Question Collins 75A-3 image problem
 

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
deaf unless I REALLY peak the antenna closely with my mfj-949
tuner(longwire & hy-gain vertical). I dont know if that means anything
or not but my other rx's(r-390,nc-300,HROs...) front ends seem to be
more broadbanded and can literally pick up signals with a piece of wire.
Also, I don't get this same interference on my other rx's using same
antenna, etc, so I'm concentrating on the receiver. I recently aligned
it via the orig manual and checked all tubes. PTO tracking isn't too far
off. Most caps were changed by prev owner. Everything else with the
receiver looks fine otherwise. I'm not sure where the "leak" is coming
in via mixers or what. I'm not an expert on rx repair so I thought I'd
ask some experts on the list to find out if this kind of problem usually
boils down to just a few things. Any help or suggestions will be GREATLY
appreciated. Thanks/73, dave austerman, N5WNM, Oklahoma


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David Austerman 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
deaf unless I REALLY peak the antenna closely with my mfj-949
tuner(longwire & hy-gain vertical). I dont know if that means anything
or not but my other rx's(r-390,nc-300,HROs...) front ends seem to be
more broadbanded and can literally pick up signals with a piece of wire.
Also, I don't get this same interference on my other rx's using same
antenna, etc, so I'm concentrating on the receiver. I recently aligned
it via the orig manual and checked all tubes. PTO tracking isn't too far
off. Most caps were changed by prev owner. Everything else with the
receiver looks fine otherwise. I'm not sure where the "leak" is coming
in via mixers or what. I'm not an expert on rx repair so I thought I'd
ask some experts on the list to find out if this kind of problem usually
boils down to just a few things. Any help or suggestions will be GREATLY
appreciated. Thanks/73, dave austerman, N5WNM, Oklahoma

Do you have the top and bottom covers on the radio? The Collins rigs
will show a substantial improvement in image rejection when all of the covers
are in place, and properly screwed down.

You can often get a substantial improvement in performance in Collins rigs
by exercising the screws and nuts that hold the intercompartment shields,
and tube sockets. These parts get a little bit of corrosion over the years,
and no longer work as good shields.

-Chuck