[Amps] Failure: SB-220 bias supply (Harbach board)

[Bill VanAlstyne W5WVO]

This morning I turned on my converted SB-220 6-meter amp and discovered it was not amplifying any more. A little checking with a voltmeter revealed that there was no operating bias being generated by the power supply. The cut-off bias (120VDC) was holding the tubes in check in transmit mode through that 100K ohm "safety" resistor on the T-R relay. Sure glad it was there!

The operating bias is generated on the Harbach replacement rectifier board, replacing the Zener diode that Heath originally used for this purpose with a diode string, if I remember correctly. (They don't provide schematics, so I will probably have to trace it out and draw one.) 

I wonder if anyone else has had this failure mode with the Harbach rectifier board? Due to lack of time, I haven't yet opened it up and pulled the board, but it can't be a very difficult fix, since there's nothing on there besides diodes and resistors. I'm just wondering why it happened, and am a little disappointed that it failed within a year of being installed.

Bill W5WVO