viking 2 mod current

[kc2etm]

hi i have a viking 2 here it had no plate current i change r28 20k resistor it was open i now have plate but when i read mod resting current i see it starts around
100 and climbs until the metter pegs  r13 is new and adjusting properly  ive slid
it to either end with now change in reading ive put a volt meter on the sliding tap with plate engaged and the voltage coming from r13 does not climb when current meter does do you think this may be just a bad shunt false reading on the meter or should i look for something else
although i do here the supply working kinda hard when im in am mode
thanks
fred

[w1vtp]

What's the control grid current? Exciter drive 5 to 6 ma would be good.  If you are losing exciter drive the 6AQ5 clamper should be shutting the screen voltage down. Is the clamper good?

OOPS - after rereading ur post it looks like the modulator is having the problem.  Is the fixed bias making it to the PP 807s?  What's the voltage at the center tap of T3, the mod driver transformer?

More info will help

GL, Al

[WQ9E]

Fred,

Have you replaced the bias capacitors in the Viking 2? If not do those now.

Does the current begin climbing in the same manner every time you throw the plate switch?  If so it sounds like instability in the modulator section.  Have any modifications been made to it?  It could also be due to a defective modulator tube.  Pull V2 (the audio driver) and see if that changes the behavior; if it does then there is a problem in the V2 or earlier stage.

The original Viking 1 modulator would take off when the 807 plate caps were reversed but I don't believe this applies to the Viking 2 and updated Viking 1.

[kc2etm]

Shorted 807 changed it now i have a new
problem internal feedback even with no
audio feed turn the mod up and it feedsback
tried swapping 807 caps nothing
any suggestions
thanks
fred

[WQ9E]

RF into the audio chain perhaps?  Does it do it with the microphone completely disconnected?

[ke7trp]

No ground.  RFI hot station. 

C

[WD8BIL]

Switch the plate caps on the 807s. Yes, even a Viking 2 will oscillate if the caps are reversed.

[kc2etm]

Found the problem bad coax jumper works now
thanks

[ke7trp]

Good deal.  RFI all over the place.  Glad you got it fixed.

C