[Amps] SB-220 Troubleshooting Question

[Mark Cherry]

Hi all.

 

I have recently had my SB-220 end its useful life.  I keyed it for CW, it
groaned, hummed and popped one of the two breakers!  During this event, it
also took out one of the two power resistors of the Harbach Soft Start.

 

I started 'shooting it in stages.  The filament xfmr and circuitry is
functioning.  There are no shorts from the rectifier board through the plate
circuitry.  All diodes have been checked with an ohmmeter and look good, as
do the resistors on that same board.  The SCR for the bias voltage tests
good as well with an ohmmeter.

 

The HV xfmr shows a secondary resistance of 56k ohms (fully disconnected)
and from everything I have found, it should be about 12 ohms.  Without any
load the HV xfmr gives about 850VAC running on 120V mains, for testing.
When connected to the Cap bank (no load after that), it resumes to blowing
the softstart power resistor.

 

I have tested caps with ohmmeter only and know that loaded, they may still
fail, but, my big concern right now is the 56k secondary resistance.

 

I am looking for any hints or thoughts as to the direction I should take or
tests I could perform.

 

Thanks in advance for any help and for your time.

 

 

Mark

WT6P - Kingman, AZ

NA6E Memorial A.R.C.

Arizona Outlaws Contest Club

 

"Borrow money from a pessimist, they don't expect repayment"