[Amps] Plate Current Flow in Standby (Titan 425)

[Joe Giacobello, K2XX]

My TenTec Titan 425 amp suddenly started showing plate current flow in 
standby shortly after powering it up.  If left on for a minute or so, 
the plate current will drift up to several hundred mA and the finals 
will get quite hot.

My initial suspicion was that there was a problem on the bias switch 
board (81219). However, I have thoroughly checked a number of components 
on that board and, after consultation with a TT technician, replaced all 
seven transistors.  While I did find one of the original transistors 
(Q7) had both junctions open, all the others seemed OK.  I also tried 
replacing the 3CX800s without any change in symptoms.

My next thought was that there was a problem with the 28v bias supply 
(board 81212), but all the diodes check OK and the 1000uF filter cap 
also checks OK. The manual's troubleshooting guide recommends checking 
D1 on the bias board and D1 in the power supply, and they're both OK.

I hate to capitulate and throw in the towel on this problem, but I'm 
completely baffled at this point.  I wonder if any of you amp experts 
out there have any suggestions for the source of my problem.


Many thanks and 73,

Joe
K2XX