[Milsurplus] Test your repair skills

[Barry Hauser]

Greetings;

While it seems as though you've tried the "tap test" with no indication, the
"twist test" may may suggest something.

Have you tightened down all mounting screws -- tube sockets included --
particularly those that are ground points?  Even though there may be star
washers which bite into the chassis and assemblies, a thin film of corrosion
might be present.  Best to loosen each one and place a drop of DeOxit (or
other corrosion treatment) alongside the screws before re-tightening.

Another possibility -- bad solder joint.  Solder joints have been shown to
fail over an extended period with slow-burn chemical or electrolytic action.
Some of this is due to rosin which did not fully boil off or some other
impurities.  This long term deterioration can produce solder joints which
have some resistance, capacitance, or even some that have turned into
semiconductors of sorts.  Possible theory:  When you twisted the chassis, it
put enough pulling (or pushing) force on a lead to move it within a
deteriorated joint.

Finally, possible broken conductor or braid on a lead somewhere, usually
near/inside a connector or where a wire fairs into a solder joint.  These
can be difficult to spot.  If you were thorough with the tap testing,
however, I would think that would have revealed it.

There also could be a film of corrosion or oxide under a stamped or cast
metal assembly where it is flush agains the chassis.  Ceramic trimmers are
prone to oxide problems or failure of insulators.  Also possible that there
is some gunk or corrosion under the wipers/contacts in the air-variables.
Inappropriate lubrication is sometimes the cause.

But that twist test makes me suspect tube socket screws and or ground
terminal screws.  A tap test might not move the parts enough, whereas the
torque might put enough force on some to make/break contact.

You say the set is somewhat microphonic -- does that show when tapping
around?  If so, then you can try to zero in on the most sensitive spot.
Often, it's a tube, but almost equally often, I've found a bad ground or bad
solder joint.  Again, the bad joints are not "cold solder joints", but may
be factory shiny on the outside.  They can fail long-term on the inside.  In
addition to becoming resistors, caps, and diodes, they can become
transducers.  Had one so bad I could easily hear the audio (not a WS-19)
without benefit of speaker or headphones.  Had an HQ-180 with a couple of
highly microphonic 6GH8's, I could hear my fingers drumming on the table
whilst pondering the difficulty.  I could also hear my own voice if I spoke
loudly toward the front panel.

Almost anything is possible much past the manufacturer's standard 90 day
warranty. ;-)

Barry



----- Original Message -----
From: "J. Forster" <jfor@quik.com>
To: "Wireless-Set-No19 @ yahoogroups.com"
<Wireless-Set-No19@yahoogroups.com>
Cc: "Armyradios @ egroups.com" <armyradios@egroups.com>; "Milsurplus"
<milsurplus@mailman.qth.net>
Sent: Monday, June 23, 2003 2:05 AM
Subject: [Milsurplus] Test your repair skills


> 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
>