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






-------- Forwarded Message --------
Subject: Re: {Collins} WE KWM-2A S-meter adjustment, and need correct manual
Date: 	Mon, 22 Dec 2014 19:58:24 -0500
From: 	Robert Thomas <ve3tou@xxxxxxxxxxxxxxxxxxx>
Reply-To: 	ve3tou@xxxxxxxxxxxxxxxxxxx
To: 	Bill Carns <wcarns@xxxxxxxxxxxxx>



Agreed -- actually, a factor of 2:1 in voltage is 6 dB, so a 25 dB
signal would be S-8. But if the gain in the RF, mixer, and IF sections
is away too low, the ACTUAL threshold will be a lot higher than the
CORRECT threshold. The radio would still be surprisingly usable because
the human ear has a huge dynamic range, and you'd still be able to hear
signals well below S-8. This is not to suggest that one should leave the
radio in this condition, just an idea as to where the problem might lie.

Hopefully not going too far OT  -- to give an example of the marvelous
capability of the ear -- in a typical classroom situation, the teacher
may be 3 feet away from the kids at the front, and 30 feet from those at
the back. The audio energy available to a kid at the back is 100 times
less than that at the front (20 dB down) because of the inverse square
law. But they all hear OK.

73
Bob


On 2014-12-22 6:22 PM, Bill Carns wrote:
Just a quick correction as you guys work through this issue. The signals
that we work with are almost always (Weak or strong) ABOVE the AGC
threshold. The AGC threshold is reached when the properly adjusted S-Meter
starts upscale and that is almost always the case.   25 uV is well above the
AGC threshold. 50 uv is S-9 in the normal situation and that is just 3 db,
or a little over half an S-Unit above 25 uV.

So, at 25 uV, you are well into the threshold.

Bill

-----Original Message-----
From: Collins [mailto:collins-bounces+wcarns=austin.rr.com@xxxxxxxxxxxxx] On
Behalf Of Robert Thomas
Sent: Monday, December 22, 2014 4:22 PM
To: Rick Poole WA1RKT; collins
Subject: 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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