Hello, AMPers. Long time, no see.
Someone asked me a question about hot switching today. I thought I
understood this subject, but while I was thinking it through, I realized
that I'm not entirely sure. The context was the unfortunate design flaw in
the Yaesu FT-990 that causes it to apply RF a mere 3ms after keying the amp
(it actually delays the RF envelope until 10ms after key closure, but
inexplicably waits 7ms after key closure before keying the amp.) The guy is
looking at a QRO HF-2000, and got this off their website:
"We have designed the transmit/receive (T/R) switching so that
the relays are momentarily "hot-shotted" with a large voltage spike
to speed switching. Also, the same circuit that controls the cathode
bias switching also controls the T/R switching. This means that
whenever the T/R relays are in transmit, the correct linear operation
cathode bias will be applied. The T/R and cathode bias relays are
sequenced so that no "hot switching" can occur. A fast open frame mechanical
relay
switches nearly as fast as some vacuum relays."
Previously, I understood "hot switching" to mean that RF is applied to the
tubes before the main relay has a chance to close. The tubes then transmit
into thin air, causing arcing, pitting and burning of the T/R relay
contacts. The way I read it, QRO's sequencing circuit makes sure that the
main T/R relay is closed before cutoff bias is removed from the tube, thus
protecting the T/R relay contacts. True? However, since the 3ms provided by
the FT-990 is not enough time for the T/R relay to close, won't the exciter
RF be "hot switched" between the antenna and cathode input circuit? Does
this present a danger to the relay contacts that switch the exciter RF? If
so, is this still called "hot switching?"
BTW, the guy asked if he could use the amp in QSK. Seems to me that even
though they claim to have speeded up the open frame relay to be "nearly as
fast as some vacuum relays" (5-8ms?), it still won't be fast enough for QSK.
True?
73, Dick WC1M
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