Hello to the group,
I am repairing a TL-922A amplifier that keeps blowing the fuses. The first
thing I found was a shorted 7.5v zener in the cathode/bias circuit. I
replaced the diode, and checked the usual stuff, but the fuses still blow.
No visible trouble (burned components, etc). I then pulled the pair of 3-500
tubes, checked the plate circuit, and surrounding components. I then fired
the amp up with the HV shutoff switch active, and the filaments and low
voltage items worked fine. I then dug in deep and pulled the diode board,
filter cap assembly, and surrounding assemblies for test and inspection. The
caps and diodes all checked ok. I did find a cold solder joint on the -HV
pin of the diode board, but that may have happened during disassembly.
Everyting looked/checked good. The bias zener survived this time.
I put every thing back together, double checking everything, then fired the
amp up again. This time the HV meter came up to 2500v or so and no blown
fuses! However, this time the CW/SSB switch was in the CW position. As
soon as I placed the switch into the SSB position, the buzzing started, and
the fuses blew again. I then took some resistance measurements, and traced
the wiring of the switch to the metering board, where everything checked
good. Even the transformer secondary windings look to be good as far as the
resistance change between the CW and SSB taps.
What next? Is it possible that a diode or HV cap is breaking down when
subjected to the higher HV potential of the SSB tap? Is it possible that
the transformer is shorting when the SSB tap is used? I am at a dead end
this evening, and have quit for now. Maybe tomorrow something that I missed
tonight will jump out and hit me in the face!
73
Eric
KE2D
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