of rust removal and repainting), and it was not very appealing. Never the less, the price was right, and I thought it might be a fun project to get on the air during the cold winter months!

The Cheyenne -- case removed for repainting and
needing an original knob (hint hint) for the Drive control.
(Click on image to enlarge.)

Hmmm...under the hood, not so pretty

The Heathkit Cheyenne transmitter was a mobile AM and CW transmitter that Heathkit marketed in the late 50's and, I believe, early 60's (its matching receiver was the Heathkit Commanche (MR-1)). With a design including a single 6146 PA, 12AX7 Mic amp, and 6DE7 as a "controlled carrier" modulator, it is similar (although not identical) to the later DX-60 series transmitters.

I powered up the transmitter with an HP-20 power supply and quickly discovered that its audio in AM mode left quite a bit to be desired -- noticeable distortion and a restricted audio passband. So I wondered...what could I do to improve its performance?

Well, the first thing to do: check to see if someone else has already been down this path. Unfortunately, a google search revealed no internet articles for improving the Cheyenne. But, because of the similarities between the Cheyenne and the DX-60, I wondered if I could apply any of the DX-60 modification articles to the Cheyenne...

First Steps...Make the Cheyenne more like a DX-60...

Electric Radio magazine has several very interesting articles by Bill Breshears, WC3K, on improving the DX-60 audio. I thought they might be a good starting point for modifying my Cheyenne, but to do so, I'd first need to correct those few differences between the Cheyenne's audio/modulator stages and the DX-60's stages.

Upon comparing the Cheyenne schematics with those for the DX-60B, the significant differences in the modulator section seemed to be:
  1. The DX-60B's 6DE7 Cathode Follower has a 33K ohm resistor from its cathode (pin 9) to ground. This resistor was lacking in the Cheyenne (and indeed, its lack prevents the AC signal on the Cheyenne's cathode-follower cathode from going below about 50 VDC).
  2. The grid of the first stage of the DX-60B's 6DE7 modulator (pin 7) has a 22 Meg ohm resistor to ground, compared to the Cheyenne's 10 Meg, and this grid is driven by the previous 12AX7 stage via a 5 nF cap, instead of a 510 pF cap in the Cheyenne.
  3. The DX-60B's PA screen voltage is driven by the 6DE7 Cathode Follower through a 47K ohm resistor paralled with a 0.1 uF cap. The Cheyenne uses a 10K ohm resistor and a 0.25 uF cap.
I incorporated the changes in first two items above (although I used a 6.8 nF cap in lieu of 5 nF in step two, because that's what I had in the junk box). I left the third item for later, until I could incorporate the Carrier-Level adjustment pot that WC3K described in his articles.

One of my goals was to drive my AL-811 linear amplifier with the Cheyenne. I don't feel comfortable running this amplifier in AM mode at more than about 100 to 120 watts carrier output power. For this level of power output from the linear, I needed the Cheyenne's idle-carrier power output to be to be in the range of about 9 watts or so.

I initially incorporated the 47K ohm resistor in step 3 above (keeping the cap at 0.25 uF) and added a 25K ohm pot in series with it to the PA Screen Grid (the 0.25uF cap paralleling both) -- similar to the carrier control pot described by by WC3K in his DX-60 mods. Unfortunately, I felt I had a bit too much drop in power, and I instead replaced the 47K with the original 10K. (This would later change again. See below...)

The new pot (mounted conveniently in the rear-panel's Key Jack hole (after all, who needs a key for AM operation?) allowed me to easily adjust carrier level. But during testing I wasn't satisfied with audio performance -- there was a still a bit of "fuzziness" on the audio (pointing to distortion) that bugged me.

Examining the audio chain, it became quickly apparent that part of the problem was with the 6DE7 "controlled-carrier" modulator itself. This modulator adjusts the carrier level such that the carrier level is low for low-level signals and higher for high-level signals. To accomplish this "dynamic" carrier-level adjustment, the first stage of the 6DE7 modulator, in addition to being an AC amplifier, "clamps" the input AC signal on its positive peaks, thus causing an additional DC voltage to be impressed across the coupling cap (that couples the signal from the second 12AX7 stage to the input grid of the modulator). This DC voltage is proportional to signal level and thus drives the modulator grid (6DE7 pin 7) more negative with higher audio levels.

As this grid is biased more negative (relative to the grounded cathode), the tube conducts less, reducing the DC plate current. The plate voltage goes up, thus raising the grid voltage on the next stage (cathode-follower) and consequently, of course, its cathode voltage.

As this cathode goes up, the PA Screen Grid voltage goes up, and more carrier appears at the output.

BUT -- the key here, and the source of the distortion, is the clamping action at the grid of the first 6DE7 section. This clamping action essentially flattens the positive peaks of the audio signal at this grid, which in turn results in flattening of the "troughs" of the modulation on the output RF signal.

This flattening of the audio signal is easily observable at the 6DE7 with a scope (monitor the plate of the first stage, for example), and is quite obvious on a 1 KHz test signal. Not good. My rule of thumb is...if you can see the distortion, you can hear it.

I also had a problem in which, as I tried to adjust the Cheyenne's audio level towards 100 percent modulation, I'd get compression (i.e. distortion) on modulated RF envelope "peaks". Again, this was readily apparent by comparing the modulation on the RF signal with the audio signal driving the modulator.

Here's photo showing both of these two distortion mechanisms (exaggerated to make it clearer) . The top trace is the modulator output. You can see the clipping on the largest negative peak due to clamping by the modulator input grid. The bottom trace shows peak compression on the output RF envelope, which you can see by comparing the different levels of the positive peaks of the modulator output with the peaks of the RF envelope -- they're all the same level!



The Next Step...Improving the Cheyenne's Audio...

The fuzziness on the audio was just enough to make me want to keep working on the transmitter. One source of this distortion, as discussed above, was from the clamping action of the "controlled carrier" modulator in order to dynamically adjust carrier level.

Hmmm...suppose I eliminated this clamping action and thus the distortion that it created? Would audio be improved?

Why not? I only needed a carrier to be somewhere in the range of 8 to 12 watts to drive my linear. There's no reason why the 6146 PA in the Cheyenne cannot handle this. In other words, why not remove the "controlled-carrier" feature of the modulator (and thus the distortion that it introduces) and keep the carrier at a fixed level?

And I wanted the carrier level to be adjustable so that I could adjust the level to give me my 100 watt "sweet-spot" output from my linear.

