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 lots of cables that cannot be conveniently routed to where the meter resides.

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 and old toothbrush - were in excellent condition, as was the "SWR/2000/200" switch that selected between displaying the VSWR and forward power after it had been cleaned.  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 combination front/panel SWR "Set" switch and potentiometer.

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 is a very good idea as it prevents possible damage to the radio and it simplifies measurements by minimizing interaction with the radio's protection circuit. ²
• Set selector switch to "SWR" and put the potentiometer in "set" 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 long shot, 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 by my attempts to free it, if not having been submerged in water.

For a few days I wasn't sure what I might do to find a replacement:  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 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 parted out and/or 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 works well and 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.  Setting the radio to 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.  Practically speaking, keeping one eye on the reverse power - and minimizing it - while keeping another on the radio's wattmeter will make quick work of adjusting whatever manual antenna tuner you might use.
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 at some power levels.  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".  If you have a meter like this but can find only the 500k version of the potentiometer, go ahead and use it anyway - just place a 330k fixed resistor across its two connections to limit its maximum value to about 200k:  It, too, may be a bit "touchy", but it will work.
    

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This page stolen from ka7oei.blogspot.com

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A solid state replacement for an old radio's "vibrator" (Wards Airline 62-345)

Figure 1:
The front of the Wards Airline 62-345 with its rather
distinctive "telephone dial" tuning dial.
It's powered up and running from 12 volts!
Click on the image for a larger version.

Quite some time ago - a bit more than a decade - a friend of mine came to me with an old "Farm" radio - a Wards Airline 62-345.  This radio - from the 1930s - was designed to run from a 6 volt positive ground battery system  such as that which one might find in tractors and cars of that vintage.

How high voltage was made from low voltage DC in the 30's

As the technology of the time dictated, this radio has what's called a "vibrator" inside - essentially a glorified buzzer - that is used as a voltage chopper along with a transformer to convert the 6 volts from the battery to the 130-150 volts needed for the plates of the tubes within.  Not only did this vibrator do the chopping for the high voltage, but it also performed the duty of synchronously rectifying the AC waveform from the transformer as the pulses from it would naturally be in sync with the motion of the moving reed, briefly connecting the output of the transformer to the input of the high voltage DC supply when the voltage waveform from it was at the correct polarity.

These devices, as you would expect, don't have a particularly long lifetime as they are constantly buzzing, making and breaking electrical contact and causing a small bit of arcing - something that will inevitably wear them out.  Even if the contacts were in good shape, the many decades of time that have passed will surely cause these contacts to become oxidized - particularly since these devices are in rubber-sealed cans (to minimize noise and vibration) and the out-gassing of these materials is likely of no help in their preservation.

Figure 2:
The chassis of the radio.  The vibrator is in its original
can in the far right corner.
Click on the image for a larger version.

Such was the case with this radio.  Often, the judicious application of percussive repair (e.g. whacking with a screwdriver) can get them going and if the contacts are just oxidized, they will often clean themselves and work again - at least for a while.  In this case, no amount of whacking seemed to result in reliable operation, so a modern, solid-state approach was needed.

The solid-state replacement

As mentioned earlier, the job of the vibrator was to produce a chopped DC waveform, apply it to a transformer for "upping" the voltage and then use a separate set of contacts to perform synchronous rectification - and our solid-state replacement would need to do just that.  That last part - rectification - was easy:  Just two, modern diodes would do the job - but chopping the DC would require a bit more circuitry.

The owner of this radio also had a few other things in mind:  He changed it from 6 volts, positive ground to 12 volts, negative ground so that it could be readily operated from this more-common power scheme.  The change to 12 volt filaments required a bit of work, but since all of the tubes were indirectly heated, the filament supply could be rearranged - but some tubes had to be changed to accommodate different filament voltages and currents as follows:

• Oscillator and detector:  This was originally a 6D8 (6.3v @ 150mA) and it was replaced with a 6A8 (6.3V @ 300mA).  Other than filament current, these tubes are more or less the same.
  

• IF Amplifier:   The original 6S7 (6.3v @ 150mA) was retained.
  

• 2nd Detector/AVC/1st Audio:  The original 6T7 (6.3V @ 150mA) was retained.
  

• AF Output:  The original 1F5 (2.0v @ 150mA) was replaced with a 6K6 (6.3v @ 400mA).  The latter is a pentode, requiring a bit of rewiring and rebiasing to replace the original triode.

• Magic Eye tube:   The original 6N5 (6.3v @ 150mA) was replaced with a 6E5 (6.3v @ 300ma) - which is also more sensitive than the 6N5, giving a bit more deflection.

The 6T7 (150mA), 6A8 (300mA) and the #47 dial lamp (6.3v @ 150mA) are wired in parallel on the low side with one end of the filament grounded while the 6K6 (400mA), 6S7 (150mA) and 6E5 (300mA) are wired in parallel on the high side with one end of the filament connected to +12 volts.  You might notice a current imbalance here (600mA on the low side with 850mA on the high side) but this is taken care of with the addition of 30 ohms of resistance between the midpoint of the filament string and ground to sink about 200mA, getting us "close enough".

He also did some additional rebiasing and other minor modifications - particularly for the rewiring of the AF Output from the original 1F5 to a 6K6 as he swapped a triode for a pentode - which was then  wired as a triode.  The total current consumption of the radio at 13 volts is 1.6 amps - a bit more than half of that being the filament and pilot lamp circuits meaning that about 10 watts of power is being used/converted by the vibrator supply and consumed by the idle current of the audio output and other tubes.

