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Re: [Amps] Harbach Heathkit SB-220 Soft Start Update

To: Larry Carpenter <aa4vi@outlook.com>, "amps@Contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Harbach Heathkit SB-220 Soft Start Update
From: gudguyham--- via Amps <amps@contesting.com>
Reply-to: "gudguyham@aol.com" <gudguyham@aol.com>
Date: Wed, 9 Oct 2024 22:02:39 +0000 (UTC)
List-post: <mailto:amps@contesting.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@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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