----- Original Message -----
From: "Steve Wedge" <w1es1...@earthlink.net>
To: "Drake List" <drakelist@zerobeat.net>
Sent: Tuesday, July 05, 2011 5:52 PM
Subject: Re: [Drakelist] R-4A PTO, Round 1
The frequency jumps when stopped and gets worse when it's
fully warmed up.
Re the "dog-bone" caps: can these be replaced by silver mica
units? Or do they have a temperature coefficient that's
different?
I would still love to get a working PTO to swap in so I can
not fear wrecking this one.
Thanks, all, for your help. I'll probably rip it apart
again shortly.
Steve Wedge, W1ES/4
I think maybe there is come conclusion jumping as well
as frequency jumping. Before shotgunning parts I may be
helpful to see if its possible to isolate the problem a
little. If it appears to be temperature sensitive the
approach is to use alternative applications of freeze mist
and a heat gun. While Freeze Mist or its clones, have gotten
expensive they are still available. You don't need much. Its
also worth seeing if there is an intermittant solder joint
or other connections. Just poking at the solder joints with
an insulated stick may find the trouble.
I am not sure about the dog-bone caps in the Drake, most
I've seen were ceramic caps of some sort. They may be
temperature compensating caps but those also come as ceramic
cased units with axial leads.
Silvered mica caps in resonant circuits can cause
jumping and warbling. The effect is known as scintillation.
Its caused by oxidation of the silver coating. That should
not happen but does due to various causes. Its mostly found
in the older style of cap in a molded Bakelite case. Dipped
capacitors seem to be free of the trouble or perhaps just
are not old enough.
Of course voltage variations will cause jumping. Zener
diodes have been pointed out as a possible cause. It may be
possible to monitor the output of the Zener on an
oscilloscope which will show any variation instantly. Again,
the alternate application of heat and cooling may show up a
bad guy. A caviet about Freeze Mist or similar, the cooling
will tend to condense moisture from the air. The moisture
can cause leakage paths which can not only mask the problem
but disable many circuits until it evaporates. So use the
spray with some caution.
Also, check for poor ground connections. They can become
intermittant too.
Check all this stuff before firing up the soldering
iron.
As far as a mechanical instability that is something
else you may be able to find by poking around with a wooden
stick.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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