> 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.
73
Rob
K5UJ
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