[Collins] Question re:75A-3 image problem

[Glen Zook]

The 75A3 is a pretty sensitive receiver so I expect
some alignment and/or component problems.

Are the bottom shield and the large top shield box in
place?  If not, then you definitely need to get the
circuits properly shielded.

Make sure that someone has not added diodes across the
antenna input.  This was sometimes done to help
prevent damage from a nearby transmitter (whether or
not this really was needed is debatable).  But, those
diodes cause all sorts of problems with intermod, etc.
 If there are diodes across the connections then cut
them out immediately.

Tighten EVERY machine screw, nut, etc., that comes
through the chassis.  Remember that all of the grounds
in the unit are made through these.  Over the years
they definitely work loose and/or become corroded. 
Tightening these fasteners takes care of about 90
percent of the intermittent and/or "strange" problems
that I run into when working on boatanchor equipment
for others.

My 75S-3A started acting "strange" a couple of months
ago and I had to put it on the service bench. 
Although it has been in basically the same position
for 30 years (hardly ever even moved) the machine
screws and nuts had almost all worked loose.  About 15
minutes spent tightening all of these including all of
those that hold the shields in place as well as making
grounds through terminal strips got the receiver "back
on track" and working like it should.

Glen, K9STH

--- daveaust at pol.net wrote:

I have a 75a-3 that I haven't really ever used until
recently. It works but has very serious images at
about 3.3  and 3.6 a few hundred kc wide. Also same
thing up on 20Meter band in wide spots across the
band. Sounds like AM bcb bleeding through but haven't
been able to match the distorted signal to anything on
my am radio. When away from the noisey areas on the
dial, the real signals sound fine although the rx is
fairly def unless I REALLY peak the antenna closely
with my mfj-949 tuner(longwire & hy-gain vertical).

Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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