One way to get around the "control carrier" feature is to bias the first stage of the 6DE7 modulator so that there is a fixed negative grid-to-cathode voltage that is large enough to prevent clipping on the positive peaks of the incoming audio, yet provide sufficient carrier to drive the linear. I already had a 25K pot mounted on the back panel of the chassis that I had intended to use to adjust carrier level (and had been wired in series with the original 10K power resistor to the PA screen grid -- see discussion above). I wired it instead into the cathode of the first stage of the 6DE7 so that I could adjust its operating point.

Through a process of iteration, I adjusted the PA screen grid resistor value and the position of the 25K pot so that, for the carrier output power that I wanted (8-10 watts), the cathode of the first 6DE7 was at a high enough voltage that full-modulation audio wouldn't be clamped by the grid. Thus, the PA screen grid resistor (from V6 pin 9) was changed from the original 10K ohms to 50K ohms. Although I used a robust power resistor (it was in the junk box), there's no reason why, say, a 2-watt resistor couldn't be used. And you can play around with the value of this resistor -- lower values of resistance will increase the maximum carrier power, while higher values will lower the maximum carrier power (maximum carrier power occurs when the 25K pot is set to its maximum resistance).

[Important Note: there's a trade-off when selecting the value of the PA screen resistor: for a given carrier output power, lowering the PA screen grid resistor value means that the resistance of the 25K pot in the cathode of the first 6DE7 section must also be lowered to maintain the same carrier output level. This in turn will bring the cathode voltage closer to the grid voltage (which is essentially at 0 volts), which means that it's more likely there will be audio distortion introduced at this 6DE7 grid due to grid "clamping" the positive peaks of the audio signal. I found that a 50K ohm PA screen grid resistor worked well for my application.]

I found that the value of the pot is about 8K-9K ohms for about 9 watts carrier (no modulation) RF output from the Cheyenne. Given this value of resistance, I added a 1 uF cap in parallel across the pot to bypass it for audio frequencies. (Note: This cap can be made larger, if it's desirable to run the Cheyenne at lower power (and thus a lower potentiometer resistance, which means you need a larger cap to maintain the low-frequency cutoff), but increasing its value will also increase the amount of time that it takes for this stage to reach its bias point each time PTT is pressed.)

By the way, with these mods made and the Cheyenne set for about 9 watts carrier output (no modulation), I measure the following DC voltages during Transmit:
  • V6.5: 6 volts (6DE7, first cathode)
  • V6.2: 110 volts (6DE7, second grid)
  • V6.9: 200 volts (6DE7, second cathode)
  • V4.3: 52 volts (6146, screen grid)
These changes gave me the ability to control the output carrier power from about 4 watts to about 12 watts. Note -- at low powers there may still be some peak flattening at the grid to the first 6DE7 stage (this occurs when the cathode-grid bias voltage is less than the audio peak voltage at the grid), but I've found that there's no limiting when the 25K pot is set to give me 8-12 watt carrier power output.

OK! Now that I had the distortion reduced, I next tackled the frequency response, which was a bit too restricted in the stock Cheyenne.

This was accomplished (in addition to the changes above) simply by :
  1. Changing the 0.001 uF cap feeding the grid of the first 12AX7 stage to 0.01 uF.
  2. Changing the 510 pF cap feeding the Audio Level pot (from the plate of the first 12AX7 stage) to 0.01 uF.
This gave me an audio passband with -3dB break-points at 100 Hz and 5 KHz.


The new schematic:

(Modulator Modifications -- Click on schematic to enlarge...)
[10 July 12 -- Please note that there is an error in this schematic!  The 1uF cap paralleled with the 25K pot should connect between pin 8 of the 6DE7 and ground, not pin 5.]

Other problems:


1. 6.3 VAC reading low on DVM at the terminal strip: only about 5.5 VAC (causing the relay to chatter):
  • Bypassed the fuse in the filament line with a wire soldered to the fuse-holder's terminals (this fuse is not needed, after all, the power supply is fused, and this fuse added a few additional tenths of a volt of voltage drop).
  • Cleaned the Function switch contacts.
2. The 0.02 uF cap attached to V6 pin 1 (600v bypass) "popped." Replaced.

3. AC Hum on AM signal which gets louder as mic gain is increased. The PTT signal of the Cheyenne's mic jack directly keys the Cheyenne's relay, which is powered by 6.3V AC. A mic with wired to a 4-pin plug to mate with the Heathkit mic jack shouldn't have an issue with this, because, assuming the mic and its cable have separate grounds for the PTT return and the audio return.

Unfortunately, a number of my mics have common PTT and audio grounds (and are terminated with PJ-068 plugs), which means that the 6.3VAC on the PTT line runs on the same ground line as the audio return and thus contaminates the audio signal with AC hum.

I really didn't want to rewire a mic with a Heathkit-compatible plug -- I preferred to keep them terminated with PJ-068 plugs so that they're interchangeable among a number of my transmitters. Instead, I made an adapter with a PJ-068 compatible jack and a 4-pin Cheyenne compatible plug. Because the common ground would create a hum problem, I decided to have the PTT switch control a DC, not AC, signal, thus removing AC crosstalk from the common return line.

To do this I added a second, 5VDC relay, and rectified the 6.3VAC filament voltage to provide the voltage to drive this relay (see the schematic above). The mic's PTT button now switches this DC voltage. The new relay then switches the 6.3VAC signal to the original Cheyenne relay.

The new relay also has an additional benefit -- I wanted some way to mute an external receiver during transmit as well as key an external amplifier, and the extra contacts on the relay now provides these functions. (A previous owner of the Cheyenne had rewired the 6-pin connector that connects to a receiver (such as the Commanche) and several of these pins were left unused, so I brought these two new signals (Receiver Mute and Amplifier Key) to these unused pins on this connector).

(Additional note: the 6.3 VAC relay has a coil resistance of only about 8 ohms. This means that it draws about 0.8 A when ON. The only 5VDC relay I could find in my junk box has contact ratings of 1A at 30 VDC. OK, 1A is greater than 0.8A, but personally, I'd prefer a bit more margin. It seems to be working well so far, though.)

(Mounting of the 5V relay)

4. The SPOT switch did not work in STBY mode. Incorrectly rewired by someone in the past, I connected it to pin 5 of the relay (300V when not transmitting, although it really should go to pin 4 of the relay (per the schematic), but the remaining wire wasn't long enough, and pin 5 is a good compromise).

