Tuesday, October 26, 2010

WRL Duo-Bander 84

I came upon a pair of these radios (plus an AC power supply) a few months ago while visiting a ham-radio store that specialized in older, used gear. It's a Duo-Bander 84, manufactured by WRL (World Radio Laboratories, Inc.) sometime in the late 60's.

(Click on image to enlarge)

The Duo-bander 84 is designed to be a sideband-only rig for 75 and 40 meters, and thus coverage on 75 meters is 3.8 to 4.0 MHz, and coverage on 40 is 7.1 to 7.3 MHz.

Tuning-up the transceiver's Transmitter is a single knob operation (rather than the usual peak-the-grid, dip-the-plate, and adjust-loading). Simply insert carrier into the Transmit signal by un-nulling the carrier balance (using the "NULL" control), and then adjust the "TUNE" knob for maximum output on the meter. (The "NULL" control (carrier-balance) will then need to be re-nulled).

The PA consists of a pair of 6HF5 sweep-tubes. With about 700 volts on the plates (the radio requires a separate power supply, by the way), I find that my peak power out is a bit more than 100 watts.

(A copy of the manual can be found via the link in the Resources section, below.)

Top view
(click on image to enlarge)

Bottom View
(click on image to enlarge)

Bottom view of a later version.
Note the new holes in the chassis under the IF board, and the crystal filter is sans case. I don't know if these changes were done in the factory, or were made later in the field by an end-user.


Notes:

1. Adjusting PA bias. Per the manual, one should set the PA bias so that, when the carrier is properly nulled and there is no voice-excitation (and the meter switch on the back panel is set to "Bias"), the meter needle, when transmitting, is on the "bias" calibration mark on the meter scale.

Unfortunately, one of my two radios had a broken meter. How should I then set its PA bias?

Well, I wasn't too sure how accurate the meter was on the rig that was operating properly (and note, the meters are not calibrated in mA), so I didn't want to use it as a calibration reference. I did a bit more research and discovered that the Swan 350 also used a pair of 6HF5 tubes in its PA. Their manual is a bit more explicit, and they state that the PA bias should be adjusted to 50 mA idle current when in Transmit mode.

I decided that if 50 mA is good enough for the Swan, it's probably good for the Duo-Bander, too! So I verified that the PA cathode resistance to ground was 2.5 ohms (there are four 10 ohm resistors in parallel), and adjusted the bias-pot on the back panel so that the PA cathode-to-ground voltage was around 0.125 volts.

And with the PA Idle Bias adjusted for 50 mA, the meter needle sits at the "Bias" marking on the meter faceplate (when the meter is in BIAS mode)!

2. No ALC circuit. The Duo-bander 84 does not have an ALC circuit to limit voice-peaks. To prevent excessive overdriving, I prefer, while monitor the output RF waveform with a 'scope, to adjust the mic's gain until the peaks are just at the peak-power out (this point will become evident as you adjust the mic-gain past this point -- the RF peaks will not get any higher, and you'll see more flat-topping).

I've thought about adding an ALC circuit, but decided that it wasn't worth the effort. For those who are interested in experimenting, a good place to start would be to look at the schematics for the Swan 350, the Galaxy V Mark II, as well as other radios (Heathkit HW-12A, Galaxy GT-550, etc.) that use sweep tubes in their finals. You'll see an ALC circuit that's common to all of these radios and which consists of a pair of diodes used as a negative peak detector to generate a negative ALC voltage based upon the PA grid voltage.

3. Distortion on transmit audio audio.

Both of my radios exhibited significant distortion on their transmit audio when I was first testing them. On both, I traced the cause back to a bad C9 capacitor. This is a 2 uF, 50V cap that acts as an AC ground for the collector-load of Q4. The balanced-modulator (Q6 and Q7) requires that the audio-drive to it (from Q4) consist of two signals 180 degrees out-of-phase and of equal amplitude. Thus the phase-splitter's (Q4) emitter and collector loads should be identical. If C9 is not a good AC ground for R11, then the amplitudes will not be equal, and there can also be a phase difference between the two that is not equal to 180 degrees.

On both radios I replaced their C9 caps with 4.7uF, 63V axial electrolytic caps that I had in my junk box, and the distortion problems greatly improved. (Note: it's OK to use a 4.7uF to replace the 2uF in this application. Larger value caps provide a "stiffer" AC ground for the audio signals, due to their lower impedance at audio frequencies).

