[Amps] Input relay for QSK amp

[Victor Rosenthal 4X6GP]

I agree about the need for "lock on". I was planning on implementing an 
SWR protection circuit, in which high SWR would open the amp key line, 
when I suddenly realized that this is exactly what NOT to do! An antenna 
malfunction in such a case would hot-switch at least the input relay. A 
safer way to do it would be to have the SWR detector apply a voltage to 
the transceiver's ALC input, or open the transceiver's key line.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 15 Dec 2018 00:23, MU 4CX250B wrote:
> Jim, I believe Paul W9IC addressed your issue, but I thought I'd weigh
> in, too. You're absolutely correct that there are many situations (eg,
> running VOX SSB with some transceivers) where the transceiver jumps
> the gun and sends RF to the amplifier input before the amp's T/R
> relays have settled down. The resulting hot-switching can quickly
> damage the relays.
> 
> So far as I know, there are only two solutions to this problem. The
> first, as you note, is to swap out the amp T/R relays with relays that
> switch faster than the transceiver's. Often this not practical, and if
> the transceiver uses diode switching may not be feasible at all.
> 
> In my QSK circuit, I do what Paul suggests and sense RF at the
> amplifier's input Jack. (I think maybe the old Alpha 77 used this
> approach.) If RF is sensed, the amp is locked out and the T/R sequence
> is never started. This "lock-out" feature prevents the relay contacts
> from closing.
> 
> Just as important is a "lock-on" provision, which prevents closed
> relay contacts from opening if RF is present. The resulting arc when
> contacts open is probably more damaging than when they close. This
> feature is also inplemented by sensing input RF.
> 
> In practice, these circuit provisions are interesting to observe. The
> lock-out feature essentially keeps the amp in standby mode if the RF
> can't be adequately delayed. The lock-on feature keeps the amp on-line
> so long as RF is present, even if the amp relay cable is unugged.  In
> either case the amp is fully protected.
> 73,
> Jim w8zr
>