[Amps] VOX Tripping and Bandswitch Damage

[Michael Tope]

Between the two Alpha 78's that we have at the local club
station, we have had a number of mysterious bandswitch
failures over the last few years. As these are costly and
or time consuming repairs to make, I have made an
effort to get to the root cause of the problem. Unfortunately,
given that we have a number of operators who come and
go at the station, its been rather difficult to ascertain exactly
when the failures occurred, let alone who was operating at
the time, and what, if anything, they were doing wrong that
might have caused or contributed to the failure.

Recently during a discussion of this problem, a fellow
club member suggested that perhaps it was related
to people using VOX. He had noticed that the bandswitch
on the Alpha 78 which makes a rather loud "clunk" when
switched, can easily trip the VOX on our Kenwood
TS-950SDX if the VOX function happens to be on at the
time. This observation raises the question, is the timing of
the bandswitch clunk and the RF rise time delay in the
exciter sufficient to cause hot switching in this scenario
(e.g. have the switch contacts closed by the time the RF
envelope from the VOX event reaches them)? It seems like
a plausible explanation, but without making measurements,
I don't know if the timing relationships are right or not.

Anyone have thoughts on this?

Thanks,

Mike, W4EF..................................