My Collins KWM-2
Alexander D. Schapira, 2006-2014, All rights reserved
thanks to all who provided suggestions and responses to my
original request for help,
I am providing this account of
how I got
my newly acquired KWM-2 back on the air.
I hope this will be of
some interest to other KWM-2 owners.
I bought this rig from its original owner, a retired Collins
engineer, who had it in storage for several decades.
It was exactly as delivered by Collins, and according to the seller,
had never been out of its case.
With any vintage equipment, there are several things that need to
be done before applying power for the first time. After a visual
inspection and a little cleaning, a resistance check was in order.
Following the chart in the KWM-2 manual, I checked every tube pin
resistance to ground. Everything was reasonable except for V11 pin 2.
This is the BFO grid. I measured 1.1 Meg
(roughly agreeing with the schematic) but the chart in the 1962
KWM-2 manual said 95k. This turned out to be a typo -- the 1978 manual
specifies 1 Meg.
Satisfied that there were no dead shorts,
I powered the rig up for the first time, and the receiver came
to life. With a short wire antenna, I was able to hear WWV on
The Dummy Load:
In preparation for transmitter testing, I examined the
dummy load that came the KWM-2. It was built by the original
owner and consisted of fifty
100 ohm 2 watt resistors in series/parallel to comprise a nominal
50 ohm 100 watt load (five series banks of ten in parallel.)
Some of the many solder joints had come apart resulting in an
intermittent resistance. I re-flowed the solder joints to solidify
the load. It now measures a solid 47 ohms -- a little low, but close
The Power Supply:
The PS was also built by the seller in 1962, using Collins parts, and
similar in circuitry to a PS-2.
listening to the rig, after about an hour, one of the
40 uF/450V dry electrolytics in the power supply began to snap,
crackle, pop, sizzle, and overheat. Uh-Oh.
If you look closely, you can see the full "One Year" guarantee on the
I replaced all ten electrolytics with
modern equivalents from Mouser.
While I was at it, I added a pilot lamp and a jumper to conveniently
disable the high voltage. The rectifiers are solid state diodes so
"removing the rectifier tube", as the Collins manual suggests, to
disable the HV is not an option.
I also added a dropping resistor to reduce the B+ slightly,
since it was about 50 volts too high.
I plan to enclose the PS in a perforated metal cage.
Perhaps I'll look for a 516F-2.
After repairing the power supply, the
receiver seemed to be working
properly, but not the
transmitter. I originally observed two problems. Although the KWM-2
did appear to tune up properly into the dummy load:
- There was
hardly any audio gain, barely enough to budge the
ALC meter even with the mic gain all the way up, and
- The VOX either
pulled in too late or dropped out too early.
I ruled out the microphone by trying
mics which exhibited similar behavior.
In order to help characterize the transmitter problem,
I hooked up an RF ammeter in series with the dummy load,
and connected the probe of an HP 410B RF VTVM across the load
to monitor the output of the rig. When tuned up in the LOCK
position I measured about 60 volts RMS and 1.3 amps RMS into the load,
for a power output of 78 watts (continuous).
These measurements are consistent with the 47 ohm dummy load
(I^2R and E^2/R).
(The load remained cool for the short time I was driving it.)
I then knew, much to my relief, that the finals, driver,
and most of the preceding stages were okay.
While working into the dummy load, I
listened to the signal on
another receiver and heard severely distorted and very weak audio.
Since the rig tuned up properly, but had very low and distorted
audio, I concentrated on the audio chain.
to do some homework:
I downloaded copies of all of the KWM-2 Service Letters and
Service Bulletins, from the CCA website. (This alone was reason enough
to join the CCA.)
I also searched the CCA archives for 'audio' and 'VOX' and printed
the relevant articles.
At the suggestion of a reader of my original post, I downloaded
the latest KWM-2 manual, even though I had the original 1962 manual.
I was pleasantly surprised to find all the modifications from the
service bulletins listed in the revised manual and indicated on the
schematics. (The original service bulletins sometimes provide
step-by-step instructions and parts locations for making the
modifications; the updated manual only lists the changed components,
without instructions or locations. So it is good to have both.)
cross-checked the relevant modifications listed in the 1978 manual
against the service bulletins looking for changes that might affect
either the audio or the VOX. Several numbered service bulletins were
relevant, so I made the indicated changes:
changed from 0.47uF
to 1uF, replaced with 600V Orange Drop
changed from 0.047uF to
0.068uF, replaced with 600V Orange Drop
R201, 220k, added
C160, replaced with
Changes #7 and #8 require the addition
of a terminal post to allow for the insertion of a series
resistor. I tried to be as faithful to the quality of the
original Collins work as possible.
These changes did not cure the audio problem, but did
solve the VOX problem. So audio was getting through to the VOX
circuits but not to the modulator. (I later removed change #8, R199,
made the minimum VOX time constant much too long.)
In the course of a very thorough visual inspection, I spotted a
R169, 1k! (No, it wasn't in an area I had worked on.)
I replaced it, but that was not relevant to the audio problem.
To troubleshoot the audio problem, I wanted to hook up an audio
source to the mic (or phone patch) input, but what audio level to
use? I measured the audio output from each microphone (while whistling
at a constant amplitude!) and obtained
results as follows:
The first two mics came with the KWM-2.
1 Euphonic C-47 Hand
the smoke out?
I set my audio source for 1200 Hz, 80
(an intermediate value) and connected it to the phone patch (mic)
I then began to carefully scope out the audio chain starting from the
microphone input and working inward.
Just to make things interesting, the audio chain is fully energized
only when in TX mode. So I disabled the PA screen and HV, and keyed
the PTT line.
