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 - 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 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
• 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.  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 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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Making LED headlights RF-quiet

TL;DR

 If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem:  You will probably need to put the offending switch-mode LED controller in "RF Jail" as described below.  While VHF/UHF is the target of the efforts noted below, they should be equally effective at HF.

* * * * *

We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy.  These devices have proliferated in the past several decades and surround us at home, work and in our cars.

Figure 1:
The vehicle in question with LED headlights -
which are now RF-quiet on all bands.
Click on the image for a larger version.

Generally speaking, modern internal combustion vehicles are in RFI terms "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home.  For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.

Effective mobile operation is therefore contingent on a vehicle that is intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well.  These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers¹ , GPS receivers and extra lighting ², to name but a few.

Real-world case - Aftermarket LED headlights

A friend of mine recently installed aftermarket LED headlights in his older Honda CRV ³.  It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters:  Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when communicating simplex (e.g. direct - with no repeater) with a weak/distant station - turning on the headlights dramatically reduced received ⁴ signal quality and range.  As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.

LED headlights typically consist of two modules:  The LED module itself and the "controller" - which itself is a switch-mode power supply to regulate the current to the LEDs as well as switching arrays of LEDs for high/low beam - and even color temperature in some models.  Sometimes these units are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.

Even knowing that it was futile, we tried putting clamp/snap-on ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful".  This was not unexpected:  Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant:  It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!

Methods of filtering

As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed and ferrites alone are likely to be effective only when the interference is very slight to begin with.  Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations:  Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.

As discussed previously on this blog ⁵ one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every wire to prevent RF energy from being conducted and the metal enclosure can prevent direct radiation from leads themselves - particularly important at VHF/UHF but less-so at HF.  Done properly, this method can be "completely" effective ⁶ in preventing interference.

Figure 2:
Simplified diagram of the method of filtering.  The inductance - provided by the ferrite beads ("L")
provide "choking" impedance to the RF currents being carried on the wires from the LED controller
(the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize
their efficacy.
Click on the image for a larger version.

Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT").  The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor.  The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.

The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device.  By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure.  The above configuration is easily capable of attenuating RF energy from HF through UHF by 30dB or better (more likely 50-60dB at some frequencies) - a value far higher than ferrite alone.

The metal box containing the electronics and filtering offer another important benefit:  As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.

An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors.  As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy.  If the ferrite beads - or some sort of inductor - had not been installed, the shunting of the low-impedance RF energy from the noisy device would have been far less effective.

Figure 3:
An assortment of feedthrough capacitors.  The top two
rows are of the "screw-in" type, typically mounted to chassis-
walls and bulkheads while those on the bottom row are the
"solder-in" type as used in the partition in this project.
Click on the image for a larger version.

In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a sensitive device FROM RF energy from a nearby transmitter we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.

Feedthrough capacitors and the partition

A bit more needs to be said about "feedthrough capacitors".  Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past.  Figure 3 shows an assortment of feedthrough capacitors:  The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.

A feedthrough capacitor has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself.  By being constructed this way, there are no wires or leads between the "capacitor" part  and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.

Figure 4:
Solder-in type feedthrough capacitors soldered
to the brass partition.  This large sheet of metal
provides a low-impedance RF path to the
common "ground" (e.g. case) to contain RF
entirely within the metal case.
Click on the image for a larger version.

Compared to a "normal" capacitor with wires, such a capacitor is far more effective at bypassing RF energy to "ground" and it also suffers much less from parasitic issues like self-resonance - a property in which the capacitor and its internal inductance form a resonant circuit can cause it to practically "disappear" from the circuit (e.g. cease to be effective) at certain frequencies.  For this application - where it's particularly important to reduce RF interference at VHF and UHF - the use of feedthrough capacitors is - along with some inductance - a nearly foolproof method of attenuating such energy without resorting to surface-mount components and/or a specially-designed PC board. ⁷

Figure 4 shows nine feedthrough capacitors soldered to a brass partition (the soldering to the plate is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them.  As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal.  This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.

Putting it in the box

Figure 5:
The LED controller in the box w/filtering.  By
containing RF currents within the box, both
common-mode and differential RF currents
on the leads are reduced to near zero.
Click on the image for a larger version.

To eliminate direct radiation from even the very short leads, the LED's controller which can cause problems at VHF/UHF, ferrite beads ⁸ and feedthrough capacitors are contained within a box as seen in Figure 5.

Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.

Below the partition (see Figure 5) are the wires that connect to the outside world:  On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module.  The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition and install the rather large ferrite beads.

The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering.  As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.

Figure 6:
As the original cables were very short (about 5", 13cm) longer
wires had to be spliced to allow placement of the large die-
cast box.  This shows the male headlight connector and the
LED module spliced to the cable and covered with nylon web.
Click on the image for a larger version.

At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.

The result

With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.

The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID:  The return signal was very weak and noisy - as hoped and expected - but there was no difference in the way that it sounded when the lights were switched on and off.  While admittedly unscientific, this test tells us pretty much everything that we needed to know:  Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!

* * * * *

Footnotes:

1. The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see:  "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article:  "A 'quiet' 5 volt USB car power supply" - link.
   
2. This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards.  Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
3. Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
4. A low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
   
5. Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits.  This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system.  This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
6. "Completely" eliminating conducted RF is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
7. The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation.  "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.  With the solder-in feedthrough capacitors, a brass partition was used as it was easier to drill and solder to than one of copper.  If screw-in feedthrough capacitors were used, the partition could have been aluminum.
   
8. The ferrite beads used in this project were Fair-Rite 2643000801.  These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller.  This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing.  I used them primarily because they were on hand.
   

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This page stolen from ka7oei.blogspot.com

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