My kenwood TL922 has developed a strange fault. When I switch it on (via an
Ameritron Soft Start module) I get a negative reading on the Ig meter of -50mA
and a positive reading of around +100mA on the Ip meter with no key down.

Keying the amp has no effect and the relay does not energise. I have checked
the ALC and relay connectors and the coaxial cables

Easy diagnosis: one of the two 3-500Z tubes has developed an internal
grid-to-filament short (quite a common fault).

The short causes the tube filament to effectively stay at ground potential even
during stand-by, and that is the reason why the tube conducts and the plate
meter then shows some idling current.

The grid shows negative current because, due to the short, the positive DC
voltage applied to the filament to keep the tubes interdict during stand.by
founds a path to ground through the grid.

WARNING: you are running the risk to blow the service transformer (T2). This is
a Kenwood design fault: when a grid-to-filament short occurs, the 80 Volt
generated by T2 is shorted to ground. DO NOT SWITCH THE AMPLIFIER ON AGAIN until
you have replaced the faulty tube.

73

Tony I0JX