[Amps] Harbach Heathkit SB-220 Soft Start Update

[gudguyham at aol.com]

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 at 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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