[Amps] SB-220 diode failure

[R.Measures]

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
>
> __________________________________________________
> Do You Yahoo!?
> Tired of spam?  Yahoo! Mail has the best spam protection around
> http://mail.yahoo.com
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org