SB-221 Blown C4 cap and reduced output

[Jan Sluchak]

Hello to the group. This weekend my SB221 sustained a loss in power. While the amp was not transmitting (listening mode), my C4 cap blew like a piece of popcorn. I checked it and discovered that it was a 22mfd 250 volt replacement unit.  What was the original value of the axial version?  I replaced it with a 22mfd 450 volt unit and the amp is operational but down in power.  With my ft1000D putting out 75 watts, and on rtty, the 20m output of the amp is barely 500 watts. Down from maybe 700 or 800.  Both Graphite and new (from RF Parts) 3-500z are glowing evenly. What could have caused this reduction in power?  Where to check next?  The Harbach parasitic mods on the tubes are a little browned or blackened.  At the moment of destruction, I saw smoke from the tube area but again, I don't know if it chimneyed up from the C4 cap on the underside of the chassis. That cardboard debris was everywhere. Could the damage of the C4 cap have taken the harbach resistors with them? or the other way around?  Why did it blow while it was just at rest.  I had not transmitted for a few minutes and then it was a short burst.
I'm learning my way around this amp so please any help would be appreciated.
Thanks
Jan N6YV 




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