5. VFO tracking way off. Re-adjusted, but during this readjustment I discovered that the bottom-end of 80 meters would quickly diverge despite the rest of the band tracking well. Because I intend to use the transmitter for AM only, I decided to leave well-enough alone and I adjusted the bandspread to track the VFO dial over the range of 3.7 - 4 MHz.


Still to be Resolved:

1. Oscillator: the 1.8 MHz fundamental at the plate of the oscillator 6AU6 (that's later doubled to provide the 80 meter signal) looks terrible, as can be seen in the top trace below (the bottom trace is the RF output (sampled via an attenuator)):


I'm surmising that the signal on the oscillator plate looks this way because the 8.5 uH inductor in the 6AU6 plate circuit is differentiating a 1.8 MHz plate-current pulse-train from the 6AU6 (after all, v = Ldi/dt). But is this the way it really ought to look? I've no idea.

Never the less, the transmitter seems to perform OK and I cannot find anything obviously wrong in the oscillator circuit, so I'm going to reserve judgment...

(Additional note: The waveform at the grid of the oscillator looks great -- a nice sine wave. Just the plate signal looks weird. Ought to have a resonant circuit to make it look nice, I think.)

2. There is a bit of audio roll-off from about 1.5 KHz (down 1 dB from 1 KHz) to 5 KHz (down 3 dB from 1 KHz). The first place this roll-off appears is at the plate of the second 12AX7 stage (yet it looks fine at this stage's input grid). I've yet to identify the cause -- I suspect it might be roll-off caused by the RC network formed by the AUDIO pot and the 12AX7 grid capacitance at pin 2, but...

3. I'm not sure if this is a problem or not, but carrier power does increase a bit as I approach full modulation, even though I've disabled the "control-carrier" feature of the modulator. I'm not sure what the cause is. Perhaps a non-linear PA Screen Grid transfer function? Or...?

4. VFO drifts.


Tuning up the Transmitter:

It's important that the transmitter's loading be properly adjusted. If the loading is too light you'll get peak compression on the RF envelope. I find that adjusting loading for peak power in CW mode actually puts it about where it needs to be for AM.

Here's the procedure I use. It seems to work.
  1. Rotate LOAD and AUDIO controls fully counter-clockwise.
  2. Place function switch in GRID position. Press PTT and adjust DRIVE for 3 mA (or for peak reading if 3 mA cannot be reached).
  3. Place the function switch in PHONE and the meter switch in the PLATE position. Press PTT and dip the plate using the FINAL control.
  4. Switch the meter switch back to GRID. Press PTT and ensure grid drive isn't exceeding 3 mA. (Important note: I've found that as I rotate the DRIVE control through 360 degrees, I hit the 3 mA level at four positions. And for two of these four locations, the output power is greater than for the other two locations, despite the equivalent 3 mA drive level. When performing the final DRIVE adjustment, be sure to select one of the two DRIVE positions that results in greatest output power (at 3 mA drive)).
  5. Switch the function switch to CW. Press PTT and advance LOAD to peak the power output. Dip plate current again, just to be sure.
  6. Switch the function switch back to PHONE. Press PTT and adjust the new CARRIER LEVEL pot (on the back panel of my Cheyenne) to the desired carrier power out (no modulation). Then, while talking into the microphone and monitoring the RF envelope on a scope, advance the AUDIO control until the "troughs" of the modulation envelope are just on the cusp of flat-lining. If you find that the peaks of the envelope reach their max level before the troughs reach their min level, you have too much carrier and you should back down the CARRIER LEVEL pot -- you're just wasting power.
You're done!


Other Notes:

1. One goal was to make my modifications without drilling new holes in the transmitter, so that if someone, at a later date, wished to return the transmitter to its original condition, they could. Fortunately, it was fairly easy to add additional terminal strips using existing screws, and the key-jack hole on the back of the chassis was an ideal place to mount the Carrier-Level pot.

Additional terminal strip for new components. No holes drilled!

2. For proper AM operation, the Loading control must be adjusted for peak power out (and perhaps even a bit beyond, to be on the safe side), otherwise the positive modulation peaks will be greatly compressed and your audio will sound lousy.

3. The 1 uF cap across the new carrier-level pot can be made larger if it's desirable to run the Cheyenne at lower power (and thus a lower potentiometer resistance) and to keep the low-frequency cut-off. But increasing its value will also increase the amount of time that it takes for this stage to reach its bias point when PTT is pressed.

4. At higher carrier levels the Cheyenne has somewhat less measurable distortion at close-to 100% modulation than it does at lower carrier levels (as long as you aren't exceeding the capabilities of the PA on voice peaks, of course). I attribute this to non-linearities in the PA's screen grid transfer function (hypothesized, but not proven).

5. Here's a transfer curve that I've made, using my Cheyenne transmitter, showing RF output voltage (attenuated by my RF "sampler") versus PA Screen Voltage (note: I'm using a 6293 tube in lieu of a 6146). Test conditions: PHONE mode, 80 meters, no modulation.

You can see the curve bending at the high voltages (this results in compression of RF envelope peaks). It looks pretty linear at lower voltages (the slight burbles are most likely due to measurement error (of either screen voltage or RF amplitude) on my part).

(Click on image to enlarge)
6. WC3K's articles in Electric Radio (regarding the DX-60, see below) describe a neat modulation monitor using an LED. There's no reason why this can't also be used with the Cheyenne. I didn't install it, because I didn't want to drill a hole in the front panel and, besides, I use a scope to monitor my modulation. But I recommend taking a look at it. (You can find similar circuits in some of the web sites discussing DX-60 mods, too.)
7. A reminder: Update (2 January 2010): New information on my Cheyenne can be found here.

Resources:

Heathkit MT-1 "Cheyenne"Information HERE

Heathkit Schematics HERE

Unfortunately, I couldn't find any information regarding modifying the Cheyenne on the web. However, its design is similar to the DX-60, and there are articles that discuss improving AM performance of the DX-60...

Electric Radio articles on DX-60 improvements:
  1. "Fun with a DX-60," Bill Breshears, WC3K, Electric Radio, Issue 133, May, 2000
  2. "More Fun with a DX-60," Bill Breshears, WC3K, Electric Radio, Issue 138, November, 2000
Websites with DX-60 improvements or discussions:


Let's see...where did I put that screwdriver?

Standard Caveat...

I hope you find this information useful, but please, use these modifications at your own risk -- although they worked for me, I cannot guarantee that they'll work for you. (After all, I could have made a mistake in transposing them from my lab notebook to this post.)

