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

[Gerald]

On Sun, 2005-08-21 at 13:59 -0700, Glen Zook wrote:
> 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 noisy 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

A couple hundred KHz wide sounds like FM broadcast, not AM. That can
come from a local FM station and lack of shielding, especially on the RF
stage and mixer stages. It can come from the RF or mixer stage having
parasitic oscillations. Sometimes those parasitics can easily come from
bad grounds, sometimes from using hotter tubes than the original tubes.
There are many tubes made for TV VHF tuners with several times the gain
of the original 6CB6 but that means they can also easily oscillate at
VHF. And when they are oscillating they don't amplify so well at HF. The
oscillating tube may be sensitive to a finger or pencil approaching or
touching the grid. The classic cure for such parasitic oscillation is a
47 ohm resistor in series with the tube grid. That makes the grid
capacitance more effective as a low pass filter, or looking at it
another way, the 47 ohm resistor limits the current into the tube grid
capacitance and lowers the gain at VHF.

Searching the CCA archives, I found a service bulletin about gain
distribution in the 75A-2A and 75A-3 but one place it calls for adding a
100K resistor and another it says 10K. Since the object is lowering the
gain of V-18 by loading its plate circuit, I suspect the 10K is the
proper value.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer