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
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