If you do find any errors, or if you have any questions, please let me know. Thanks!

- Jeff, K6JCA

Thursday, October 29, 2009

R-105A/ARR-15 Receiver

[Update (4 January 2010): Additional info on modifying the R-105A to improve selectivity can be found in my new blog posting here.]


I picked up this receiver, along with a companion ART-13 transmitter, a couple of years ago. Both are in "well-used" (beat-up) condition, but...what the heck. I'd been looking for an ART-13, and the ARR-15 intrigued me. And no, the tuning knob isn't original.

Here's a picture of them in the radio operating position of a military plane. (Photo is from this website: 51H-3.)

(Click on image to enlarge.)

Although I'm well familiar with the ART-13 transmitter (having disassembled one for parts back when I was in high-school), I've never seen (nor heard of) the R-105A receiver. It's the military version of Collins 51H-3 receiver, manufactured post-World War II (mine has a 1951 contract date). And, apparently, it was Collins first remotely-tunable receiver (tunable to 10 preset frequencies).

Although intended to be used on 10 preset frequencies, the receiver can also be tuned the "normal" way via a tuning-knob and band-switch on the front panel. Frequency coverage is 1.5 - 18 MHz in 6 bands, and modes are MCW (AM) and CW.

The R-105A is designed to be powered from 26.5 volts DC, and it uses an internal dynamotor (DY-34) to convert this voltage to 220 VDC for tube B+ voltage. If using the dynamotor, I believe an external power supply should be rated at 15 amps, 26.5 VDC.

My radio did not have the dynamotor installed. Instead, a previous owner had wired the B+ line to one of the spare pins on the back connector. My receiver's power requirements are:
  • 26.5 VDC (filaments/motor): 1.4A normally, 5A (or a bit more) when Autotuning.
  • 220VDC (B+): about 70 mA.

Here are some photos. Despite the relative shabbiness of the exterior, the interior is actually in nice shape.

(R105A, Top View)

(R105A, Bottom View)

(R105A, Right Side View)

(R105A, Left Side View)

Getting It Up and Running...


OK, the only documentation I had was a schematic that I downloaded from the web (see "Resource" section, below). The radio had no dynamotor, but the previous owner had brought B+ out to pin 18 of the rear connector. So I attached a 220 volt supply between pins 18 and 9 of the rear connector ("plus" to pin 18), and 26.5 volts between pins 17 and 9 ("plus" to pin 17). I attached a pair of headphones and an antenna, then switched on the power supplies, turned on the radio's front-panel switch, and...

Nothing. The dial-lights were lit, but I couldn't hear anything -- it was as if the receiver was dead.

I looked at the schematic again and noticed that the resistors in the cathodes of the RF Amplifier and the First IF Amplifier weren't grounded, but were instead going to pin 3 of the rear connector. Clearly they needed to be connected to something (such as ground), but what exactly should this be?

One of the websites I visited mentioned that, in CW mode, the front-panel Gain pot is used to control RF, rather than AF, gain. Hmmm...RF gain as in, perhaps, the cathode of the RF amplifier? Ah ha! A clue!

I noticed in the schematic that there was one section of the gain pot, R139C, that, when the radio was in CW mode, was connected to pin 20 of the rear connector. Could it be as simple as connecting pin 3 to pin 20 on the rear connector?

Yes! I connected these two pins together, applied power, and...signals!!!

There were still some issues, though. I could hear distortion on AM signals, and I could see that, for whatever reason, there was way too much gain -- so much so that the AF Amplifier was being driven into distortion for reasonable-level signals.

When I looked at the R-105A schematic that I had downloaded from the BAMA site, I quickly realized it did not match my receiver. In fact, that schematic is for the R-105 (non-A) version, and there are some significant differences, particularly in the Audio stages. So I traced out my receiver's circuit from the detector up to (but not including) the AF Amplifier. Here it is:

(Click on image to enlarge)

Regarding the distortion and gain issues, my primary suspects were the limiter and the AVC circuits. But I looked at my schematic and quickly realized there were some strange things in the design and that I had no idea how the limiter and AVC were really supposed to function. I poked around with a scope and DVM for a few days but didn't make any headway. What I needed was a good description of how these circuits were supposed to operate. Usually the military tech manuals contain some sort of theory-of-operation descriptions...it was time to try to round one up...

After a bit of searching, I found someone on the web that could sell me a manual reprint (see "Resources" below), and I ordered it. It proved to be quite useful...

The first thing that I discovered upon reading it is that pin 3 of the Limiter stage (V110), during normal operation, should be higher in voltage than pin 8 of the same tube (that is, both diodes are conducting). In my radio pin 3 was lower than pin 8 (despite the fact that the plate voltage of V105A was higher than V107A) and the diode of V110B wasn't conducting all of the time. Oh oh. Cap C133 looked fine -- must be a leaky 12H6. Unfortunately, I didn't have a spare tube in my tube-stash, so I made a solid-state replacement using an octal plug, two 1N4006 diodes, and an 80 ohm, 3 watt resistor (to mimic the tube's filament load -- I made this using 3 power resistors I found in my junkbox). The octal-plug was wired as follows:
  • 80 ohm resistor between pin 2 and 7
  • 1N4006 Anode to pin 3, Cathode to pin 4
  • 1N4006 Anode to pin 5, Cathode to pin 8
I plugged it in and...the voltages were now OK! (This mod should suffice until I can find another working 12H6 tube.)

But there was still a gain issue -- during modulation peaks, loud signals would flat-top at the output of the AF Amplifier. (Note: the front-panel gain control does not control the level of the signal fed to the AF amplifier, it actually controls the gain (via attenuation) right at the headphones. Thus it's reasonable to expect the AF amplifier to operate at a high level (to get the best dynamic range), but...it should never go into clipping!)

I spent quite a bit of time exploring the AVC and audio stages...was there too much gain in the audio? Was there not enough gain (or leakage) in the AVC circuit? Or...?

Although there's quite a bit of gain in the audio stages, it looked to me, from the component values and from what I was measuring, that the gain I was seeing was reasonable (and I reduced the gain of the AF driver as much as I could by setting R156, an internal pot, to its max value). I checked the AVC line for leakage or loss (the AVC line drives the grids of the RF amplifier the 1st IF Amplifier) -- it looked fine. Finally, after much poking around, the only explanation I could come up with was that there simply wasn't enough AVC control-voltage being developed to keep loud, highly modulated signals from clipping.

