The relays on that board operate off the 120v supply that biases the tubes off
in standby and operates the relay and the soft key if you have one. If the amp
is on and you lose the 120v supply voltage, the soft start will open up and the
resistors will be on series with the 240 supply voltage. It short order the
resistors would burn up. The breakers won’t open up, the resistors burn up and
open.
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On Wednesday, October 9, 2024, 5:46 PM, Larry Carpenter <aa4vi@outlook.com>
wrote:
I have had some time to investigate my Harbach Soft Start failure and found
some very unusual results. First while the amp was in an idle state I heard and
arc saw smoke from the top and side of amp and it powered off. I have checked
all the rectifier diodes, filter capacitors and bleeder resistors and found
them all in tolerance and no diodes or caps shorted or open. The Harbach board
itself is completely failed. By this I mean both ceramic 20 ohm resistors open
and the two 48 vdc spdt relays have open coils, one trace on the Harbach board
between the relay and 240 line is burned open and the resistor in series
between the two relay coils is open. The bridge rectifier that supplies voltage
for the Harbach relays also seems to test open. I am wondering if I have a
shorted 3-500Z or something else. Neither circuit breaker operated and power
switch is still functions when tested with an ohm meter. I see about 2.5 ohms
of resistance looking at the primary of the power transformer. I a
m a little hesitant to apply power even with the tubes out. Has anyone else
seen such a catastrophic failure of a Harbach Soft Start module? Thanks, AA4VI
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