[Top] [All Lists]

Re: [Amps] [Fwd: Re: LK 500 ZA heat]

To: "'jeremy-ca'" <>,"'alexander a. aimette'" <>, <AMPS@CONTESTING.COM>
Subject: Re: [Amps] [Fwd: Re: LK 500 ZA heat]
From: "Dick Green WC1M" <>
Date: Wed, 15 Aug 2007 12:55:09 -0400
List-post: <>
> BTW, my LK500ZC has many hours behind it including several ARRL
> and CQ > DX and WPX contest awards. I never once had a "parasitic" problem
> lost a tube since it was new in 1987. It has also tuned just fine into 3:1
> higher VSWR.

I haven't had my LK500ZC as long as Carl, but I never had a "parasitic"
problem with mine, either. I had lots of other problems, most of which were
the result of errors and mishaps on the part of previous owners.

Someone had changed the grid and plate meter shunt resistors from the
factory values, which I had to get from Billy at Omega. Not sure, but maybe
the values were changed to avoid tripping the ALO circuit, which I later
disabled because it constantly tripped on 10m. 

The most serious problems were the result of damage sustained when a
previous owner's wife threw a chair at him, he ducked, and the chair hit the
amp, knocking it off the operating desk (I'm not kidding.) In addition to
cracking the meter faceplates and breaking a cap lead in the 40m output
circuit, the tubes were damaged. Every time the guy switched on the amp, the
tubes would arc and it would blow a fuse. I bought the amp for a song and
replaced the melted plate choke, all three tubes, the mis-valued resistors,
the broken faceplate and a cracked tuning knob. Also added a surge resistor
to the HV output to prevent future damage from arcs. I also replaced the
parasitic suppressor resistors because they were looking a bit charred,
probably the result of bad tuning practices. I remember fixing the QSK
circuit, too, but don't recall what was wrong with it. I think it was a bad
zener diode, perhaps fried during one of the arcs.

As for fan noise, yes, the LK-500 fans sound like a freight train when they
switch to high. The ZC has a temperature-sensing probe that automatically
switches the fans between high and low as needed. Problem is, the thing
constantly switches to high, even when the amp is idle. The noise is
deafening. I modified the fan circuit so that the fans would run somewhat
faster in low mode. This increased the cooling substantially with only a
modest increase in noise. The result is that the amp switches to high mode
much less frequently, even during transmit. It never switches to high during
idle. I don't recall now, but I might have slowed the fans a little in high
mode to take the scream out of them. I spent a lot of time measuring the
temperature of the tube exhaust, plate cap, pins and even glass to make sure
that the cooling was as good or better after the mods than before.

73, Dick WC1M

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>