Hi John:
As I recall, with zero bias voltage, that is about what the two tubes would
idle at, with a blown zener diode.
I would replace the zener with a string of decent diodes, to get the voltage
sufficiently high to limit the current at idle to about 160 mils total for
the two tubes.
This level provides both good linearity on SSB, as well as requiring a
somewhat lower level of drive to obtain full output.
The zener diode has little ability to withstand spikes, and probably failed
when your diode failed in the power supply string. The use of the diode
string for the bias supply, will prevent that occurrence in the future.
I build the new bias supply board on a small piece of perforated board, and
mount it in front of the existing rectifier/metering board. This is quite
an inexpensive fix. Optionally, you can spend a bit more, and replace the
board with the Harbach retrofit. If you want more info on the design of the
bias supply board, please contact me off list.
By the way, the HV meter's voltage divider may be a little off tolerance at
this point. The HV normally shows about 3,200 Volts unkeyed in SSB when the
line voltage is adjusted to approximately 245 VAC. The Harbach mod will
recalibrate things and make the reading more accurate. If the voltage is
still a little low, the capacitor bank may need eventual replacement, and
Harbach has a new bank for your amp as well.
Regards,
Mike Schatzberg
WB2AJI
----- Original Message -----
From: "John Adkins" <w4kv@yahoo.com>
To: <amps@contesting.com>
Sent: Tuesday, August 09, 2005 1:05 PM
Subject: [Amps] SB-220 diode failure
> My sb220 cooked one of the diodes on the pc board a couple of days ago. I
replaced the faulty one
> with a 1kv piv 2.5 amp substitute that was in my junk box. Resoldered the
wires back to the pc
> board, reassembled it, and fired it up. When I went to tune the amp up,
with no exciter power, I
> get a plate current of 200 ma instead of the 120 ma the manual indicates
should be seen.
>
> HV is 2.2 kv on low power, 2.8 kv on ssb.
>
> The manual suggests that the zener may be blown.
>
> What do you think? Should I just get the Harbach rectifier/metering board
substitute for the
> original pc board and eliminate the zener altogether? Other suggestions
for troubleshooting this?
>
> Thanks
>
> John
> w4kv
>
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