On Aug 9, 2005, at 10:05 AM, John Adkins wrote:
> 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?
If the zener is shorted, it can be replaced by 7, fwd biased, 1A Si
diodes in series, mounted on a perfboard. If the resistance of V2's
parasitic suppressor R is more than 30% high, odds are that you had a
parasite at c. 110MHz - which caused the zener to fail. The fix is to
either replace the tubes with low on emission old, tired ones, or
decrease the Q of the VHF suppressor to reduce VHF gain.
> 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?
Check the R of the 0.82-ohm, grid-I meter shunt resistor on the circuit
board.
Put 2, 3A diodes across each meter movement.
>
> Thanks
>
> John
> w4kv
>
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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