[Collins] KWM2A receive alignment - USB/LSB

[Gerald]

On Mon, 2006-01-30 at 08:32 -0500, Steve Garwood wrote:
> I have noticed that the receive audio on my RE KWM2A is noticeably different
> between USB and LSB.  LSB has fuller audio, USB is sharper.  In reading
> through the alignment procedures, I'm not too clear on where I should be
> looking to correct this.  I have to admit I have more experience with the
> Drake tube receivers in adjusting the carrier oscillator (C130) so that
> there is little difference when switching back and forth between USB and
> LSB.
> 
>  There doesn't appear to be anything as simple as this in what I have been
> reading.
>
> I'm assuming that the KWM2A alignment procedure ultimately covers this, but
> is there a simpler way to accomplish this?  I'd rather not dig into it until
> I know more about it.
>
> Thanks for any information..
>
> Steve Garwood, N0CZV / AFA1GQ
> 
> Attica, Ohio 
> 
>  
The filters and the crystals are expected to have been perfect. There 
is NO adjustment in the alignment procedure, and nothing adjustable in the radio.

As has been already noted, the filter or the BFO crystals can have
drifted. Filter drift has to come from either contaminants inside the
filter loading the disks or from couple wire welds coming loose. I
expect broken welds are most common. You can check the filter by
checking its response curve using the crystal calibrator and PTO with
the S-meter. The 3 db down points should be 2.1 KHz apart and the ripple
not quite 3 dB. There should be deep notches out a little further, then
a bit of leak out a few KHz. A filter change should make the ripple
greater, and the response curve less symmetrical. Though that can be
affected by the particular wire(s) broken. If the filter response curve
matches the original specifications, it probably hasn't drifted leaving
the crystals.

It may be possible to adjust the crystals by tweaking the fixed
capacitors from grid (C53, 22 pf) to ground and screen to ground (C55,
47 pf). To keep the oscillator output the same their ratio ought to
remain constant. The crystals are both affected by these capacitors. The
switching leaves no room for adjusting one crystal at a time.

The problem is common enough that International Radio stocks replacement
crystals (http://www.qth.com/inrad/crystals.htm). $20 each, $35 a pair.
There is no reason to believe the original crystals might not drift more
or that the new crystals won't drift eventually.

New crystals can be bought from International Crystal Manufacturing for
similar or higher prices.

If a frequency counter confirms the crystals are off frequency,
replacement is the best repair, capacitor tweaking is a cobble that can
be hard on the rest of the components in the BFO.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer