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Re: [Amps] Input relay for QSK amp

Subject: Re: [Amps] Input relay for QSK amp
From: Victor Rosenthal 4X6GP <>
Date: Wed, 19 Dec 2018 06:28:45 +0200
List-post: <>
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.

Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
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.
Jim w8zr

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