How could I develop more negative AVC voltage?

Looking at the schematic for the AVC circuit, it is is unlike any I'd seen before. Although there is a diode detector (V106A), this is only used to change the signal-level threshold at which the AVC begins operating, rather than, as is typical, developing the AVC voltage itself.

Instead, it is the second section of V106 (V106B) that actually develops the AVC voltage.

It does this by acting as a variable load on the AC-coupled IF signal (coupled to the tube via C123). If there is no AGC action, this IF signal sees R121 (1 Meg) as its load, and R125/C129C low-pass filter the signal across this load.

With small signals, the cathode of V106B sits at about 17 volts (this level is set by the voltage divider formed by R132, R122, and R133). For signals whose amplitude, at the plate of V106B, is less than 17 volts, the tube is in cutoff and, effectively, out-of-circuit. Thus the IF signal only sees R121 as its load, and because the IF signal is AC-coupled and R121 is unchanging, the AVC voltage, after the IF signal has been low-pass filtered, is 0 volts. (That is, the low-pass filter is essentially an "averager", and the average of an AC signal that is symmetric and centered on 0 volts is...0 volts.)

If the signal amplitude on the plate of V106B exceeds the voltage of the cathode, V106B begins to conduct (the amount of conduction is determined in part by the cathode-grid voltage: note that the grid is tied to ground). When the tube conducts, it acts like a finite-valued resistor in parallel with R121, the 1 Meg load resistance, and thus the load resistance seen by the IF signal (coupled via C123) is lowered. Because the tube only conducts on postive peaks, the IF signal sees this smaller load (and thus more attenuation) only during its positive peaks, but not during the remaining part of this signal's cycle. Thus, there is more attenuation for positive peaks than for negative peaks.

Because the positive peaks are attenuated compared to the negative peaks, the "average" of the signal is no longer 0 volts, but instead it is a negative voltage. And this is the AVC voltage.

V106A is used to lower the cathode voltage for strong signals to drive the AVC voltage more negative -- if the cathode is lower than 17 volts, the tube will begin conducting at a lower positive signal amplitude, and thus more of the positive peaks of the IF signal will be attenuated compared to the negative peaks, and thus the AVC will become more negative.

Essentially, V106A acts as a diode detector, detecting the IF signal coupled to it via C132 and developing a negative voltage which, when fed to the cathode of the second stage of V106 (via R123), subtracts from the 17 volts that is normally there (fed to the cathode of V106B via R126). C186 filters out the high-frequency IF signal, leaving only its negative audio envelope.

I needed to develop more negative AVC voltage during loud signals. After experimenting, I was able to get reasonable performance with this simple mod (which can be easily backed-out if one is a purist and wishes to keep their receiver in original condition) :
  • Parallel R123 (470K) with a 47K resistor.
  • Parallel C186 (470 pF) with a 4.7 nF capacitor.
This modification drives the cathode of V106B lower (on average) than occurs with the stock 470K resistor in R123 (because R123 is smaller, there is less voltage "lost" across it, due to the voltage-divider action that takes place with R126, and hence the cathode of V106B is driven lower (but never less than about 0 volts).

The change in the value of C186 matches the change in R123 and keeps unchanged the time constant of the filter formed by R123 and C186 (which filters out the IF frequency, leaving only the modulation envelope).

It seems to work well. In my listening tests (and measuring with a scope) there is certainly less distortion with the mod than without it.

That's it! Besides that, I haven't changed anything else in the receiver.

Other notes:

Althought the receiver really isn't designed for SSB use, it can be used in that mode, although tuning is a bit too fast.

The autotune is really very cool! (There are 10 channels you can preset.)

The IF is quite broad. It reminds me of using a Command Set receiver.

To Mute the receiver during transmit, add an SPST switch between pin 3 and pin 20 on the rear connector. This switch should be closed during receive and open during transmit.

With its octal-tube sockets and well laid-out design, the R-105A is a real pleasure to work on, especially when compared to typical ham boatanchors in which components are often buried under other components, making access difficult, if not impossible.


Resources:

Schematics here (BAMA site). Yes, they are small and difficult to read. But...they're the only schematics I could find on-line, and they're better than nothing at all. IMPORTANT NOTE: Although the BAMA site lists these as being schematics for the R-105A, they are actually for the earlier R-105 (non-A) version! There's a crystal rectifier detector shown in the schematic in lieu of a detector implemented with 1/2 of V105 (as my R-105A has). And V107 is shown as a 12SJ7 instead of a 12SL7. (By the way -- there's a mistake, too, in the schematics: they incorrectly show R107 connected to the same line as R111 (First Mixer's Cathode resistor). Instead, R107 should connect to B+. And I've no doubt there are other differences...)

AN/ARR-15A feature summary.

51H-3 Good information and a great picture of an ARR-15 / ART-13 pair aboard a P2V anti-submarine patrol bomber.

More pictures here.

Tech Manual: AN 16-30ARR15-3. [You can purchase reprints of this manual (as of 30 Oct 09) from WA5CAB.]

Rear-Connector Pin Assignments (traced from the schematic: click on image to enlarge):
And finally, a reminder that my later post (on improving the R-105's selectivity) can be found here.

Standard Caveat -- take everything I've written with a grain of salt. I could have easily made a mistake.

Thanks!

- Jeff, K6JCA

Sunday, October 18, 2009

KW Atlanta Transceiver


This cute little radio followed me home from last weekend's De Anza Swapmeet. It's an "Atlanta" transceiver manufactured by KW Electronics, Ltd., of Dartford, England.

KW Electronics manufactured radio equipment for the British market. I believe the Atlanta transceiver was their one foray into the American ham marketplace and was sold here back in the early 70's (although I must admit that I never heard of the company at that time).

As you can see from the photo, the radio isn't in "original" condition. There's a toggle-switch just to the right of the band-switch (discussed below). There's also an additional DC "accessory" connector in the upper left-hand corner of the Power Supply front panel, and I'm not sure if the meter is original, or not (I suspect it's not).

The radio uses two 6LQ6 sweep tubes in its PA (also compatible: 6JE6 tubes). PA final voltage is spec'd at 800 volts, but mine measures 700 volts in receive, and 650 when loaded in transmit mode (I don't know if the power-transformer is original, or if it has been replaced).

So, given the lower PA voltage, the output power isn't quite as high as I would have expected it to be, but it suffices.