Here's C9 in the schematic:

(Click on image to enlarge)

4. Excessive Transmit Audio Low Frequency Roll-off.

Per the alignment instructions, the filter passband can be "shifted" in frequency by adjusting the Carrier Crystal frequency with trimmer C39 (mounted next to the carrier crystal at the back of the radio). I found that, even with the crystal adjusted to shift the filter passband as close to the carrier as I could, I still had excessive roll-off in the low-frequency audio, so much so that it seemed as though the low-frequency cut-off was around 500 Hz.

Poking around the audio path with a 'scope, I discovered that there was excessive low-frequency roll-off occurring just after the mic-jack coupling capacitor C7 (0.01 uF). I paralleled C7 (0.01 uF) with a 0.1 uF cap (or you could simply replace C7 with a 0.1 uF cap), and this removed the excessive low-frequency roll-off.


Bandwidth is now 300 - 3000 Hz.

5. Carrier Crystal Oscillator stops oscillating in Transmit.

While I was trying to adjust the Carrier Crystal oscillator frequency to shift the filter passband so that the low-frequency cut-off was around 300 Hz rather than 500 Hz, I discovered that, as I rotated trimmer C39 towards one of its limits, the oscillator would stop oscillating when I was transmitting (but there was no problem in Receive mode).

As an experiment, I paralleled C15 (150 pf silver mica cap) with a 100 pf silver mica cap, and this seems to have cured the problem. My reasoning for adding this cap is: the oscillator would stop oscillating as its frequency was lowered, and so I assumed this meant that the trimmer cap was approaching its maximum capacitance. I decided to add capacitance across C15 (that is, to the fixed-cap side of the voltage divider formed by C15 and C39) in order to return the capacitance ratio between these two caps back to a value where the oscillator still oscillated. It seems to have worked, but I can't say that this is an optimal solution. Consider it a band-aid which fixed the problem for this particular transceiver.


(Note that you can easily add this capacitor by simply mounting it across the coax (from the Carrier oscillator) connected to the two pins at the back of the printed-circuit board, very close to the trimmer cap C39.)

6. Receive Distortion due to AGC:

While operating this radio I noticed that, for some signals, there was some subtle distortion on the receive audio. This problem seemed to manifest itself with stations who were using wide-band audio (e.g. lots of low frequencies). Fortunately, most signals I copied didn't seem to have this problem.

But the distortion was noticeable enough on a couple of stations with whom I talk regularly, and so I decided to look into it.

I noticed that, per the schematic, there are actually two AGC lines: an "AVC RF" line (to the grid of V6) and an "AVC IF" line (to the grids of V3 & V4).

The AVC IF signal has a very fast decay time constant (essentially, the decay of C21, a 0.02 uF cap, is controlled by R22, a 33K ohm resistor, which is C21's discharge path into C22, a much larger 0.22 uF cap).

AVC RF, on the other hand, has a much slower discharge -- C22's decay is controlled by R23 in series with R24.

(Click on image to enlarge)

Looking at the AGC (or AVC, if you prefer) signals with a 'scope, I noticed a potential problem with the AVC IF line. If you look at the top photo below, you'll notice there are a lot of "spikes" on its waveform (as the AGC goes more negative, there is more attenuation). These spikes are voice peaks at C21, and they quickly decay via R22.

One often sees this sort of AGC response in older receivers. I believe this fast-decay AGC is to limit the gain of fast transients (e.g. static crashes?) yet not have them affect the longer-delay AGC. Unfortunately, in my experience, these sorts of spikes can "modulate" the received signal and produce perceptible receive audio distortion, and often the receive audio will sound better if one can eliminate this sort of AGC modulation. (Of course, recognize that, in doing so, there can be a trade-off with limiting the gain of fast-transient noise).

For the WRL Duo-Bander 84, one way to clean up this distortion is to add more capacitance to C21 -- that is, make it larger. But rather than add another cap, one can simply short-out R22, the 33K ohm resistor between C22 and C21. This means that the AVC IF line's decay time constant becomes the same as the AVC RF line's time constant, and you can see the result on the AVC IF line in the lower photo, below.


(One issue with this sort of mod, though, in which the capacitance is greatly increased, is that the attack time will slow down (because you're charging more capacitance). The Duo-Bander's Receive AGC is audio-derived, and therefore, as with many other receivers with audio-derived AGC, you can get an audible attack "pop" at the start of strong signals. In theory, the mod I've made should exacerbate this type of popping, but I haven't noticed much difference, if any, in attack pops with or without this mod.)

