Viking II puzzler

[Paul Litwinovich]

I've got an interesting problem with my Johnson Viking II. It started with an oscillation in the modulator which I could see on the scope when monitoring the RF and sounded like a buzz on the receiver. Then the mod tubes (807s) started drawing a lot of current, over 400 mA. Here is where it gets interesting. With either 807 run by itself I get about 50% of normal idle current and the rig will produce somewhat normal positive or negative mod peaks depending on which tube is pluged in. The rig has new low voltage and bias filter caps, is producing -55 volts bias on the tubes, 300 volts on the screens. I also replaced the screen by-pass cap with no results. The rig will not run with both 807s socketed. I also pulled the 6AK6 that drives the 807s with no results. My first guess was a screen to grid short buit this would not explain why either tube will run by itself. I also disconnected the mod xformer secondary and ran it into a 2K ohm load to eliminate a py to sy short with no results. the Mod xformer tests ok with audio, resistive and hipot tests. I did not have a spare set of 807s but I made up a couple of adapters to plug in a pair of 6BG6s and got the same results. I just got a new set of 807s but I am a little nervous about damaging them, not having found proof positive of what caused the failure. Again, the modulator draws wicked current when both tube are plugged in, with or with out audio applied.

Any one have a similar problem?

73

Paul A.Litwinovich
Chief EngineerWSHU AM&FM, WSUF FM
N1MUC
"RF IS GOOD FOR YOU"