[Hallicrafters] S-38 BFO and hum problem

[Patrick A. Thompson Sr.]

Don't overlook heater to cathode leakage as a source of hum when all else
has been checked.  Most tube testers will check for heater/cathode leakage
or you could check by substitution of known good tubes.

Pat
wa4tuk

-----Original Message-----
From: hallicrafters-admin at mailman.qth.net
[mailto:hallicrafters-admin at mailman.qth.net]On Behalf Of Craig Roberts
Sent: Wednesday, December 24, 2003 10:34 AM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] S-38 BFO and hum problem


Merry Christmas fellow 'crafters!

I've successfully restored several S-38-series receivers (fun n' easy
projects, eh?) and am presently finishing up a former basket case "original"
S-38. It now plays well with the usual and surprisingly excellent
sensitivity and selectivity, but has two aberrations that have never shown
up in any of my later S-38 models. though:

1) There's a low-level ambient hum that's not affected by the volume
control. I've replaced the electrolytics, of course, and checked all solder
joints, ground points, etc. I'm thinking it may be because the S-38's
circuit uses smaller value filter caps (40/30/30/20) than the later
60/40/40/20 types. Could that be?

2) The BFO injection is insufficient. I've played with the "gimmick"
capacitor and installed a new 12SQ7 and related components (except the mica
caps) and there's still no oscillation to speak of (certainly not enough to
copy CW or SSB). I know I'm missing something obvious, but what could it be?

Any suggestions would be most welcome.

Many thanks, 73 and Happy Holidays!

Craig
W3CRR


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