The "chopper" circuit

The other issue with the 6 to 12 volt conversion is that of the primary of the high voltage transformer:  This transformer is center-tapped with that connection going to the "hot" side of the battery (which was originally at -6 volts) - but what this really means is that there's about 12 volts from end-to-end on the transformer at any instant.  We can deal with this difference simply by driving the transformer differently:  Rather than having the center tap "hot" with the DC voltage and alternatively grounding one end or the other as the vibrator did we can simply disconnect the transformer's center tap altogether and alternately apply 12 volts to either end, reversing the connection electronically to preserve the original voltage ratio between primary and secondary.

This feat is done using an "H" bridge - an array of four transistors that will do just what we need when driven properly:  Apply 12 volts to one side and ground the other - or flip that around, reversing the polarity.

Consider the schematic below:

Figure 3:
Solid state equivalent of a vibrator supply.  This version uses an "H" bridge, suitable for
the conversion of a 6 volt radio to 12 volt operation as detailed in the text.
Click on the diagram for a larger version.

This diagram shows a fairly simple circuit.  For the oscillator we are using the venerable CD4011 quad CMOS NAND gate with the first two sections wired to produce a square wave with a frequency somewhere in the 90-150 Hz region - the precise value not being at all critical.  The other two sections (U1c and U1d) take the square wave and produce two versions, inverted from each other.

Figure 4:
The top (component side) of the circuit.  This is built on a
piece of phenolic prototype board.
Click on the image for a larger version.

The section of interest is the "H" bridge consisting of transistors Q1 through Q4 wired as two sets of complimentary-pair Darlington transistors.   Here's how it works:

• Let us say that the output of U1c is high.  This causes the output of U1d to be low as it's wired as a logic inverter.
• The output of U1c being high will cause the top transistor (Q1 - a PNP Darlington) to be turned OFF, but at the same time the bottom transistor of this pair, Q2, will be turned ON, causing the connection marked "PIN 1" to be grounded.
• At the output of U1d - being low - we see that the bottom of this pair of transistors, Q4, is turned OFF, but the top transistor Q3 is turned ON causing V+ (12 volts) to appear at the connection marked "PIN 5".
• In this way, the low-voltage primary of the transformer has 12 volts across it.
• A moment later - because of the oscillator - the output of U1c goes low:  This turns off Q2 and turns on Q1 - and since this also causes the output of U1d to go high this, in turn, turns off Q4 and turns on Q3.  All of this causes "PIN 5" to now be grounded and "PIN 1" to be connected to V+ - thus applying the full 12 volts to the transformer in reverse polarity.

Also shown are D1 and D2, the solid-state replacements for the synchronous rectifier of the original vibrator.  While this could be a pair of high-voltage diodes (>=400 volts) we simply used half of a full-wave bridge rectifier from a junked AC-powered switching supply.  Finally, resistor R3 and capacitor C2 form a filter to keep switching noise and high-voltage spikes out of the power supply of U1 to prevent its destruction - a sensible precaution!

Now some of you might be concerned about "shoot through" - the phenomenon when both the "upper" transistors (Q1, Q3) might be on - if only for an instant - at the same time as the "lower" transistors (Q2, Q4) as the switching is done.  While this may happen to a small extent, it has negligible effect - particularly at the low switching frequency where this effect would constitute a very minuscule percentage of the switching period:  This circuit is efficient enough that no heat sinking is required on transistors Q1-Q4 and they get only barely warm at all.  Were I to build it again I might consider ways to minimize shoot-through, but this would come at the expense of simplicity which, itself, is a virtue - and since this circuit works just fine, would probably be not worth the effort.

Figure 5:
The bottom (wired side) of the circuit with flying leads
connecting to the original base socket.
Click on the image for a larger version.

These days one might consider building this same type of circuit using MOSFETs instead of Darlington transistors (e.g. P-channel for Q1 and Q3, N-channel for Q2 and Q4) and this should work fine - but the Darlington transistors were on hand at the time that this circuit was built and very easily driven by U1 - and the bipolar transistors are - at least in this case - arguably more rugged than the MOSFETs would be - particularly since there was no need to include a "snubber" network to suppress switching transients that might occur.  It's also worth noting that while standard MOSFET transistors would work fine for a 12 volt supply, you'd have to be sure to select "low gate threshold" devices to work efficiently at 6 volts or lower - something that would not really be an issue with the bipolar Darling transistors shown here.

This circuit is simple enough that it was wired onto a piece of phenolic prototyping board, snapped down to a size that will nicely fit into the original can that housed the vibrator.  To complete the construction, the top of the can - which was originally removed by careful filing and prying - was glued into its base using "shoe goo" - a rubber adhesive - keeping the board protected, but also allowing it to be easily disassembled in the future should modification/repair be necessary.

To be sure, the Internet is lousy with this same sort of circuit, but this version has worked very well.

What about the center tap version of the solid state vibrator?

You might ask yourself "what if we don't want to rewire a 6 volt radio to 12 volts?"  As noted previously, the boost transformer in the radio had its center tap connected to the "hot" side - which, in this case, would have been the negative terminal (because many vehicles had 6 volt, positive grounds at the time).  This circuit could be easily modified for that as you'd need only "half" an "H" bridge and the resistors driving the transistors would be changed to a lower value - perhaps 2.2k.  Depending on whether the it was positive-ground or negative ground, or whether the center-tap was grounded or "hot" - this would dictate whether you needed the PNP or NPN halves of the H-bridge.

(If you have a specific need, feel free to contact me by leaving a comment.)

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This page stolen from ka7oei.blogspot.com

 

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