[Collins] Problem with a KWM-2

[Dr. Gerald N. Johnson]

On Wed, 2006-12-27 at 10:38 -0800, Glen Zook wrote:
> One thing to do is to tighten all of the machine
> screws that go through the chassis.  These definitely
> come loose and/or become corroded over the years.  All
> of the grounds in the unit are made through these
> connectors and many "screwy" problems can be traced to
> this hardware.

Yes, as the aluminum tries to expand, the steel screws won't allow it so
it has to extrude and when it cools its a little bit looser. I wonder if
a spec of Alnoox as used for electric panel aluminum connections might
help the longevity. The other problem is that the screw is stainless,
the socket lug may be nickle plated, and the ground lug is sure to be
tin plated. Lots of dissimilar metals to add corrosion in a humid
environment.
> 
> The feed through from the i.f. is the most probable
> cause for your problem and tightening the hardware can
> definitely help.  Of course there can be other
> problems.  But, I have found that about 90% of the
> "screwball" problems on equipment that I service for
> others can be traced to loose and/or corroded
> hardware.

Including the grounds for the coil shields.
> 
> My 75S-3A receiver has been located in basically the
> same position on my main console for over 30 years.  A
> few months ago it started having some intermittent
> problems.  I put it on the service bench and found
> that at least 20% of the machine screws had loosened
> over the years.  After retightening the screws and
> checking the alignment ("just in case") the receiver
> was performing just like new.  It is the heating and
> cooling cycle that happens every time you turn the
> equipment on and off that causes the problems. 
> Frankly, it is going to happen no matter what although
> it takes years generally for anything to become a problem.

I shows up a lot more rapidly when a radio's grounds depend on squeezing
a PC board where the plastic can flow much more readily than aluminum.
In the old days Collins wouldn't allow that construction, but most
recent rice boxes (and Tentec) work and fail that way. I've lost track
of the number of radios I've fixed by tightening the board mounting
screws or making that recommendation remotely.
> 
> Glen, K9STH
> 

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