[Milsurplus] Test your repair skills

[Joe Foley]

An old Nolan Lee post suggested making up a selection
of wooden dowels with small notches in the end to poke
and prod around inside the radio to test for loose
connections.  

I agree that if there is a difference made by tapping
or twisting the chassis it is an indication of a loose
joint of some kind, somewhere.

Cleaning, while doing a very careful visual
inspection,....

Joe




--- Barry Hauser <barry@hausernet.com> wrote:
> 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. ;-)


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