Heathkit SB-1000 problem

[Christopher Hall]

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.

73
Chris
VE9ZX

 

[Ed Clark WD4ED]

responding to
http://www.electrondepot.com/repair/heathkit-sb-1000-problem-125718-.htm , Ed

ve9zx 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.

73
Chris
VE9ZX

Did you ever get this figured out? I have exactly the same amp with the exact
same problem.

Thanks,

Ed

 

[Michael Black]

Since that's a fairly old posting you may not get a direct response.

And since he wants help, he should have just posted instead of replying to
an old message. Not that replying to old messages is ever good.

Looking at the thread in the archives: it was suggested that this
symptom probably means that there's a fried component, between the HV
filter board, and the tube anode. According to the schematic I see,
the red wire from the filter board goes to a small bypass cap to
ground and into a choke (RFC3). From the output of the choke, it goes
to the anode/plate of the tube via what shows on the schematic as a
resistor (100 ohm 2 watt?) in parallel with a choke or coil of some
sort. At a guess, this may be a "coil wound on resistor" parasitic
suppressor. There's also a connection at this point to the HV
blocking caps and then out to the output-match circuit.

If the choke (RFC3) or this coil/resistor combination is burned or
damaged, it would prevent the tube from receiving any high voltage.
This wouldn't be apparent from the meter reading (since the meter
pickoff is on the HV filter board) but you'd see no plate current
being drawn. Careful inspection of this current path should show you
where the continuity ceases to exist ...

... IF YOU DO THE INSPECTION WITH PROPER CARE AND PRECAUTIONS. This
is a DANGEROUS high-voltage part of the circuit, and contact with it
while it still holds charge can leave you dead, dead, dead in very
short order.

For what it's worth, I understand that Ameritron offers a professional
repair service for this amplifier (it's a "licensed copy" kit version
of the Ameritron AL80A, designed by W8JI).

See http://www.ameritron.com/heathkit.php for details.

And it's not like an RF power amplifier is a complicated thing. I've seen
people build them that would be fearful of building something that does
more. An amplifier has a tube or two, and a power supply, and because it
is a power amplifier, everything is big and easy to work on. One could
just check the components one by one if they had no skill in
troubleshooting.

Michael

 

[[email protected]]

FWIW there appears to be a schematic at :

http://www.vintage-radio.info/download/download.php?dir=heathkit&file=sb-1000.zip

I got it and unzipped it and it looks clean. A PDF result form Google comes wuith a warning from google about "harm your computer" so I didn't get that.

If there is plate voltage then there is, that part works. If something went poof it is probably in the cathode circuit, if anything is. If not then in the grid circuit. If so, that tube is probably shorted of has alot of leakage of some type.

In fact without drive it probably should be redplating.