The AM Forum

..ok..now, my viker2 is losing output power...starts out at abt.100watts, then starts to drop-off...mod current rises, as well...power supply?...
..help...

...sk...

 Bias supply filter cap leaky and drawing current after it heats up, miss-matched, bad, or gassy 807 causing strong/good tube to overheat and draw current, weak 6AL5 bias rect....
  You can spot the 807 issue easy enough from the top but need to put the thing on it's side and carefully measure/monitor the bias and other voltages. The bias will be there without switching the plate voltage on but be careful anyway.

 Those seem the most likely issues but you have to get the voltmeter in there to isolate the problem.

  Mine has been real stable after replacing the electrolytics and solid stating the rectifiers, etc.

  Bill, KB3DKS in 1 Land

Sounds like your RF amplifier is drawing less current (power going down) and the plate voltage is probably rising as a result which causes your modulator current to rise.

Could be bad final(s), screen voltage issue, etc. How much plate current does the rf final draw? When you tune off of resonance how high will it go?
Does it do this on all bands?

Just a few things to check.

Dave

How is the grid drive itself, does it drop?  If so suspect C-25 (the coupling cap).

R-35 (in the screen lead) does not dissipate much power but if it has become temperature sensitive this could be your problem, I have seen that happen before. 

So try to narrow it down, does the grid drive drop or remain steady.  What is the screen voltage doing.  It could also be one or both of your 6146 finals although one of the more typical failure modes for those is increasing plate current as it heats due to a damaged control grid from previous excessive grid current events.

Rodger WQ9E

..ok, well, i'll look into this further...but firstly, what is the value of c-25? (voltage, etc)...and which is the bias supply cap?....i do have the viker manual, but of course, it is in a box "somewhere"...

..sk..

..ok..checked all the toobs...all ok...but grid current is dropping, and output decreases the longer i transmit...this can be a 1 minute transmission, or 5 mins..no difference...

..sk..