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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
From: wb0gaz via Amps <>
Date: Thu, 12 Oct 2023 23:53:43 +0000 (UTC)
List-post: <>
 Rob -

You are correct in your assessment. It turns out the LK-550's step-start 
circuit is a small PC board which includes the (missing) high power resistor (I 
have a 25 ohm, 25 watt ceramic as a substitute); the cause of that resistor's 
failure was the simple circuit that controls the relay. It takes 12VDC from an 
auxiliary DC supply (which is part of the overall power supply so rises in 
voltage while the load stabilizes), passes this through a resistor then into a 
relay (across coil is an electrolytic cap.) In a nutshell, on closer inspection 
I found that the RC network charging resistor (appears to be 100 ohm, probably 
1 watt, may be carbon composition) had fractured. That left the relay 
permanently in the "step" stage, cooking the power resistor.

How/why the 100 ohm resistor died is unknown (I'm dealing with an amp that has 
unknown, multi-owner history), however, a replacement (metal film 100 ohm 3W) 
is on the way, and before I re-assemble, I'll measure the relay coil resistance 
and see what the circuit's normal timing should be.

Will report back later if/when this is worked out.


> Working on an Amp Supply LK-550 (three 3-500Z, external HV transformer).

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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