[Collins] 75A-2 question

[Dr. Gerald N. Johnson]

On Thu, 2007-08-23 at 12:56 -0700, Glen Zook wrote:
> You are thinking of the 75A-1, not the 75A-2 or 75A-3.
>  The 75A-1 has all sorts of oil filled capacitors but
> the later receivers have "brown beauty" paper
> capacitors and not the oil filled "bathtub"
> capacitors.  The "brown beauty" capacitors are just as
> bad as the "black beauty" capacitors in the 75A-4 but
> have brown colored bodies and not black.

Those molded capacitors are also oil filled kraft paper insulation. The
paper is impregnated with oil, otherwise there would be gaps between the
foil and the dielectric and those gaps would arc first burning the paper
to carbon shorting the capacitor.
> 
> All of the "brown beauty" capacitors need to be
> changed out, replaced by orange drops or similar
> "modern" capacitors.  It has been my experience that
> none of them can even "think" of passing a "leak"
> test.  

They will pass a short test on a Simpson 260 on the R x 1 range, maybe
the Rx 10,000 but that's not enough resistance for AVC or audio coupling
service. 30 some years ago, I went through an NC-300 that had rested a
decade or two in Florida. I think two of the molded paper capacitors
passed my leakage test, but by the time I got them out to test their
leads were too short to go back in.
> 
> While replacing the capacitors one also needs to
> tighten all of the machine screws, nuts, etc., in the
> radio.  All of the grounds in the receiver are made
> through these and over the years these definitely work
> loose.  After doing that the receiver will definitely
> operate better.

Good point, then the aluminum electrolytics also have not aged perfectly
and will test with a poor power factor, lots of series resistance which
destroys their bypassing effects. I like Sprague Atoms as sold by
Mouser. They were good capacitors 50 years ago and appear to still be
made by the same formulae.
> 
> When using the 75A-1, 75A-2, or 75A-3 receivers on CW
> or SSB one needs to run the audio gain "wide open" and
> then use the r.f. gain as the "volume control".  This
> is the way to operate all superheterodyne receivers
> that do not have a product detector.  The "S" meter is
> not going to work but the receiver will receive the
> signals MUCH better.
> 
> Glen, K9STH
> 

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