The AM Forum

.. cant seem to find my manual just now... could someone email me, or post the drawing for the viker please....output power is dropping during transmissions...

..tnx...tim..

..sk..

Tim,

The manual etc. is available from BAMA and it is a decent copy.

Does your grid drive remain stable during long transmissions? If not the problem is probably in the exciter section or perhaps the coupling cap from the driver.

 Do you need to touch up final tuning during the transmission? If so that could be a bad fixed loading cap or a problem with your antenna (or tuner if used).

Rodger WQ9E

..rodger...i wont go on bama, just because i'm on dial-up, and you can imagine how slow this would be...i do have a full manual, somewhere!..grid drive is good, no change there...plate current drops, and output on power meter...antenna is ok fb...swr is flat, etc...

..tim..

..sk..

Tim, I extracted the Viking II schematic from the bama manual and sent that page to your email.  Hope this helps!  73, Rick

Check the high voltage rectifiers or screen dropping resistor if the pi network seems stable.

Lots of good ideas already in the thread Tim. Another place to check is around the clamp tube components. A dirty clamp adjustment pot or flaky 6AQ5 clamp tube can cause plate current changes and power output fluctuations. As a test, you can simply yank the 6AQ5 out of the socket to take that circuit out of the equation.

..power still dropping...replaced the 6aq5 in the clamper, and the 6146s...
  what/where is the driver fer the viker?

..sk..

When facing the front panel, the low level RF components are on the left side of the chassis. Right up front, you'll find a 6AQ5 driver tube with its associated tuned circuits and in the compartment behind the driver, you'll find the 6AU6 oscillator tube.

You might find that the front panel drive potentiometer (R25 - 25K) is failing. That supplies screen voltage to the 6AQ5 driver and has a history of failing. I took the advice of Pete (W1VZR) and installed a simple pass tube with a 6AQ5 to fix it and that eliminated the whole issue. Usually you will see fluctuations in grid drive, or a complete absence of drive when this is on its way out. Since your grid drive appears to be stable, this might not be the area that is giving you trouble. With one of my Vikings I found that several of the resistors under the oscillator and driver tubes had changed value and eventually just replaced every resistor under the chassis. It was not an expensive task and only took about an hour to get in there and do the surgery. That pretty much took care of any intermittent issues I experienced. They were very brittle and no doubt were changing value under load as they warmed up.