[Collins] KWM-2A help (long)

[Dr. Gerald N. Johnson]

I'm working from the last Rockwell manual, stored on line at CCA.org. In 
the extensive list of revisions, it hints some vintages used the 6U8 for 
the crystal oscillator, then maybe 6U8A.

Now I've also found my yellow book with a 1959 version of the schematic. 
I sure do wish someone would have compared schematic to radio and to 
production drawings. This vintage schematic shows V12A calibrator is 1/2 
a 6U8A, but V12B, cathode follower is a 1/2 a 6U8.

The bandswitch circuits are all wrong. They show a rotor tab that 
contacts only one pole at a time and I'm sure the tab is wider 
contacting two so those open poles aren't really open. Then in the 1959 
schematic of the crystal oscillator on 20m it shows an 8-50 (C65) by 
itself and then a 47 fixed (C66) with the wires crossed on an adjacent 
switch pole. THEY SHOULD BE SHOWN CONNECTED with a dot. The last 
schematic was redrawn and neatened making it worse. I'm pretty sure the 
8-50 won't tune down to 3 MHz because for 3.5 with the slug way deeper 
in the coil it took a 240 fixed in parallel with the 8-50.

The transmit second mixer is kind of balanced for the LO from LO signal 
going to the grid of one triode and the cathode of the other. The 3 MHz 
signal is neutralized by the capacitive unbalance of the two capacitors 
inside L6 and the 6 pf capacitor C26 from mixer output to the bottom of 
that coil. If C26 is missing there would be more 3 MHz on the output.

If L5 has been replaced with a choke having a series resonance in the 14 
MHz range it would kill off the 14 MHz output.

These 8-50s have been known to go open. Its where the shaft hits the y 
shaped conductor on the back. DeoxIT and exercise is a solution, 
providing they will still turn. Sometimes they lock up and there is no 
modern replacement that will stand the RF voltage and current.

We don't know the vintage of your M-2, but with 6U8 crystal oscillator 
its older rather than newer. Can't go by the serial number, they were 
issued in random order (by decree of Art Collins), but you can get an 
idea by the date code on the crystals and the mechanical filter and 
maybe the electrolytics if they can be decoded. Early M-2 used a metal 
cased (Y) filter, later used the plastic cased (FA) filter with a 
different mounting under the chassis.

On 9/19/2010 8:43 AM, Al Parker wrote:
> Thanks Jerry & Dave,
> I'll try to comment on Jerry's & add my observations:
> 1. Yes, the 14mc range is doubled in the osc. plate, it uses a ~8.7xx mc
> xtal doubled to ~17.4 to mix with the ~3.1xx mc variable IF signal to
> get ~14.3xx. I'm getting abt 20v p-p out of the osc. on 14 vs abt 30 on
> 7mc, so it is less. Is that too much less? I'm not sure, because---
> 2. At the plate(s) of the 2nd mixer I see the mixed signal, but the LF,
> ~3mc, signal is by far the dominant one, the 14mc (or 7mc) is much less,
> appears as a modulation freq. on the LF signal. I didn't record the
> amplitude of that signal, but I'd guess that the HF component was less
> than 1/4th of the LF. (I'll see if I can get a look at it with the spec.
> anal.) At the following RF amp grid, the tuned ckt has eliminated the LF
> component, and just leaves the wanted final freq. But it's only ~400mv
> for 14mc, vs 2v at 14mc. That tuned ckt seems to be peaking properly,
> but I will ck to see if there's another peak in the cap's range (not the
> mirror peak). I'll do that this AM. That may be the solution, I sure
> hope so, but then it's mis-tuned on 15 & 10m also. Those trimmers have
> an 8-50 range, so it's just possible on 14 & 21mc to peak both the
> wanted & the 3mc IF signals.

Not likely to tune to 3, but maybe to tune to 20.5 MHz.

When a mixer isn't balanced or neutralized, the gain at the input 
frequencies to the output is always higher than to the converted output.

> I did ck the final & driver neutralization, it's not perfect, but
> they're stable.
> I have 2, from different manuals, schematics that show the 6U8 osc.
> tube. I wish they'd date coded the schematics.
> many thanks for the insight, I'll let you all know what the outcome is.
> 73,
>
> Al, W8UT
> www.boatanchors.org
> www.hammarlund.info
>
> "There is nothing -- absolutely nothing -- half so much
> worth doing as simply messing about in boats"
> Ratty, to Mole
>

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