On Fri, 08 Apr 2011 09:20:18 -0400, Christopher Hall wrote:

I am the second owner of a Heathkit SB-1000 that was built in the late
1980s. It was working great until I accidentally tried to tune it into a
mistuned antenna and I saw a flash through the ventilation screen on the
left side somewhere around the rectifier/filter section and there is now
no power output. The amp powers up, the fan runs, the meter lamps light
and the tube lights up. The amps multimeter shows 3400 volts on the high
voltage position (a little high maybe but it has always been like that).
With no drive, when the amp is keyed there is no plate current and no
grid current. Keyed with drive there is grid current, but no plate
current and no power output. These conditions are the same regardless of
which band the amp is set to. I had thought that the zener diode might
have failed so I replaced it, but nothing has changed. I would
appreciate any thoughts as to what may be the problem and how to fix it.

Some basic things to do before you start trying to measure 3500 volts,
with equipment that's either inadequate or expensive:

One: Something obviously burnt. So look for something that's obviously
burnt. If the tube isn't flowing current then there's a good chance that
the current path between the power supply and the tube is broken. So
look carefully at every component between the power supply and the tube.
Trace the path from the transformer secondary to the tube, and check for
char. Because you're reading voltage at the meter, you can start by
going from the meter pick-off point to the tube.

Two: If you don't see anything, start measuring for continuity (still
with power off). Measure the resistance from the meter pick off point to
the tube anode -- it should be low (probably less than one ohm, because I
think those things used a coil wrapped around a resistor for UHF
suppression).

Three: Maybe you did blow up the tube. I'm not familiar with the
SB-1000. If the tube has a glass envelope, inspect the area from the
anode to the anode cap -- look for things that appear to be burnt and/or
melted.

Four: (really, this is part of one). Sniff around. Did you notice "that
expensive smell" after the bang and flash? If so, it should still be
lingering, and it should be strongest at the burnt component. Use your
nose to find the burnt part.

If none of this locates the problem, then go ahead and power it up and
start taking live measurements -- but do observe all precautions. 3500V
will have you in a philosophical discussion with St. Peter about the
meaning of the phrase "suicide by stupidity" before you ever realize that
you're dead. I'd have to dive into my ARRL handbook to learn about
safety at those voltages, but I would imagine that you'd start with a
decent high voltage probe, and go from there.

--
http://www.wescottdesign.com