I found the audio signals to be normal until I got to the *input* to
where the audio signal was severely clipped and distorted. Ah-Ha!
Using a DMM with a diode test range, it looked like two of the four
diodes in the balanced mixer ring were bad. (One open, one shorted.)
I unsoldered one end of each diode to confirm this. Yes! I replaced all
four diodes CR1,2,3,4 with the modern 1N4454 equivalent.
CR1, 2, 3, 4
Voila! Plenty of clean audio right through.
This was the
main problem all along!
I quickly readjusted the carrier balance, reset the ALC zero,
ran coax from my workbench up the basement stairs, and connected up my
antenna, a trap dipole for 80/40/20.
I heard W8ERN in Michigan on 7.184 MHz, called him,
and he came back with a 5x5 report.
The band was extremely noisy, but I was ecstatic!
I asked W8ERN to indulge me while I switched from
the hand microphone to the D-104. The report was "Very, very good
K3MRG on frequency also heard me and said I was 30 over S-9,
with great audio (using the D-104) .
Having seen the failure of the
50-year-old dry paper electrolytics
in the power supply, I decided to replace the paper electrolytics in
Referring to the Collins 1978 KWM-2 manual, in change #3, C264 was
from 4uF/350V to 20uF/350V,
so I used the new value. I also replaced C254, 4uF/350V, another dry
paper electrolytic, C102, 100uF/16V, and C259, 8uF/25V.
I decided to leave C106 the 3-section metal can (wet) electrolytic
alone since there was no hint of trouble (hum or excessive ripple)
I noticed that C102 had not been the proper value according to the
and didn't look to me like it was a Collins part.
And it had no red glyptol on its connections.
Back on the air, a few more contacts confirmed that nothing had
gone wrong while changing the electrolytics.
While changing bands, I noticed that I had to 'diddle' the band switch
to get it to make contact reliably in the three lowest (3 MHz) band
positions. An inspection showed that the 'rotor' of the wafer switch
came to rest with its sweeping contact not quite centered under
the outer contacts.
In this switch, the shaft is removable (in order to allow the aluminum
coil covers to be installed and removed) and so the shaft angle is not
fixed relative to the mechanical detent. So I loosened the coupling of
the shaft to the knob detent, rotated the shaft ever-so-slightly to
rotor contact under the nearest fixed contact, and re-tightened the
coupling. Now the band switch engages reliably in all positions.
Although the band switch contacts look tarnished, I resisted the
temptation to clean them just to make them look pretty; but I did apply
a little DeOxit.
The KWM-2 service instructions suggest a complete alignment if the rig
hasn't been used in a long time, so I proceeded to do the 'Laboratory
Alignment'. I followed every step of the procedure. The rig was now
calibrated, aligned, neutralized and back on the air.
Several KWM-2 owners recommended adding a fan to keep the rig cool.
This sounded like a good idea. Following some of the published
articles, I fabricated a bracket and mounted a 3"
12 volt DC fan in a completely non-destructive and reversible way.
It is powered with a full wave bridge and filter from the 6.3 VAC
available on filament pins on the noise blanker socket. It runs fast
enough to move hot air, but slowly enough not to be too noisy.
A few days after moving the rig off the bench and into the shack,
I was listening to a QSO while working on something else nearby. Then
a whiff of something burning
and smoke pouring out
of the KWM-2.
I immediately shut it off. Another "Oh no."
Back down in the shop, and on the bench, there were no obvious signs of
anything burned. I knew that the problem must be in the receiver
I disabled the high voltage and screens by popping out the jumpers.
Since the power supply was solid state, I gradually brought up the
line voltage using a variac and soon found that the source of the smoke
was inside the enclosed relay compartment. The 68 ohm 1 watt, series
resistor, R157, in the receiver B+ line, had burned up.
It was clear that
something had shorted out causing the
overload that burned out R157, but what?
I turned again to the pin-by-pin resistance chart in the KWM-2 manual,
and proceeded to check each tube pin's resistance to ground.
(This was the second time I did this. The first time was
before I powered the rig on for the first time, at which time
everything checked out within reasonable tolerances.)
This time, everything checked out okay until I came to V12, pin 1,
which was a almost a dead short to ground instead of 10k.
This pin is connected to R159, R160, and C237. It took me a long time
just to find R159 hidden behind several wires. I carefully moved the
wires to get a better look and found that R159, a 1k 1/2 watt, had
R160, 120k was good,
so that left only C237, half of
a dual 0.01uF ceramic disk. I clipped out the half of C237
that was connected to V12 pin 1, and found that it read a solid 7 ohms
confirming the culprit! I removed the dual C236/C237 disk
it with two 0.01uF, 1000 volt disk ceramics, and I replaced the burned
out R159. Now V12 pin 1 had the proper resistance to ground.
I inspected R68, 15k, which would have blown if the other half of the
dual disk, C236, had shorted, but it was right on. Only one half of the
0.01 had shorted. Finally, I replaced R157, the 68 ohm, 1 watt resistor
in the relay compartment.
I powered up and the receiver came to life. All the smoke was back
inside the components where it belongs.
In the week or so since completing the repairs, I have worked eight
countries and about 20
domestic stations using a trap 80/40/20 dipole only thirty feet up.
All-in-all, I enjoyed every minute I spent trouble shooting this rig
forward to meeting you all on the air.
-Al Schapira, W2ADS
Most of the elapsed time taken to repair the rig was taken up by
for parts to arrive from Mouser. They're very fast, but unfortunately,
they can't ship parts until *after* you place your order. :-)
Please feel free to contact me at
Page last modified 12/07/15