Viking Valiant VFO Problems

[w8fax]

OK. I have read everything I can find on this common problem with the dang thing. I have done all the grounding that W8JI recommends. I have changed out power supply caps and the wax tubular caps. I have changed out the dreaded R in the vfo compartment. I have cleaned the switches and made sure that everything that was supposed to be tied to ground, really was. I have installed a fuse and three wire power cord. I can measure no AC from chassis to ground anymore. HOWEVER....the vfo STILL wants to jump and just generally be unstable on CW from 40 meters and up. I dont notice it on phone, and have had it loaded into a dummy load for long periods of time. It just seems to be on cw. ALso, have plenty of drive, plenty of plate current, at rest current is w/in manual specs, tubes are all good, AM modulation sounds good and looks fine on scope, and I have changed the 866's out for 3b's. The vfo tracks correctly and is on frequency as indicated on the dial. Any more ideas??? About ready for a mallet adjustment...lol. Anybody.......help help...thanks for your help....Al/W8FAX

[WQ9E]

40 and up uses a different oscillator circuit in the VFO.  If you are sure that the VFO is dead stable on 160 and 80 then you are probably looking at a defective silver mica cap or poor variable capacitor contact (either main or trimmer) in the 40 meter oscillator section.  When you cleaned switches did you also clean the switch sections inside the VFO housing?

If critical listening indicates some problem also on 160 and 80 then there are a lot of possibilities.  Have you tried new tubes in place of the oscillator and VR tube in the VFO?  A tube checker, even a very good one, is not going to catch tube related problems like what you are experiencing in the VFO.  VR tubes, especially with age, are far from perfect so don't accept it as being good just because it glows.

The problem could well lie in the keyer circuit.  Issues with the keyer tube circuit are much more noticeable when operating CW.

Have the bias filter caps been replaced?  Have you tried a new tube in place of V11?  Cycle R-39 a few times to clean it.  Of course the passive components in this circuit can also fail.

[w8fax]

Finally got back into it. Work takes up a lot of my time. ANyway, thanks for the suggestions. I did change the tubes and cleaned the switches inside the vfo. I thought I had it. It sat for a long while running, and other than some slight drift, seemed OK. I tried a couple of CW QSO's and it seemed to hold fine. Put it all back together and it worked OK for about a day. Then it started again. It will just sit there cooking along fine, and then take a jump. It is not really a slow drift, just a jump. I am going to have to get into it again and do some more testing of voltages. Now I notice that when I "zero" the vfo and then key the xmtr, the frequency shifts. I also note that the keyed wave form looks kinda strange. Oh well, it's a hobby? I wonder if these things were all that stable to start with??? Seems like a lot of info floating around about vfo troubles. Has anyone tried an external vfo?? Seems like that would be "surrendering"....lol

[W2ZE]

as stated before, you need to replace the silver mica caps in VFO. this is a relatively easy fix, and takes about 5 dollars worth of parts.
Just replace these, no need to go through an excercise in futility with cleaning switches and checking voltages, etc....

[w8fax]

Yes..I was afraid of that. This means taking the cover (top) off the VFO. I have had the side off a few times but not the top. Any "hidden" tricks to this??? BTW..are 5% micas good enuff??  Thanks for the help and comments...AL/W8FAX

[W2ZE]

I replaced these when I worked on the rest of the componenets in the VFO, which reqired me to take out the entire cover. Not an easy job as you know. I also had the shaft coupling broken, so it made sense for me to do that.  Also check for out of tolerance resistors as well, I found several, so it is still worthwhile to do this work.

[w8fax]

OK...thanks for the info. I guess there is no "easy" way..lol...Al

[w8fax]

One thing I fergot to mention. Somewhere in the past, somebody put a large hole (about 1 inch) in the top of the vfo cover. It is right over the 6AU6 tube. Then they fitted a metal panel plug in the hole. The metal plug was drilled full of 1/8 inch holes. Also, a Bakelite tube was slipped over the tube. The tube has about 3/16 clearance on the sides. This makes sort of a "smokestack" for venting heat out of the vfo compartment, while still retaining the shielding. I took the "stack" off the tube when I replaced it, thinking that maybe the compartment actually needed the heat to stabilize, but it made no change. Interesting concept tho', and somebody was thinking when they did this......AL/W8FAX