Undersized diodes IMO Bill. I only use 1N5408's when replacing that zener as
they are impervious to something like a little arc.
Carl
KM1H
----- Original Message -----
From: "Bill VanAlstyne W5WVO" <w5wvo@cybermesa.net>
To: "[Amps]" <amps@contesting.com>
Sent: Wednesday, January 13, 2010 1:13 PM
Subject: [Amps] Failure: SB-220 bias supply (Harbach board)
> 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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