[Hammarlund] HQ-170 Audio

[k2cby]

Ray’s note that turning down the audio gain control reduced but did not kill
the noise
suggests to me that the source is something riding on the B+ buss.

With the audio gain up, the noise would be amplified by the detector and 1st
audio stages 
and passed through to the output stage as a large signal.

With the audio gain fully down, the noise would be greatly reduced but it
would still be 
present on the plate and screen of the output stage only and would not
totally disappear.

In locating hum & noise troubles, I have had good success hanging a very
large value mylar 
(.1 µF) or small value (2 µF) electrolytic with appropriate voltage ratings
from grid to ground, 
plate to ground, and B+ to ground at each of the various stages. When the
noise vanishes, 
you have found the stage to troubleshoot. Just be sure to discharge the
capacitor each time 
before reconnecting it. 

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net