HQ129X and Speaker Hum

Paul Litwinovich paull at WSHU.ORG
Tue Aug 29 10:13:02 EDT 2000

----- Original Message -----
From: Dave Noon <david.noon at ODYSSEY.ON.CA>
Sent: Saturday, August 26, 2000 10:52 AM
Subject: HQ129X and Speaker Hum

> I have just finished recapping (including filters) my HQ129X - it works
> just great. However, there is a low level hum coming from the speaker when
> the audio and sensititvity gains are both turned down. There is nothing
> heard through the earphones (150 ohm impeadance). Any thoughts on where
> hum may originate or how to find it?
> Thanks
> Dave  VA3DN

Dave, I just aquired a 129X but did not notice this problem, I'll check more
carefully for it tonight.
Did you try replacing the 6V6 output tube? A heater to cathode short can
cause this type of problem.
I have not been into the rig yet so I can't say if the headphones are
connected before the 6V6. On many of the National HRO receivers the phones
are connected to the audio driver and not the output tube. This could
explain why there is no hum in the phones. Also check the cathode and screen
bypass caps associated with the 6V6.

Paul Litwinovich
Chief Engineer, WSHU AM&FM, WSUF

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