• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Weird Hum from Heathkit SP2 Preamp

david24x7

david24x7

New Member
Hi. I just finished recapping all the electrolytics in this preamp that I've been using for some time (better late than never). I had some issues with recitfier diodes but I've worked that out and I had eliminated the 60 cycle hum that given me trouble.

After I got it back on the shelf (and since), I'm getting this unusual noise from the speakers. It's less of a constant buzz and more of a 60 cycle rhythm. I'm not sure how to describe it, so here are 3 short (10-15 sec) videos so you can hear the sound for yourself.


Can anybody help me figure out what might be causing this sound? Volume is at about 3 on the dial so it's pretty loud and annoying.
 
Register to hide this ad
Yes. I let my Dynaco ST-70 warm up first and no hum until I turn on the preamp and after it warms up, it starts this weird hum. Do you think it could be a tube? I had tested all tubes last week before this started happening and they all tested good but maybe something got knocked lose while I was working on the recap and troubleshooting a different hum (which was fixed when I resoldered all the joints on the new caps).
 
If it is in both channels, the power supply. If in one channel, something in the feedback loop.
 
Test it without a connection to main amp. Perhaps via headphones. Always good to first confirm it is just one device, and not related to any example external connection.
 
I have seen it on a different preamp a long time ago,I think the selector switches were dirty on that one.If it was ok before the recap,I would check the values of the replacement caps against the schematic, or a poor ground on one of the downstream electrolytic power supply caps
 
Last edited:
The hum is in both channels but much louder in B channel and the oscillation is only in the A channel. The new caps are marked with same/similar value as the ones that I took out but I'll break out my capacitor meter and verify. I did clean the contacts with contact cleaner before all this noise started so maybe that does have something to do with it - esp since it does not affect the AUX inputs and only the Mag Phone and Tape/Mic to a lesser degree.

Thx for all the good tips. I'll keep you posted on the results.
 
JURB said:
If it is in both channels, the power supply. If in one channel, something in the feedback loop.
Click to expand...
Hmmm. It is in both channels though greater in Channel B than Channel A. And very little hum in the AUX inputs - only Mag Phono, Mic and tape.
 
FYI - I've exhausted what I can do so I've brought her to my local tube stereo repair shop. Hopefully, it's something simple. Cross your fingers.
 
You must log in or register to reply here.
Back
Top Bottom