Viking II puzzler

Gary Schafer gschafer at MEDIAONE.NET
Mon May 15 14:39:36 EDT 2000


When you say "with no results" does that mean it did not oscillate in that mode or it did not fix it? Does it oscillate with the mike gain turned all the way down ? You might try shorting the grid of the audio driver tube to ground and see if that cures it. If so look for rf getting in the audio chain. When you put the 2 k load on the mod transformer did it oscillate then? I assume at that point you had removed the 6146s so no rf? Check to see if the resistors are in series with the grids of the 807s. Could be a parasitic in the 807s if someone took them out. Also check the b+ bypass caps for the low level audio stages. If one is open it could cause feedback via the B+. Also check to see if someone has added negative feedback from one plate of an 807 or from the secondary of the mod transformer back to the mod driver or second speach amp. If so and if the plate caps are reversed on the 807s it will make a good oscillator.

Good luck
Gary  K4FMX

Paul Litwinovich wrote:

> 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

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