[Hallicrafters] Hummmmmm.......

[Mike Everette]

I have noticed a similar phenomenon with my SX-25,
when tuning in a strong signal on the AM and SW
broadcast bands.  If the RF gain is wide open and the
AVC is on, I note a hum on the signal.  Turn off the
AVC, back off the gain, and it subsides or disappears.

The same thing happens with my S-22R, but not to the
same degree.

This seems to point toward the AVC line, but I haven't
had a chance to do much troubleshooting.  It seems
like I read something a long time ago in an old RCA
tube manual or the "Radiotron Designer's Handbook"
about this problem.  Wish I could remember the source.

I also remember reading that a continuous subtle hum
could be a first-oscillator/mixer issue.  If it's just
on CW or SSB, look at the BFO and second detector as
well. Perhaps the B plus to the oscillator, mixer or
BFO is insufficiently decoupled.  Did you really
replace all the paper caps?  Some of the ones in the
SX-25 are very difficult to get at, especially around
the oscillator-mixer, and the temptation is great to
leave them alone.  The SX-24 is quite similar.

Both these radios have been recapped, and pretty much
re-resistored as well.

I'd wondered if it might be due in part to the power
supply feed to the audio output stage in the SX-25,
since the stage is supplied with B plus straight off
the rectifier and not through the filter choke.  This
was done for two reasons -- to improve the regulation
of the B plus to the rest of the receiver; and (I
rather suspect but it's not admitted in the manuals)
to reduce the size and cost of the filter choke.  The
S-22R audio stage B plus, however, is filtered through
the choke.  

This hum may just be evidence of a "design problem"
but I tend to think it isn't.  Wish I knew the answer
but at this point I don't.

73

Mike
WA4DLF




 
____________________________________________________________________________________
Expecting? Get great news right away with email Auto-Check. 
Try the Yahoo! Mail Beta.
http://advision.webevents.yahoo.com/mailbeta/newmail_tools.html