On the receive side there can be "popping" on the starting edge of loud signals. The AGC is audio-derived (rather than being derived prior to the detector) and it has a rather slow attack time, so some amount of leading-edge popping is to be expected.

But, despite these small problems, overall it's a nice package.


Here are some of the issues I ran into getting it on the air...

1. No Power Switch -- Wired Permanently ON. There was a "goof plug" on the front panel, just to the right of the band-switch. I opened up the radio and discovered that, at one time, there had been a power switch mounted to the back of the AF Gain potentiometer, but it had been removed by someone. Apparently they then drilled a hole in the front panel for a toggle switch, but by the time the radio got into my hands the toggle switch had been removed and the radio wired to be permanently On. I removed the "goof plug" and put a toggle switch into the existing hole.

2. Very low speaker volume -- someone had added a cap in series with the speaker and also ran a separate ground for the speaker from the radio to the power-supply unit (which they left unconnected). Why, I don't know, but I suspect they made these two mods to reduce speaker hum. But the cap was much too small, only 1 uF or so (thus presenting a very large series-impedance at AF frequencies). I removed it and wired the seperate ground to the speaker, and it works fine now.

3. Intermittent receive signal strength. Receive signals would sometimes be loud, and other times weak. If I touched the rear panel, I could make them fluctuate: clearly an indication of a bad connection somewhere. After hunting around, I finally found a lose connection at the bottom of L2 (inductor in the ouput pi-network). A very awkward location. Luckily, I had a very narrow soldering iron, so I could get to the bottom of the coil and repair it!

4. Low Power Output -- only 50 watts or so. I brought the power up by peaking transformer T1 per, I thought, the instructions in the manual (they're a bit ambiguous). However, carrier suppression was now terrible. I was able to get better carrier suppression, but I had to change T1's alignment procedure. Here's what I did:
  • T1 has two cores. Move the core closest to the chassis all the way to the chassis-side of T1. Let's call this the "bottom" core.
  • Transmit and insert some carrier, then adjust the top core for max TX signal.
  • Stop transmitting, and then adjust the bottom core for max RX signal.
  • Repeat, if necessary.
5. Poor Carrier Suppression. First, I adjusted the frequencies of the two carrier-oscillator crystals so that the bandwidth in both modes (when operation, say, on 80 meters) was close to identical (BW about 300 - 3100 Hz, measured using a white-noise source fed into the mic inputand a spectrum analyzer on the output). This put the carrier level for both sidebands at about the same level (when viewed on a spectrum analyzer). Then, with the Carrier Balance knob pointing straight up, I adjusted C114 for minimum carrier in both sideband modes. Then use the front-panel pot for fine-adjusting. You might need to do this several times before getting a good null. (Note, I'd tried to null carrier by nulling with the pot first and then adjusting C114. I could never get good suppression this way.)


Ongoing Problems...

1. Carrier does not remain suppressed. Don't yet know why...
(Update, 25 October 09: While poking around, I discovered that the two 100K resistors in the balanced modulator (7360) bias-network circuit had drifted an enormous amount -- one measured 232K, the other measured 335K! I ran a quick calculation (using the measurements of voltages that I'd made) and discovered that, if they had been 100K resistors, they each would have been dissipating more than 0.5 watts. Not good (given that they're 0.5 watt resistors), and perhaps the cause of the enormous change in resistance value. I've replaced them with 100K ohm, 1 watt resistors. Brief testing shows promise -- the carrier suppression seems to be a bit more in line, now.)
2. Noticeable distortion on very loud receive signals. Surely AGC related, but I don't yet have a fix...


Notes:

1. Voltage Chart Errors. Take the voltages listed in the manual's "Voltage Chart" with a huge grain of salt. Some are clearly wrong, such as the voltage on V16 pin 6. There is no way it can be 210 volts -- it comes, via a resistor, from the 150 volt regulator!

Similarly, some of the "positive" voltages are actually negative (unless I am really screwing up my measurements!).


Resources:

Yahoo Group: KW-Radios This is a great resource for schematics, manuals, etc.

KW Atlanta Photo: Atlanta

Thursday, October 15, 2009

Yaesu FT-1000D AGC Mods

Some years ago I purchased an FT-1000D. After using it for awhile I began to notice subtle distortion on SSB receive audio that, over time, I found more and more annoying.

The distortion artifacts were subtle but noticeable. They sounded like a slight "crunching" or "crackling" sound (very noticeable when someone says "ahhh."), and could be made to stand-out (when looking for its presence) by rotating the "Shift" knob to accentuate high frequencies. With the shift in its normal position, the distortion was still present, but it tended to be masked (in most cases) by the higher level of the voice signal.

(One quick test I use for locating the cause of audio distortion is to reduce a receiver's RF Gain. If the audio sounds clearer with less RF gain, then, in my experience, there's a very good chance that AGC action is creating the distortion.)

I reduced the FT-1000D's RF Gain. The the audio sounded clearer. Ah ha! There was a very good chance that the distortion, therefore, was related to AGC action, and so I started experimenting...

My main concern was SSB operation (for which I usually use either SLOW or MEDIUM AGC rates). Looking at the schematic, I noticed that there was a 10K resistor (R2140) in series with the 2.2 uF cap used for Slow AGC. Shorting-out this resistor reduces SSB distortion. But it will distort the CW envelope. My feeling was that I never use Slow AGC for CW, so this was an acceptable compromise to make.

Even the 4066 analog switch introduced some distortion artifacts (per its datasheet, its resistance is about 500 ohms for a 10V supply voltage). I replaced this analog switch with a relay, which has a much lower "ON" resistance.

I also replaced the analog switch used for switching in the MEDIUM AGC circuit (the 0.47 uF cap) with a relay. Experiments for FAST and MEDIUM AGC settings revealed that, for CW, a 30K resistor actually worked better than the original 10K ohm resistors used for these time-constants. So I added a 30K resistor (replacing the two 10K resistors) that is common to both the FAST and MEDIUM caps (0.22 and 0.47 uF).

However, the 30K does produce distortion artifacts on SSB. So I decided to compromise: only for strong signals is the 30K ohm resistance switched in -- for normal or low-level signals, the resistance is very low. (For low-level signals, the AGC line sits high. This turns on a 2N2222 which in turn shorts out the 30K resistor. For strong signals, the AGC line is driven lower, until, for very strong signals, it drives the 2N2222 into cut-off (threshold set using the two series-diodes attached to the base), which then places the 30K back into the circuit.)

