[Collins] KWM2A V13

[Dr. Gerald N. Johnson]

Pin 6 is the plate of the crystal oscillator section and that resistance
includes many load circuits plus the power supply. And the leakage of
several electrolytics. Its not anchored in stone, it will vary. Check
the two 68K resistors in the bias circuit to the cathode of the VOX tube
for being high in resistance to make that B+ resistance look high. They
tend to be over heated and to have drifted high. A 5 watt 33K power
metal film resistor is a higher quality replacement for those two that
are in parallel.

V13 triode is used for receiver mixer and cut off by negative bias on
transmit. V13 pentode is the crystal oscillator, it should draw more
plate current on the high bands because its doubling in the plate there
and the crystals are on a higher frequency. The glow may be from
electrons missing the plate, not uncommon in power tubes, but not so
common in tubes that size. If its a sample of the plate current striking
the glass, its intensity may change from band to band. The plate current
will be greater when the crystal activity is poorer and that is
certainly a possibility though the crystals should have aged at
different rates. Check the pin voltages on that tube while operating too.

For those following the tone oscillator discussion, I see in the SSB
Yellow Book that early KWM-2 used a twin-tee tone oscillator, rather
than the phase shift oscillator shown in the 32S manuals and the later
KWM-2 manuals. Still there's a high pass side to the twin tee circuit to
encourage feedback at VHF and so parasitic oscillations.

For the audio distortion, check for a leaky cathode bypass capacitor on 
the first audio and for leaky coupling capacitor between the audio 
stages that can up set the grid bias on the following stages.

Over time there have been changes in that cathode circuit and connection 
to the AVC using some of that first audio stage cathode bias to set an 
AGC threshold.

The calibrator can be set smack on. I improved the stability of the 
calibrator in my 75S-3B (dating from 1964) by encasing the crystal in a 
block of white styrofoam and I glued a bit of aluminum foil to face the 
nearest tube to reflect radiated heat. Today I might want to use Dow 
blue board for a bit sturdier insulator than white bead board.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 1/31/2012 7:34 PM, Rick Gouge wrote:
> I have been working on my 1964 KWM2A for the last few days. Just your
> basic clean up and tweeking you have to do when its been sitting on
> the shelf for too long. Everything seems to be working very nicely
> right now. I only have one concern. V13  6U8A has a slight blue glow
> to it like its gassy. I have change the tube out a few times and its
> still there. So my next guess would be that something in that circuit
> needs to be look at. The first thing i did was get the resistance
> chart for the tube sockets. the only thing that i could find out of
> res was pin 6 which should measure 8K. It mearsures 10.51 K  Could
> that be my gassy effect or should i dig deeper or just don`t worry
> about it? The only thing i could find wrong that could be a little
> problem in that area would be that my audio on the 4 Ohms speaker is
> not the nice perfect audio that you should get from a KWM2A and my
> calibrator is a wee bit off, I will look on my schematic again.  What
> do you think Boys am i just being a
>
> worry wart? Rick VE7RiK
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