[Johnson] Trouble shooting valiant dead on AM

[radioman]

take it step by step
Nobody can guess what happened.. you gotta measure stuff.

1. Normal Grid Drive in AM mode  7.5 ma ?

RF Output will fall to zero with no grid drive, as the clamp tube shuts
down the Finals to protect them.  A mode switch failure can cause this.

2. Modulator resting current normal ?  ( I wouldnt try to modulate it)

3.  take Xmtr out of case..   power off.. all filter caps discharged, ac
line cord unplugged :

clip one end of ohm meter to bottom of plate choke. Watching for
continuity
.Work backwards. following the B+-..to the output side of the secondary
of the mod transformer.. then the other side.. then follow the B+- path
untill you loose continuity.

There's your problem.

As others have said.. if the mod xfmr secondary is open or the mode
switch gets funky.. you'll lose B+- to the finals.

Most common failures in mod xfmrs are short to ground ( blown fuses and
smoke) or open secondary winding ( nothing).  Primary to secondary
shorts also happen..with arcing, frying distortion..and eventual death
of the xfmr.

What were the operating conditions when it crapped out?
idle current ?
PA plate current ?
Mod Peaks ?

Any mods to your transmitter, or old caps still re-maining ?

I would like to avoid this +ACE-
Maybe it was just fate.


Larry W6WUH