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Re: [Amps] Input relay for QSK amp

To: Jim Thomson <>
Subject: Re: [Amps] Input relay for QSK amp
From: MU 4CX250B <>
Date: Fri, 14 Dec 2018 16:23:05 -0600
List-post: <>
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

Sent from my iPhone

> On Dec 9, 2018, at 12:54 PM, Jim Thomson <> wrote:
> Date: Sun, 9 Dec 2018 08:22:45 -0700
> From: Jim Garland <>
> To: Bob Maser <>,
> Cc:
> Subject: Re: [Amps] Input relay for QSK amp
> <Jim, You're right in principle, of course. The problem is that a
> <well-designed amplifier relay circuit shouldn't depend on what
> <transceiver is used. The amplifier itself should be immune to
> <hot-switching in all situations.
> <Jim W8ZR
> ##  Ok, what happens on VOX  SSB ??   Some xcvrs output  RF
> a lot faster than  others.   And the  0-30 msecs  delay adjustment on my
> yaesu  FT-1000-MP  MK-V  only works on CW  mode.
> ##  One fellow on another forum mentioned that his  new yaesu  FTDX-3000
> when run on VOX  SSB, has a tendency  to  fry the relay contacts on his  L4B
> and L7 amps, which are  3PDT mech types, with  15 msec operate time.  His
> conclusion is the 3000  xcvr outputs RF on ssb  vox, a lot quicker than any 
> of his
> other xcvrs.   The only  fix is faster TR relays in the amps  with  3 pdt  
> relays.
> Jim   VE7RF
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