The AM Forum

So, just operating tonight on 3880, all
Of a sudden my valiant decides to steadily drop to 3877.   Key up after that sometimes on 3880, and sometimes not.    Keyed with no power zero beat, everything seems to be on 3880, so I think this only happens when hv is applied.    Thinking this could be a host of things, but curious where to look first.

If it stays on freq with a xtal in the socket, then the problem is in the VFO.

That would likely be the infamous "chernobyl resistor".

The one that is burning up inside the VFO case... that one.
Or it may be something else in the VFO like a bad VFO tube, etc... a voltage that is moving under load.

Others will have better info than I.

                       _-_-bear

I had one that did that.  I opened up the VFO compartment, replaced the tube, Uprated the Resistor, replaced the caps and she was back in service.  What a PITA to get the VFO cover off.  Make sure not to lay the rig over as you can break the VFO coupler.

C

Thanks.    She's on the bench now.    We shall see!

Which tube did you replace, the regulator or the oscillator?

Both. My reg tube had the glass sucked in slightly.

Someone in the past has taken 2 , 2 watt resistors in parallel and created a 18 k resistor still running 18k.   I'll change that, but I doubt it's the issue.    I kinda doubt the caps are bad.   But the tubes could be an issue.    I kinda sounds like a voltage regulation issue.    I just don't know.

On the resistors, make sure to use over 2 watt ratting.  I thing i used 4 watt. Swap tubes i guess.

As I recall the Valiant VFO has two LC circuits band-switched into the tube. One for 160/80 the other for 40/20/15/10. Try warming up a dummy load on 40 and see if the problem exists on that LC or not. That should narrow your search to one, the other, or neither.

It sure could be a bad cap though. I had a postage stamp cap in the voltage divider of a Johnson VFO crap out. The frequency would shift a few hundred HZ now and then. It kept getting more often. I doped it out by substituting cheap disc ceramics. That found the problem but they were too drifty to use. A silver mica from the junkbox made it all good again.

Update....   Got this on the hp rf analyzer and could watch the vfo drift quite a bit.... Ended up changing out the 6au6 oscillator tube and all seems well.   Also cleaned switch and variable caps contacts, etc.   grounded the radio with 3 wire plug, etc.    So far minimal drift.   In fact, I tried rapid cw and couldn't get it to chirp much, so I think it is ok!

Ver nice.  Cant wait to hear it sometime

One other thing I was able to track down with the rx analyzer on the HP.....   Sometimes, the vfo would drift 20-75 Hz at a time, rapidly.   This ended up being caused by the bandswitch wafer in the very back of chassis where it connects to all those micas not making good contact.   The switch was kind of hard to turn, and a little corrosion back there.   I cleaned it good and sprayed with a little De Oxit, and now the switch is good and smooth, and functions properly, and the drift is very stable every time, not some of the time.    Just wanted to post this in case others run into this issue later.   Yes, love to work anyone on the air.   Matter of fact, I'm gonna tune it up on 15 or 40 later on.   Had a lot of fun the other day on 15 meters AM.   73 to all, and thanks for the help and comments.   What a fantastic group on here.  - Joe

Great tip.  On a valiant, I use a toothbrush and de-oxit. I rotate the switch and scrub the contacts until they are clean and shiny.  Some of them need to be tensioned. You can watch the contacts rise and fall.