SB-301 problem

[Schichler, Alfred]

I have been working on an SB-301, trying to restore it to like-new operating
condition, but I still have a slight hum problem that won't go away. I
replaced just about all the electrolytics, but the beat note on CW or SSB
mode is still not crystal clear when using an external unmodulated carrier
or the built-in crystal calibrator. It's not that bad, but I would like it
to be as clear sounding as my Kenwood R-1000.

I'm not positive it's from AC hum or not, but it looks like a pretty low
frequency mixed with the audio if I look at the audio out on a scope. I
notice it more when looking at a low frequency beat note, around 50 to 200
Hz or thereabouts. It seems like it originates around the product detector,
but it's hard to be sure. The BFO and LMO look pretty clean. Maybe this is
common with SB-301's and I'm being too fussy. I was wondering if anyone else
has experienced this problem.
Any help would be appreciated.

Thanks,
Al, NE2D

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