I need some sleuthing to remote diagnose a failed amplifier.
This past weekend I had my TL922A fail during the ARRL CW contest. I had
replaced a bad tube in this unit and did a number of mods. Since then it
has performed flawlessly in heavy contest duty. Until this weekend.
The unit is still out of the country as I could not bring it back at this
time but I was looking for some guidance for a potential field repair.
The amp failed while tuning on 20 meters. There was a burning smell and the
circuit breakers (not the fuses) tripped.
Upon opening up the unit. The two vitreous enamel resistors I used as plate
fuses had melted and broke. Also, the large doorknob blocking cap had split
open and was blackened. There was no other obvious damage or signs of
distress. The plate choke looked unscathed. I had previously direct
grounded the grids and removed the grid chokes. The underside of the tubes
looked fine. I regrettably did not check the bias zener.
I have my theory but I was looking for advice.
The plate resistors melting was a surprise and the were not obviously
burned. The were deformed and cracked. An alternate theory is that they
wilted from the heat of the tubes and failed. However, I can't see that
causing the subsequent chain reaction.
On the other hand, they may have done their job. I clipped out the failed
resistors and turned on the amp. The HV appeared to be intact in both the
CW and SSB positions.
Any suggestions would be much appreciated.
73, Tom W2SC 8P5A
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