My Centurion 422b amp is sick. With initial application of power, in
selected PTT and STANDBY modes (and un-keyed), the 3-500's are drawing 140 ma
plate current (and barely perceptible grid current). Panel MODE led's indicate
proper mode when switched between STANDBY and OPERATE, and the TRANSMIT led is
unlit. When placed in OPERATE mode, and keyed (at the PTT RCA jack), the
TRANSMIT led is illuminated. Keying the PTT input when in STANDBY mode does NOT
illuminate the TRANSMIT led. So, the panel MODE-indicating led's suggest that
mode control functions are OK. The RF input/output switching/routing is correct
for all modes.
Have begun troubleshooting the QSK board, and Filament-AC board (with the tube
bias circuitry), and the HV filter board, but have found that the componentry
on those boards differs from that described in the 422 (no-suffix) manual.
Although the manual available on the RKR website is identified as a 422b
manual, it appears to be for a 422 (no-suffix) amp. In that manual, there are
discrepancies between the schematic and board layout and circuit trace views
for the FILAMENT-AC, QSK, and HV FILTER boards. And, the QSK board in this amp
is a p/n 78247, not the 81542 called out in the 422 manual.