During the last CW WPX contest, the antenna changeover relay of my SB-220,
which had been occasionally sticky for a couple of years, finally got to
the point where it was sticking in the standby condition almost continuously.
I'm installing a vacuum relay QSK system, but out of curiosity I took the
relay apart to explore why it failed. The normally open contacts were all
in fine shape. The normally-closed RF contacts showed some erosion, but
weren't in bad condition. To my surprise, though, the normally-closed bias
contact, which supplies 110 VDC cutoff bias to the tubes, was very badly
eroded. I can't be sure, but it seems likely that this contact was
responsible for the sticking.
Since we usually assume RF hot-switching is the culprit, I thought this
would be worth passing along. It suggests that even if you don't replace
the relay with a QSK circuit, it might be a good idea to install an
electronic bias switch (a much simpler mod) to protect against this failure
mode.
73, Pete Smith N4ZR
n4zr@contesting.com
Sometimes a tower is just a tower
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