[Collins] 75s-3 RF gain and muting problem - Solved the good and the bad

[Gerald]

On Thu, 2005-12-01 at 16:23 -0500, David Vest wrote:
> Looks like I found the problem with my 75s-3,  it appears the RF gain
> control (10K Pot) has gone bad.  After checking the circuit and convincing
> myself that everything else looked ok, removed the pot from the front panel
> and took it loose and it did not look quite right on the meter.  I then grab
> a 10K pot out of the junk box and hooked it up and everything fell into
> place.
> 
>  
> 
> So there's the good, found the problem, now the bad is where can I find a
> replacement?
> 
>  
> 
> BTW with the junk box pot in the rig the bias voltage came back down to the
> expected -65 volts.
> 
>  
> 
> Thanks for the input.
> 
>  
> 
> Dave
> 
> __

That's exactly what I concluded at the end of my last post. Its hard to
come to that conclusion because pots don't go open end to end as often
as the wiper looses contact. But with old age that open end to end may
prove to be a more common failure mode.

Its a dual pot with concentric shafts. I don't know the taper for the RF
gain, but I'm sure the half meg audio section is a log taper. The RF
gain might work with a linear taper. CPN 376-2151-00

Used to be, you could go to the distributor and build a pot from pieces.

Then maybe, just maybe, one can acquire a new 10K pot on the mil type
RV4 style conductive plastic and disassemble it for the resistive layer
and assemble that into the pieces of the original pot.

You might try some temporary circuit mods. Wire a 12K resistor from end
to end of the RF gain pot. Then see which end of the element is open and
hook a 12k or 15 K from that end to the wiper. It won't necessarily work
as smoothly as a new pot, but it will function until the right pot can
be had.

I don't know if Surplus Sales of NEBR has that pot or if Antique
Electronic Supply might have such a dual concentric shaft pot.

Surplus sales seems to have it:
http://www.surplussales.com/Collins/Coll75S.html Not cheap but the exact
part.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer