Jim wrote:
> I just picked up a 255 power supply for my Omni D and it has a pronounced
> hum. This is not coming from the speaker, but from the supply itself. The
> supply that came with the radio (non speaker type) does not do this. Does
> anybody know what to check?
Typically, this is caused by parts of the transformer moving with the
magnetic forces generated. Sometimes the laminations are loose,
which may or may not be cured by tightening the core
bolts. Sometimes one or more windings have come loose, or the entire
bobbin is free to move. The former can sometimes be cured by soaking
the transformer in thinned varnish and letting it dry (this is best
done under vacuum, if you can arrange it). The latter can sometimes
be cured by driving suitable wooden wedges between the bobbin and the
core (or re-driving the ones originally installed, if there are any)
-- but be careful not to damage the windings. The wedges should have
a very narrow angle, sort of like the shims carpenters use to adjust
window frames.
Sometimes the transformer is loosely mounted to the
enclosure. Sometimes stray magnetic flux is vibrating the enclosure
or some other ferrous part.
A bit of investigation and persistence should at least get you to the
point that you know what is moving. Fixing it may require some imagination.
Best regards,
Charles
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