Viking II (((((HELP))))

[k5kab]

Ok guys I need some help. I have a Viking II that was working great up until a couple of days, ago.
Tune up buffer, max out buffer with drive set to 3: Worked great.
Turn drive down to 2 and with the meter on grid max out grid 5-6 with drive at 1-2.
And so on.
Now no buffer until I bring the drive to 8-9 and the same with the grid and the whole process is squirrely. I can still get 300 mils of plate and a 100 watt carrier, but only with the drive at about 8-9 and still the grid is 3-4 at that point. I normally ran the drive about 3 and that gave me 200 mils of plate now it takes the drive to be on 8-9 to get 250 mils.
Tested all the tubes and they are all good.
What happen to the thing?

[WA1GFZ]

check the drive pot it may have a dead spot

[WD8BIL]

The drive control (R25) on the V2 adjusts the screen voltage to the 6AQ5. They have been known to fail. Check the voltage across it. It should be equal to the LV supply output. Then check the voltage on the wiper of that pot (center leg) and see how evenly it changes when the pot is rotated. The bottom of the pot should be grounded. If you pull the tube with the power OFF and unplugged you should read 25K from the top of the pot to ground.

This is a place to start. If it checks out ok then it sounds like you're losing drive from the 6AU6 oscillator or a bypass cap somewhere has given up the ghost.

[W1AEX]

Check your LV supply voltage. The electrolytics may have dried up and your VFO and low level stages may be starved for plate/screen voltage. It also might be time to go through the low level stages and replace resistors. My osc/buffer/grid drive settings were very unpredictable/unstable with my Viking until I went through and rebuilt all the low level stages. Some of the resistors crumbled as I removed them. If you are using a VF-122 go right for the infamous Chernobyl resistor (R-51 18K two watt) and replace that with a much higher wattage resistor. It took less than two hours to replace them all, and that included time for two coffee breaks!

[k5kab]

You guys are awesome, broken wire on .005 cap to the bottom of the 6aus.

[W1AEX]

Nice!