The AM Forum

After a hiatus of a few years I put my Viking II back on the air. Unfortunately the oscillator seems intermittent (works about 90% of the time). Replaced osc tube (I use a 6AH6, doesn't everybody?). Checked voltages (unfortunately when I have it open and on the bench it works FB). The screen resistor R19 is supposed to be 68K. Someone put in a 33K but it has aged to 59K so it seems ok but I'll replace in a few days to 68K. Thinking screen resistor might have a crack in it I've done some judicious pushing on it with a dowel, no issue seems apparent with screen resistor.  Have recleaned the band switch and crystal switch contacts. Has anyone had any similar results with their Viking IIs?

73

Gary

Hi Gary,

I had two problems with crystals not operating in my Viking II, although I normally use the external 122 VFO.

In one case, removing the 6AU6 to test it and putting it back in solved the problem that time, and you have had the tube in and out.

In the other case, I had some crystals that did not work in the Viking II but worked in a transistor test oscillator I built.  Studying the schematic, the electron-coupled oscillator circuit has no r.f.  return on the screen grid.  I put in 10 pf to ground (worried about too much loading but you can go a little higher value) and all the crystals that didn't start, worked after this.  Perhaps aging and more leakage at the big crystal socket array and switch has something to do with it.  So perhaps they just got away with that circuit when it was new, I believe one other Johnson TX has this oscillator circuit, but not their other transmitters.

GL OM

Thanks Tom, I tried several crystals and a VFO with similar results. One thing I’ve done that may have solved the problem is loosen and tighten the tube socket mounting screws. Since then I have been unable to replicate the problem. I would have thought internal capacitance of the 6AU6/6AH6 would be sufficient for RF return. But if I have any further issues I may try your suggestion. 73
Gary W5GW