Questions On Troubleshooting SB-220

[Goss, Ed]

Hello,
     I recently experienced a failure of my SB-220; it was operating fine
one minute, then the next time I looked the meter lights were out, and there
was no output power or grid current showing.  High voltage was still
present, and the cooling fan was turning.  When I removed the cabinet, I
found that I was unable to measure any 5 volt filament voltage.  I carefully
pulled the 3-500Z finals and did not measure any filament-to-grid shorts.  I
also did not notice any evidence of filament transformer overheating or
anything oozing out of the transformer case.  I did not smell or hear
anything odd when the amp failed.
     When I disconnected the two large filament transformer leads from the
rest of the circuit, I was able to measure the correct 5 volts.  But since
the tubes were pulled, there was nothing except two pilot lamps still in the
circuit, so at first I was unsure why I wasn't able to measure the correct 5
volts with the transformer leads wired in.  Apparently, anything at all
across the 5 volt filament leads causes the voltage to drop to zero, even
just one pilot lamp clip-leaded in.
     I suspect a failed filament transformer, not due to the well-known
filament-to-grid tube short, but perhaps just transformer old age or an
internal defect (I had a main power transformer go that way once).  I would
hate to replace the transformer and have the same thing happen again though
-- wouldn't a 3-500Z filament-to-grid short remain shorted...it wouldn't
open back up again after the tube cooled down?  I don't see any other
evidence of a tube short (like fried grid chokes or plate meter resistors).
Anyone else go through this kind of failure?
     I'll probably install the Harbach mods while I have the 220 apart.
Anyone have any recommendations as to installing a QSK circuit and where to
find that mod, and are there any preferences for vacuum relay vs PIN diode?
Of course I'd like to get the amp functioning again before I install any
mods.
     Thanks for any suggestions or comments.
--Ed, N3CW--

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