Thanks to folks who gave specific suggestions again. In my first
session, I'd put weather stripping and double stick tape under the cover
and on the shield behind the front panel. But I still had a highly
irritating buzz in the 330 to 360 Hz range when on the internal speaker.
OK, so I took the top cover off again and cranked the sidetone level up
to 47, then ran it through the suspect pitch range. The speaker didn't
sound too bad if I damped it by holding the magnetic shield, but it was
pretty much OK even if I did not do that. So it's something inside the
cabinet doing a sing-along. How can I apply excitation to suspect parts
and see what's doing it? By leaning the top plate against the cabinet
with the magnetic shield touching the top edge, I was able to couple
enough audio into the box to make it happen. Turns out one or more of
the crystal filters are singing. I have all slots filled and at 350 Hz,
the one on the far right is doing Karaoke to the sidetone. When I go to
340 Hz, one or more of the others join in.
I'm not sure how to make it/them shut up at this point. It takes
significant finger pressure to quiet them down, so a bit of foam strip
may not do it. I haven't gone to the bottom to check or tighten the
screws yet. I'm going to stew on that a while first. Probably need to
remove a filter completely and examine it to come up with a plan. I'm
also reluctant to stick things to the tops as I might mess up the labels.
73--Nick, WA5BDU
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