> 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.
Dick, "hot switching" simply means operating a switch (or relay) with
voltage (DC, LF or RF) applied. In the case of the FT-990, the input
relay (or input contact) of the amplifier will almost certainly be
"hot switching" (break and make) and the output relay (or contact)
will probably be "hot switching" (on break) because RF is present at
the amplifier 3 mS after the relay activate command.
> 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?
"Protecting the contacts" - not true. The tube is only in cut off for
zero signal conditions (effectively class C). Application of drive
in cutoff will still result in RF output ... with increased IM products
and distortion consistent with class C operation.
> Does this present a danger to the relay contacts that switch the
> exciter RF? If so, is this still called "hot switching?"
Yes, this is a certainly danger to both the input and possibly the
output relay contacts.
>
> 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.
I have never seen an affordable open frame relay fast enough for QSK.
If they can handle the power, they're too large and slow ... if they're
small and fast enough they can't handle the power/RF voltage, particularly
with an off resonance antenna.
73,
... Joe Subich, W8IK
<W8IK@Subich.com>
<www.qsl.net/W8IK>
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