I found that the 30K also increases distortion on AM signals, so I also short it out for AM operation. (Therefore, it is only present for strong CW, SSB, and, I suppose, FM (it has no effect on FM)).

Here's my original markup of the FT-1000D schematic:
(Click on Image to Enlarge)


Here's a drawing that might be a bit clearer...

(Click on Image to Enlarge)


Notes:

1. If one would like to keep the changes simple and not incorporate all of the mods that I made, I would recommend the following two modifications:
  • Short out R2140 (10K ohms: Slow AGC.)
  • If you use Medium AGC for SSB, then also short out R2141 (10K ohms: Medium AGC. But note that this may create some fuzziness on CW signals.)
2. While reviewing my notes from 2003, I discovered a mention in my lab notebook of the addition of a Schottky diode across the 1.5M resistor (cathode connected to the RF GAIN side of the resistor) so that, as RF Gain is turned down, the AGC follows with little delay (otherwise there could be a long delay for the audio level to catch up with the control position. But I don't show this diode in my final schematic. I don't know if it's actually there, or not, and I'm not about to reopen the FT-1000D to find out.

3. I used Clare DSS41A05 Relays.

4. Audio envelope testing was performed by modulating the RF signal from an HP 8640B (in AM mode) with a pulse generator (100ms pulse, rep rate of 1 second, 2 ms rise/fall, 1.4v peak, -1.1v DC offset). The AGC waveform was monitored @ TP2005, and the audio envelope monitored at the headphone jack.

5. Diodes are in series with the base of the transistors to ensure that they fully turn off when the control signals go low.


Important note:
I made these modifications to suit the style of operating that I prefer (SSB ragchewing). These modifications may not be suitable for your style of operation. So, if you're also experiencing annoying distortion, please consider these mods to be a starting-point for your own experiments in improving the performance of the 1000D.

Tuesday, October 6, 2009

Central Electronics CE 100V Transmitter

My adventures bringing a CE 100V back to life!

A number of local hams have Central Electronic 100V transmitters, and on occasion I've joined them during their ragchewing roundtables on 80 meters (sans 100V on my part). I enjoyed the sound of the radio as well as its styling, and I thought it might be nice to have one of my own. And thus began my search for a 100V.

I finally found one that a local ham was selling on Ebay. I made a bid...and won it! Fortunately, because the seller was local, I was able to save shipping charges (it is a heavy radio!) and pick it up myself.

When I got it home, I discovered the radio, besides having extensive cosmetic issues, also had operational problems, and I put it to the side while I worked on other projects that were less daunting than tackling the 100V appeared to be.

Finally, I decided to bite the bullet and get the 100V on the air. First thing to do...pull it out of its cabinet and then get to work...

The radio is a marvel of design, and arguably represents the high-water mark of amateur radio transmitter design of the 50's. Which translates into a radio that is large, heavy, and complex (26 tubes!).

(Top View, with VFO Assembly removed)


(Paging Doctor Frankenstein!)

When I looked inside the actual radio, my heart sank...the chassis wasn't dirty, it was oxidized. I believe it must have originally been plated, and this plating had turned an ugly grey color. And in some places, actual rust had appeared!

(Typical oxidation/corrosion on this radio. Labels on the back panel and on the chassis are essentially unreadable.)


(Despite the terrible shape of the chassis, the front panel actually looks pretty good!)

My VFO was very difficult to turn, and felt "lumpy". Per the Tusa notes (well worth a read) on the 100V, I decided to remove the VFO assembly and take a look at what might be going on...

(Note: the VFO assembly is actually fairly easy to remove. You do not need to drop the front panel! Instead, follow the procedure in the Tusa notes (although please note that for step 7, you should unscrew the two bottom mounting posts from the VFO assembly, not from the front panel)).

After I'd removed the VFO assembly, I was curious to know how it looked inside, so I removed the back cover. Whoops! Chunks of foam (and foam bits) tumbled out. Looks like Central Electronics used this foam (3/8 " thick) to act as an insulator to minimize temperature variations within the can. And after 50 years, it was disintegrating.

(Disintegration of the insulation foam in the VFO Assembly!)

I happened to have an old mouse pad lying (1/4" neoprene), so I cut it up and glued it to the inside of the can with some RTV cement. Voila!

(Old mouse pads have many uses, such as...new insulation!)

Now to attack the difficult-to-turn VFO. The Tusa notes recommend repacking the bearings, but from the instructions I'd read (and from the stories I'd heard), it sounded like a real nightmare.

Instead, I decided to see if a shot of WD-40 into the bearings would help to loosen up the old grease...

I held the VFO so that the knob was pointing toward the floor, then applied a quick burst of WD-40 into the "well" (see photo below). The bearings are below this well, and, by holding the knob towards the floor, I hoped the WD-40 would flow down into the bearings.

It seems to have worked. The mechanism turns much more easily now. Sure, I probably ought to repack the bearings (because the viscosity of the grease might give it a bit "smoother" feel). Maybe next year...
(Click on image to enlarge.)

While I had the VFO out of the radio, I was curious to learn how the VFO tracking mechanism worked...

As the VFO frequency is adjusted, the VFO's lead screw moves a core in and out of the main VFO coil, thus changing the oscillator frequency. But there is a secondary adjustable coil, too, whose core is attached (via a rod) to a right-angle bracket that can pivot. This rod moves in and out of the secondary coil according to the height of the "VFO Corrector Adjustment Screws," and allows small corrections to be made to the VFO frequency as the the user tunes over the 1 MHz-wide range of the VFO.

You can get an idea of how the mechanism works from the two photos below:
The lead screw moves the frequency correction assembly (consisting of the screw run through the block) along either direction of the lead screw (depending upon whether the frequency is being adjusted up or down). A rod runs over the bottom of these screws (shown bottom-up in the photo above), which in turn causes the metal right-angle bracket to which it is attached to pivot.

Attached to the other end of this right-angle bracket is a rod which drives the core of the secondary coil in or out, thus correcting the frequency. In the photo below you can see both the larger main coil (on the same axis as the lead screw) and the smaller secondary coil below it (only a couple of turns of this coil are visible).
Pretty clever!

A note about the frequency correction adjustment. I would recommend that you start at the end of the VFO that has the largest "positive" (rather than negative) delta from the dial frequency. In other words: if the offsets at either end of the dial are both positive, start at the end that has the largest positive delta. If both of the offsets are negative, start at the end that is closest to the dial frequency, and if one end is positive and the other negative, start at the positive end.