The mod can be made easily without removing the PCB: To short-out R22, jumper the top of C22 to the top of R37, as shown in the photo below.


Again, I want to stress -- the original distortion that I was hearing was very subtle and only manifested itself on a couple of signals that I listened to regularly. For the most part, all other receive signals sounded fine. So this modification is certainly not necessary, and, if you do try it, it's quite possible that you won't notice any difference on the majority, if not all, of the signals you copy.

7. Other problems...

Here are some of the other problems I found in my two Duo-Bander radios:
  • Very little gain on receive. I traced this to R46 (47K, 1W plate-load for V7b) reading infinite ohms. Replaced with a 47K, 2W resistor from my junk-box.
  • Very low TX audio. I traced this to a faulty R9 (270K, 1/2 watt) in the Mic Preamp circuit (it read infinite ohms with an ohmmeter). Replaced with same value resistor.
8. Measuring resistance per the Manual's Resistance Chart. While trouble-shooting the rig I discovered that a number of resistances listed in the Resistance Chart were reading infinite ohms, despite their non-infinite values in the table.

These were typically plate resistors (or other resistors connected to the +400V B+ supply), as well as, for example, cathode-resistors in the TX path (e.g. V7 cathode). The reason why they were reading infinite with my ohmmeter was that they had no path to ground. (Note that the manual specifies its resistance tables in the chart to be "resistance to ground").

To make an accurate comparison of all resistances, per the chart, I'd recommend that you actually measure across the specified resistors, or you could ground the appropriate relay pins (e.g. pins 2 and 6 for Receive and Transmit B+, and pin 7 for TX cathodes) and then make your resistance measurements (in either case, though, be sure that all power is first disconnected from the radio -- remove the connector from the power supply to the Transceiver's P1 Power Plug!).

9. Power Supply:

The manual states that the power supply requirements are:
  • HV: 800VDC @ 400 mA
  • Low B+: 325/375 VDC @ 200 mA
  • Neg: -100 VDC @ 30 mA
  • 12VDC @ 200 mA
  • 12VAC (or DC) @ 5 A
Per one of the links in the Resource section below, three power supplies were available for the Duo-Bander 84:
  • Deluxe 400 Watt AC supply AC384A $89.95;
  • 400 Watt DC Supply DC384A $99.95;
  • 250 Watt AC Power supply AC48A $49.95.
In addition, other Galaxy power supplies allegedly can be used, too (I've not personally verified this, though), per this post on the Electric Radio Forum:

12/05/02 08:47 PM | 0 Good Guy Alerts Vote Edit Reply | WRL Galaxy Duo-Bander 84 power supply info wtd:
n3ibx
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1 Post
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Washington Crossing
N3IBX
Ignore User
Hello All, Would anyone know if the AC supply from a Galaxy 5 MkII will work with the Duo-Bander 84? Any assistance will be appreciated. Mod-U-Later, Joe Cro N3IBX
Joseph Cro


12/06/02 07:38 PM | 0 Good Guy Alerts Vote Edit Reply | Galaxy Power Supply
Stu
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Elmira NY
K4BOV
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Yes Joe,

Any Galaxy power supply except the PSA-300 (came with the Galaxy 300) will run the Duo-bander without modification. The PSA-300 will certainly run a Duo-bander; but, the 12 pin Jones plug must be rewired to conform with the later Galaxy pin configuration.

If you want to send me the serial number of your Duo-Bander, I may have some service bulletins for your particular series. Might want to give me the serial number of the power supply as well. They all work as I indicated; but, some are more capable than others.

Stu/K4BOV


10. Some other radios using 6HF5 tubes in their finals:
  • Swan 350 and Swan 400
  • Drake 2NT
  • Galaxy 300, Galaxy V, and Galaxy V Mark II
  • Galaxy 2000 (Amplifier)
  • Hallicrafters HT-46 and SR-400

Resources:

1. Specifications and more information on the Duo-Bander 84 can be found here and here. (Admittedly these are both sketchy).

2. An Instruction Manual (in PDF format, and including alignment instructions and schematic) for the Duo-Bander 84 can be found here.


Standard Caveat:

Of course, I may have made a mistake, so use my suggestions at your own risk!

Also, this radio uses high-voltages that can kill you. Always use caution when working on a radio of this type.