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. snip

Hi,

Perhaps an open RF choke in the plate circuit?

Some Ameritron models have a series HV rectifier in the plate
circuit, to serve as a fuse. If present, perhaps it opened.

If you are not comfortable working around lethal voltages,
Ameritron may service this model Heathkit amp.
(It's basically a kit version of their AL-80, single 3-500Z amp).

73,
Ed Knobloch

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.

73
Chris
VE9ZX

On 04/10/2011 12:17 AM, Edward Knobloch wrote:
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. snip

Hi,

Perhaps an open RF choke in the plate circuit?

Some Ameritron models have a series HV rectifier in the plate
circuit, to serve as a fuse. If present, perhaps it opened.

If you are not comfortable working around lethal voltages,
Ameritron may service this model Heathkit amp.
(It's basically a kit version of their AL-80, single 3-500Z amp).

73,
Ed Knobloch

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.

73
Chris
VE9ZX



My first thought was the plate rf choke as well. It would take QUITE an
overload of plate current to blow that, but a high SWR into a bad
antenna might induce enough RF current in the choke to fry it. If you
are lucky the break will be on the surface and might be repairable.
Otherwise a new choke is needed. Another possibility is the parasitic
suppressor between the tube plate cap and the plate choke may have
opened. This is easier to repair/replace.

I REALLY doubt that you blew the tube that quickly. The only way to
destroy a 3-500Z in an instant would have involved frying much of the
power supply as well. If the tube filament is not open and the plate
isn't melted (hole burned though it!) or discolored unevenly or the
glass softened in one or more spots the tube is good! 3-500Z'z die
slowly or self destruct like a supernova!