A new push-pull switch/potentiometer for the Heathkit HM-102 VSWR/Wattmeter

Figure 1:
The Heathkit HM-102 HF VSWR/Power meter.  This meter -
often found at swap meets and hamfests for low cost - is still
as useful as it was when new - and is easily repaired today,
decades after its introduction.
Click on the image for a larger version.

A few weeks ago I got my hands on a(nother) Heathkit HM-102 VSWR HF Wattmeter/SWR bridge.  This device was available from the early 1970s and into the early 1980s and was a common sight in ham shacks in the U.S.¹

This unit consists of two parts:  The front panel meter/switch and the "bridge" section and it included a fairly long cable, giving the user the ability to use it as a single, stand-alone unit with the bridge mounted in the enclosure with provided screws or to have the meter and the bridge separated by several feet - often more convenient in a busy ham shack with cables.

Repair needed!

The unit depicted in Figure 1 had belonged to a (silent key) friend of mine and due to an unfortunate set of circumstances, it was submerged for some time when the basement of his house flooded after his passing.  Amazingly, the meter movement survived unscathed - aside from a few small particles of dust that were easy to blow/brush out after carefully popping off the front  - but the rest of it needed a bit more attention.

The case itself and the knobs - once worked over with a bit of "Formula 409" cleaner - were in excellent condition, as was the "SWR/2000/200" switch that selected between displaying the VSWR and forward power.  The circuit board comprising the bridge and the SO-239 coaxial connectors looked a bit "crusty" and would require a few parts, but a few minutes with an old toothbrush, some dish soap and an ultrasonic cleaner did wonders in making it look better.

What did not fare well was the front/panel SWR "Set" switch.

SWR measurement by the HM-102 and other single-meter bridges

Using a single-meter VSWR bridge is a multi-step process:

• Transmit at a relatively low power.  If the VSWR of the load is uncertain, using a low power setting very good idea as is prevents possible damage to the radio and it simplifies measurements by minimizing interaction with the radio's protection circuit. ²
• Put the VSWR in "set" or "calibrate" mode.  This bypasses the wattmeter's calibration, allowing the user to set the meter and is accomplished on the HM-102 by pulling on the "Set" switch.
  
• Set the meter to full scale.  This sets the meter so that the full scale is relative to the current forward power to allow a direct comparison of the reflected power and thus the VSWR.
• Take the meter out of "set" or "calibrate" mode.  With the forward power calibrated, the meter will now directly read SWR on the appropriate scale.  On the HM-102, this is done by pushing on the "Set" switch and the VSWR can be directly read from the meter's scale.
  

For the HM-102, the "set/calibrate" function uses a "push-pull" switch on a potentiometer, and as Figure 2 shows, this assembly did not survive.  This type of switch was ubiquitous in the 50s through the 70s, found on radios and televisions as well as many other consumer electronic devices, but it has all but disappeared in recent decades and I thought that finding a suitable replacement might be difficult.

Figure 2:
The remains of the original push-pull switch/potentiometer.
The aluminum shaft had seized/galled to the aluminum
bushing and efforts to free it resulted in its eventual
destruction - although it was likely already a lost cause.
Click on the image for a larger version.

While I figured that it was a longshot, I initially tried several things:  Some "PB Blaster" as well as some heating/cooling cycles, but it became apparent that even if I had been able to free the potentiometer shaft that had seized to the bushing, both the potentiometer and the switch itself were likely to have been destroyed.

For a few days, I wasn't sure what I might do:  I rummaged around in my collection of potentiometers and while I found one or two "push/pull" units with an integrated switch, none of them were anywhere near the needed 200k-Ohm value.

The Internet to the rescue

On an impulse, I decided to check the parts suppliers like Mouser and DigiKey and was surprised to find out that these devices were still readily available.  On a whim, I also checked Amazon for a "push-pull potentiometer switch" and was greeted with dozens of options:  I'd managed to overlook the possibility that such switches would still be sold as replacement in other gear - namely certain brands of guitar amplifiers.

A bit of further searching showed that the two most common values of these potentiometers seemed to be 500k and 250k - the latter being "close enough" to the original 200k part that it shouldn't really matter ³.  While most of these devices have knurled shafts for splined knobs - which would have sufficed even with the setscrew knob of the HM-102 - I did find on Amazon a Bourns unit (P/N:  PDB183-GTR02-254A2 - which I also found at Digi-Key) that had a "smooth" 1/4" shaft and ordered that .  The cost:  About US$13 or so, including shipping - cheaper that the other parts suppliers, and faster delivery, too.

Figure 3:
The new push/pull potentiometer/switch installed in the re-
furbished HM-102.  This is the Bourns P/N
PDB183-GTR02-254A2 - still readily available.
Just visible in the photo is a 1.3 MegOhm resistor across the
terminals to bring the maximum value closer to the 200k-
Ohms of the original device and this may help to make it
slightly less "touchy" during adjustment.
Click on the image for a larger version.

When it arrived a few days later it was a perfect fit with the shim/spacer of the original switch/potentiometer.  As this potentiometer was intended for stereo audio, it has two independent sections - and the switch itself was a DPDT - so only half of the device needed to be wired up.

Getting it working again

The only other casualties were on the bridge's PC board:  The 2-20 pF trimmer capacitor was seized and the 50k board-mounted trimmer potentiometer was too "flaky" to be trusted.  It so-happened that I had an exact replacement for the capacitor and found a very similar trimmer potentiometer in my collection that was 60k - close enough.  Spending a few minutes with a dummy load, I was then able to null the VSWR bridge and calibrate the meter as described in the manual.

A quick test showed that the power meter was within specifications and the VSWR bridge's balance was good from at least 80 through 10 meters when spot-checked.

Conclusion

At first, I thought that this vintage HM-102 might have been consigned to the trash can, but with only a bit of work and a few parts, it was resurrected and cleaned up.  Aside from a few scuffs revealing the fact that it had, in fact, been used for some time, it looks very good for a piece of electronics that is a half-century old and had been nearly drowned!

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Footnote:

1. The HM-102 - which is designed to work from 1.8 through 30 MHz - appears to have been introduced around 1972 when the kit cost $29.95 while in 1981, the kit cost $49.95 and was apparently discontinued after that.  There was also a VHF version for 6 and 2 meters - the HM-2102 - that looks very similar and in terms of mechanical design and much of the above discussion could be applied to it as well - and it even uses the same potentiometer/switch as the HM-102.
2. Modern radios have "fold-back" circuitry to automatically reduce power if they "see" a high VSWR.  It is best to set the transmitter to low power if the VSWR is unknown (say, 5-10 watts on a 100 watt radio) as even the best protection circuit isn't completely able to protect the radio under all conditions - particularly if one is using an external antenna tuner and forgetting to reduce the power level.  Running a low power output during tuning prevents the fold-back circuit from activating as much:  If the radio had been set to high power, the output power will rise rapidly as one attains a better match (e.g. when using a manual tuner) and the user will have to frequently re-set the SWR to full scale to obtain an accurate reading and possibly confusing the operator - but at low power, this complication is minimized.
   
3. The original potentiometer/switch was 200k, but the modern replacements are only readily available as 250k devices with a logarithmic taper.  This higher value and possibly the logarithmic taper may make this control - which is simply placed in series with the front-panel meter - a bit "touchier" than the original when it is adjusted.  Placing a 1.2-1.5 MegOhm fixed resistor (the precise value is unimportant) across the potentiometer connections will reduce its maximum value to about 200k and make this adjustment slightly less "touchy"
    

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This page stolen from ka7oei.blogspot.com

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