[Milsurplus] Test your repair skills

[J. Forster]

Problem Summary:  Chuck, W1HIS has been using my WS 19 on the air for several
years now and has noted several problems, which are likely related. They are:

Occasionally there is a abrupt roughly 60 KHz transmitted frequency shift. This
appears after the set has been on some time (> 20 min) and often goes away upon
cooling overnight.

When the set was mounted on a wooden carrier last year, the CW tone became
ragged. The carrier couples mechanical vibration from the dynamotor into the
set.

The set is somewhat microphonic.

Easy, right ??  Read on.

We have spent two nights looking at this problem, and here is what we have
determined so far. It is NOT the band switch contacts and the master oscillator
tube is NOT microphonic.

To reach these conclusions, we took the set out of the case and hooked it to a
PSU and placed it in operation. A slight twist of the chassis makes the
frequency change. Tapping and poking various components does not produce
significant results.

To determine which oscillator was at fault, we tuned other receivers to the BFO
and Master Oscillator, (The Tx frequency is the sum of the two) and determined
the Master Oscillator was changing. The MO changes whether the band switch is
set to either high or low band when the chassis is stressed. Changing MO tubes
made essentially no difference.

It appears that the problem is located in the MO tuning capacitors and trimmers,
C6A, C9B, and C35A and the capacitance change is calculated to be about 5 pf.
What can be changing linearly, and sometimes stepwise, with heat and/or
mechanical stress?

Oh, I forgot to say that the problem is independent of the main tuning dial, so
the mesh of the plates is not a factor. There is no perceptible backlash in the
capacitor or drive. Hehe !!

If you need more info and/or set diagrams they are available at:

http://www.trackpads.com/webs/wireless-set-no19/index.htm

This is not a joke but a real teaser of a problem. Chuck and I are going back to
it Monday, and would like to hear from anyone with bright ideas. We really DO
NOT know the answer.

-John