Re: {Collins} WE KWM-2A S-meter adjustment, and need correct manual



If you download Section 7 of the KWM-2/2A manual from the CCA website, and look at mods 26, 26, and 27, it will shed some light on the matter. At a quick glance, it seems your radio is one of the "early" units referred to under mod 25 (the mods are detailed in the sheets before the schematic, and the places they occurred are shown by little numbered arrows in the schematic).

The voltages referenced in your manual(s) would of course refer to radios in which the circuitry agrees with the schematic in the manual, so would not be helpful in those areas of disagreement.

One possibility -- and I stress the fact that I'm guessing here -- is that the gain of one or more IF, RF, or mixer stages is away too low, so the 25 microvolt signal is below the AGC threshold even with the RCVR GAIN ADJ pot set for maximum gain. Signal levels (and frequently noise levels!!) on the HF bands are typically well above the normal AGC threshold, so the radio would work OK except it wouldn't hear weak stations. Try cranking up the generator.

The AGC seems to be basically working, since you don't report severe distortion.

Good luck & 73
Bob

On 2014-12-22 4:00 PM, Rick Poole WA1RKT wrote:
I have acquired a really-sad-condition KWM-2A that I am restoring to decent looks and functionality (my two previous threads, about getting another cabinet or repainting this one, are about this radio).

It's a winged emblem, with the open-frame relays (which seem to be in OK condition), serial number looks like 10653 although this was hand-written with a magic marker so I can't be sure it's correct.

I have two KWM-2/2A manuals, one a PDF 9th edition with a Rockwell cover (obviously MUCH too new for this radio) and one a paper copy of the 4th edition from 1964.  I'm thinking that one is probably too new also, as noted below.

I've gotten the radio working more or less; receiver sounds pretty good although the AVC decay is a little fast, and this afternoon I got some good signal reports (signal strength, audio quality, and on-frequency) on 20 meters, but there is one remaining issue in trying to complete the S-meter adjustment procedure per the manual... the meter zeroes fine with the S METER ZERO control but the RCVR GAIN ADJ screwdriver-adjust pot has no meaningful effect on the S meter reading with 25 microvolts at the antenna connector per the manual (the pot itself seems OK).

I figured the AVC / S meter issues may be related, so I poked and probed the circuitry with a lab scope and voltmeter and determined that the RCVR GAIN ADJ control has no significant effect on the IF level into the AVC rectifier.  The 1st and 2nd IF tube pin voltage and resistance-to-ground readings are all within a reasonable deviation from what the manual says.  However the AVC rectifier's plate voltage and resistance-to-ground are WAY off from what they should be according to the manual.  So is the circuitry, as best I can tell... the 4th edition manual, and the radio itself, show the plates of the AVC rectifier going through the secondary of T5, then to R82 (4.7K).  That's where any similarity between schematic and radio end.  The schematic shows R82 going to R180 (180K) and C265 (.01), and the radio shows R82 connected to a 3.3M-ohm resistor and .05 capacitor, to a wire that goes off into the wiring harness to who-knows-where.

So, first question... anyone have any idea which manual goes with this radio, and where I might find a copy (paper or PDF)?

Second question... anyone have experience with AVC and S-meter issues including the ineffective RCVR GAIN ADJ control with any of these radios and can point me to a possible known cause?

A solution for the first will probably guide me towards a solution for the second.

Thanks...

Rick WA1RKT
Londonderry, NH
Catch me on 14.300 MHz Intercon / MMSN most days
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