"Zero" your dial at this frequency by moving the black line on the clear plastic to overlay the "0" on the dial (there's a screw a few inches below the VFO knob that let's you do this). Then, moving the VFO in 500 KHz increments, adjust the frequency using the "VFO Corrector Adjustment Screws" per the Tusa Consulting note on VFO Recalibration.


Problems that I've run into:
  1. Meter not working. No movement, at all. I opened up the meter and discovered that one of the "spiral springs" that attach to the armature had opened up. It was a real pain to repair, but repair it I did. (By the way, my meter is about 1 mA Full Scale, and has a resistance of 47 ohms). I also added a pair of diodes (1n5818) hooked antiparallel across the terminals of the meter (to protect the movement from burning out), as well as a 0.1 uF cap -- if you add the two diodes you must include this cap, otherwise your meter may read low in the "Watts" position.
  2. VFO Sticking/Hard-to-Turn (See discussion above)
  3. VFO Not Tracking (See discussion above)
  4. Lack of the -120V Blocking Bias in STBY mode. A 1uF/200V electrolytic cap that was attached to this line (via a 10 ohm resistor in the power-supply section) was shorted to ground. (Note: neither this cap, nor the 10 ohm resistor, appear in the schematics). These parts were probably added to slow-down the transition between STBY and Transmit. I didn't have a 1uF with a high enough voltage rating in the junkbox, so I instead used a 2 uF cap.
  5. Inability to Null Carrier in Sideband Modes. The meter would remain pegged to the right in Null mode irrespective of any adjustments I made to the two Carrier Balance knobs. I measured the forward-voltage of the four original germanium (CK715) diodes in the modulator plug-in module, and the voltages varied wildly (from 0.234 volts to 0.632 volts). I replaced these with HP 5082-2063 (Schottky?) diodes that I had in one of my parts' bins (their Vf was 0.34 volts, and matched within millivolts for all 4 diodes). Works fine now.
  6. Wattmeter: Reads too low, and cannot adjust far enough. Resistance values had drifted over time, and one of the 100 ohms resistors had drifted to 109 ohms. Replaced with 100 ohms, and now can adjust with the pot (although it's almost at its limit).
  7. RF Ammeter: Reads too high, and cannot adjust far enough. The resistors have apparently drifted. The voltage-divider resistors are in an extremely awkward location, so their replacement is very difficult. Instead, I added 120 ohms in parallel with R147/R148, and that brought the voltage into range to allow correction using the pot.
  8. No X-Axis movement on Monitor Scope. Replaced V21 (6U8A)
  9. Low-frequency Noise in Audio, Eventual Loss of Sideband Suppression. I traced this to a leaky cap in the audio phase shifter module -- one of the symptoms was a high DC voltage at an output (pin 8) of the phase shifter. Cap C130 was leaky (but the leakage couldn't be measured with a DVM) -- I replaced this 4711 pf mica cap (actually measured 4739 pf) with 4731 pf consisting of a 4300 pf mica and a 470 pf mica in parallel. (Update: I've discovered that this problem is discussed in the 100V article by Charlie Talbott, K3ICH, in the August, 1996 issue of Electric Radio, and I've implemented one of his mods, which is to insert a 1uF, 400V cap betweenV7 pin 1 and the phase-shift network PS-2 socket's pins 2 and 6 (to isolate C128 and C130 of the phase-shift network from the B+ voltage on V7's plate)).

With these issues resolved, I've deemed the 100V ready for the air:

The 100V in its operating position!

Although the 100V is now up and running, there are still...

Problems I've yet to resolve:
  1. 8 MHz Oscillator cannot be adjusted to be exactly 8.000 000 MHz (it remains too low). Even with the adjustment cap at minimum value.
  2. FSK Adjustment does not span 100-900 Hz. Instead, it only seems to have a range of about 150 Hz.
  3. PA "Idle" Wattage (in SSB Xmit, no voice) should be in the range of 60 watts (per the recommendation of others) -- mine is more around 30 watts. (Central Electronics added adjustment pots for both the driver bias and the PA bias adjustment to their 200V transmitter, but these parts are not in the 100V, and to add them involve more surgery than I'm willing to undertake at the moment).

Other notes and Comments:
  • The schematics can be inaccurate! I've found additional parts, and I've found parts missing, when comparing the actual circuitry to the schematic.
  • Tusa Consulting has a number of notes on the CE 100V and 200V transmitters. You can find these notes here.
  • A previous owner had replaced the two batteries internal to the Speech Limiter module with two AA-size alkaline batteries, and had mounted their holder on the outside of the Speech Limiter's case.
  • To get at the tubes and adjustments beneath the fan in the audio section, just loosen the transformer screw and tilt the fan bracket up...

  • Some other useful data...
100V I.F. Mixing Scheme
(Click on image to enlarge)

Some manual copies have impossible-to-read voltage charts. Here are clearer copies (thanks to Jon, K6JEK).

Tube Voltage Chart
(Click on image to enlarge)

RF Voltage Chart
(Click on image to enlarge)

Articles on the 100V in Electric Radio magazine:
  • "Restoration of the Central Electronics 100V," Dennis Petrich (K0EOO), Electric Radio, Number 20, October 1991.
  • "Observations on the Central Electronics 100V & 200V," Charlie Talbott (K3ICH), Electric Radio, Number 88, August 1996.
(There may be additional articles in Electric Radio. These are the two that I've found.)


Phase Network Simulation [11 March 10]:

I ran a SPICE simulation on the CE 100V's Phase Network and compared the two outputs. Here's the plot (and the schematic):

(Click on image to enlarge)

The solid line is amplitude, while the dashed line is phase.

Sideband suppression versus phase error can be calculated with this equation:

- 20$\displaystyle\log_{10}^{}$|tan($\displaystyle{\frac{\delta}{2}}$)| (Where "delta" is the phase error.)

So a phase-error of 1 degree (from the ideal phase-shift of 90 degrees) will result in about 41 dB of sideband-suppression; a phase error of 2 degrees: 35 dB; while a phase-error of 10 degrees will result in only about 20 dB of sideband suppression.

(Note: I haven't included in this simulation the small-signal resistances presented by the grids of the tubes that the phase network output drives (I don't know what they are). For ease of calculation, I've assumed that they're infinite. Also, because I don't have any SPICE models for tubes, I just used a transistor as the driver.)

[LTspiceIV, the program that I used for my simulations, is free, and it can be found here.]


Standard Caveat!


I may have made a mistake in any of the above, so use at your own risk!