[Hammarlund] Oscillation in HQ 170A

[Kenneth G. Gordon]

On 6 Aug 2012 at 19:15, Mike Lopez wrote:

> I had a similar situation with an HQ 110. After much frustration, it turned
> out to be one section of the main filter, believe it or not. The problem was
> unrelated to the rest of the audio circuit. I replaced all filter caps, so I
> don't remember if it was a lower value section used at the cathode (Bias) of
> the audio out tube or one of the other sections. I don't recall if the
> multisection can had that lower value in it or not.

Yes. It does. It is C-82/A/B/C/D. C-82D is the cathode bypass for the audio output tube 
(6AQ5), 40 MFD 25VDC.

For those of you with oscillation, try paralleling C-89D with a known good 40 MFD 25 VDC 
cap.

More than likely, however, C-82D is shorted, so the above won't help.

Pin 2 of the 6AQ5 should show ~430 ohms to ground. If it is less than that, suspect C82D.

Ken W7EKB