[Collins] Re: 75S3 Questions - Update

[Dr. Gerald N. Johnson]

On Wed, 2008-02-27 at 14:11 -0500, Bill Stewart wrote:
> Once again, thanks for the info.
> To Robert Jefferis: Thanks for clearing up the mysterious C29, which I
> missed on the schematic. The one I took out has the same Collins part nr. as
> on my parts list. I did replace all Sangamo molded caps, all 'lytics and the
> B+ and bias diodes. I'm saving the old parts and maybe one day I can find
> out how to read the date codes. I suppose my rcvr could be somewhat dated by
> the fact it uses 6U8A's instead of 6EA8's plus used the S/T diodes. Your
> trans. seems to run cooler than mine. I measured the temp. after about 2
> hrs. The first 30 mins. it is just barely warm. One more note on the
> 6BF5...even on the tube tester it gets really hot. Yes, the B+ 'lytic is a
> bit tuff to get too, but I did manage and replaced with a can type. BTW, all 
> tubes cked. ok (gas & shorts too).

Some date codes are two digits week of the year, and two digits for the
year. Sprague date codes are something else. Crystals will be date coded
as well as original tubes. Does the silkscreen call for 6U8A or 6EA8?
That might hint at age.
> 
> To Jerry, K0CQ: Thanks for all your input Jerry. After replacing the S/T
> cube diode in the bias supply, the 6BF5 bias went from about -8vdc
> to -11vdc. The 6BF5 in my rcvr is V10.

So V8 in my book (with V10 as the 6BF6) is the triode section of a
6EA8/6U8A and pin nine is the control grid. That has IF signal on it,
having an electrolytic to ground makes no sense. Must be a mistake in
the socket and pin number.
> 
> To Bob, K4TAX: I think the main hum problem was taken care of with the
> replacement of C29. There is a bit of background hum, which I don't notice
> with the antenna connected...background noise masks it.  There may be some
> on phones which was addressed with a svc. bulletin. Don't recall seeing
> the shield you mentioned...may be missing on my rcvr.

If you have the plastic cased plug in filter it is sensitive to the
magnetic field from the power transformer and hum modulates the noise.
The filter wasn't added until about the beginning of 1965 to the later
75S-3B/C receivers. I found that noise source independently and have a
small piece of mumetal folded over the filter which cured the problem. I
think a bit of any steel would work, maybe even as thin as a tin can.
The larger shield box from the later factory production doesn't allow
for installing the much larger 200 Hz crystal filter either.
> 
> Now I have it hooked up with a DX-60B/HG-10B and making contacts. Thanks 
> again
> for all the info and help. This is a first class list...73 de Bill K4JYS

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