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Re: [Amps] Amp Supply LK-550 (550) need replace inrush module's resistor

Subject: Re: [Amps] Amp Supply LK-550 (550) need replace inrush module's resistor - o
From: Steve Thompson <>
Date: Thu, 12 Oct 2023 18:12:01 +0100
List-post: <>
After blowing a step start resistor I've taken to adding a fuse in
series with them. A slow blow 2A fuse will take 20A for 100ms, plenty to
cover any likely inrush but will blow fast enough to protect the
resistor if the shorting relay fails. Even 1A slow blow will be ok for
most cases.

Steve G8GSQ

Usually when an in-rush resistor fails, it's because the circuit
(relay, etc.) that bypasses it after a second fails and the resistor
stays in series with one of the 120 v. legs to the h.v. transformer
primary.  The operator doesn't know it until  the resistor gets so hot
that it cooks.  If the resistor is mounted on a glass-epoxy board,
that burning board can really stink.  Avoid the fumes.

In your case, that part of the failure happened somewhere else.  But
I'm mentioning it so you know what happens.  How do you know before
your nose knows?  By operating with a monitor scope that samples the
RF into your amplifier and the RF coming out of it to give you a
trapezoid trace.  If the in-rush resistor stays in the AC supply line,
the trapezoid will look like an arrowhead.  The peak RF power part of
the trace will be squashed -- rounded.

What resistor to use for replacement?  You can't go wrong with a 10
ohm 25 watt wire wound brown glaze resistor.  That's easy, but you
have to figure out why the old resistor cooked in the first place.
Apparently, the amp got sold because someone didn't know what to do,
or what went wrong, and how to fix it.  That's a nice amplifier but
the educational opportunity it gives you is worth a lot more.

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