Valiant VFO

[k7pp]

Howdy Friends,
Thought I'd post a question before I start re-inventing the wheel.

Got my Valiant working really well with the exception of the VFO.

Lot's of power and great modulation and all the meters work.

Problem is,  the VFO is not stable.  Heat does not seem to be a factor.

It varies in frequency about 100 hz is little jumps with just the vfo
spot activated or under full power out.
The level is fine and the meters are stable but the frequency is not.

Here's what I've done so far.

Found that the 18K 2 watt resistor was replaced with a 12 K two watt.

Replaced with 10 watt 18K.

Replaced the 6AU6 ECO.  Replaced the 0A2 regualtor.
Replaced the .005 at one end of the 100K grid bias resistor.
Checked regulated B+ and it's stable.
Checked the +300 volts to the ECO's plate and it's stable.
Temp removed the 12AU7 grid block tube so it's not in the circuit.

No change.

Noticed that this seems to happen in either range.
Looks like Johnson has the LC networks for 180 and 80 separate from the
other bands.  They have a completely different set of caps and coils.

The instability is present on any band.  Even 160. 
I'm now looking at the 43 puff cap in the 6AU6 grid (C18).  Very hard to get to.
Found a 49 puff silver mica I might try but it's gonna be a job.
I really hate to completely disassemble the VFO to get to it but I may have to.

I've got the top and side cover facing the regulators removed.
Might try to remove the other side cover to make this happen.

Last question......what the H*** was Johnson thinking?

Please give me a heads up if something occurs to you but otherwise wish me luck.
I'll change every damn part in that VFO if I have to.

Regards,

[Jim, W5JO]

Before you go for the complicated stuff try cleaning the socket pins of the VFO tubes very well and the wipers of the variable cap.  If that doesn't work, you can look at fixed caps.  Heat all solder joints but be careful of the temperature compensating caps because too much heat on low value caps can ruin them.   The mica caps do go bad, but it is not VERY common.  Those postage stamp micas are subject to failure if the transmitter has been in a damp climate and subjected to repeated warm and cool cycles.

[WQ9E]

Jim's advice is excellent and I would add to that if the problem still remains try disconnecting C22 to be sure that the problem isn't caused by an intermittent change in load in the buffer stage.

Also, how are you monitoring for changes in the voltages at the socket?  Most analog and digital volt meters are not very good for catching small rapid shifts in voltage.   This is where a DC coupled scope is sometimes the best instrument and for seeing a small change (and it doesn't take much for only 100 hz shift in a multi-megahertz signal) a sensitive differential comparator with adjustable offset is needed.   I have a 7A13 differential comparator plug-in for my scope and rarely do I use it but in the right situation it is a very useful measurement aid.

[k7pp]

Thanks,  Guys,
Good suggestions to be sure.
I never did disconnect the coupling cap from the 6AU6 as it is an electron coupled oscillator and plate load shouldn't make that much difference.  Perhaps it does.
Shouldn't be to difficult to do,  though.
I didn't get to excited about the postage stamp caps as they form two complete colpits circuits for each portion of the spectrum.
One for the 160 and 80 and the other for 40 on up.
either one switched in makes no difference in the instability.
There are about half a dozen .005 bypass caps that I just might shotgun but I'll do the isolation of the plate coupling cap first and then C18 on the grid second and then go nuts changing components.....Hi...
Did the cleaning job as a first order of business but it did look pretty clean inside to start with.

I'll update this as it evolves.


Many thanks,
Pete, K7PP

[Jim, W5JO]

Good luck Pete.  I had a DX 100 years back that quit working after about a month and I traced it to the tube socket under the VFO tube.  All voltages were present but RF would not show on the output.  Usually frequency jump is not caused by bypass caps or the like, it is usually a mechanical problem somewhere like a poor ground.

Was that a kit or can you tell if it was factory wired?

[w5omr]

Had the same VFO issue in a Viking Ranger.
Simple fix was to put star-washers under ever screw head.  That cured 80% of it.  The metal develops some oxidation build-up.   A bit of scraping with a knife around the holes revealed 'fresh' metal for the start washers to bite into.

the other part was sweating the solder joints around the oscillator and replacing the regulator and 6AU6 (I think - that was back in the mid 80's.)

I don't know the Valiant, but I seem to remember that it, too, like the Ranger, had the VFO built in, and therefore it's a pain in the butt to get to.

[WQ9E]

Pete,

An ECO provides good but not perfect isolation from the load.  The issue probably is in the VFO but as long as the Valiant is out of its case the VFO coupling capacitor can be disconnected in  a few seconds and you would hate to spend a lot of time inside the VFO if the cause is external. 

It doesn't take much to create a small shift in frequency.  As people found out with the Heath SB-300, all it takes is a slight variance in heater voltage (due to a slight change in line voltage) to cause a noticeable shift even in a crystal controlled oscillator.  Unfortunately when you start chasing these small anomalies it is often difficult to localize the root cause.  BUT you will have a wonderful sense of accomplishment after you succeed.

I never did disconnect the coupling cap from the 6AU6 as it is an electron coupled oscillator and plate load shouldn't make that much difference.  Perhaps it does.
Shouldn't be to difficult to do,  though.

Many thanks,
Pete, K7PP

[The Slab Bacon]

Make sure that the socket pins are ALL clamping good and tight on the 6AU6 VFO tube. Give the tube a good wobble to make sure there is no oxidation on the socket grippers and the tube pins. also clean the VFO tuning cap berings and shaft wipers real good, apply a drop or 2 of light oil to each and vigorously rock the shaft cw / ccw to clear off any last hard grease or oxidation. Also do the same to ALL of the little air variable trimmers. The grease that the factory puts on them will get harder than Chinese arithmetic with time. and cause flakey connections.

Make sure that ANYTHING AND EVERYTHING that is grounded in the VFO with a screw and a grounding lug has a good, clean, and tight connection. Look at EVERY solder connecion in the VFO box, if any look dull or "cold", resolder them.

Last but not least, clean the contacts in the front panel function switch (VFO, X1, X2, etc)
A bad connection there can also cause the VFO to do strange things. My Ranger will do that if it hasn't been used for a long time. (A Valiant is just a Ranger on steroids)

If these dont do it, it's time to start replacing some parts.................................
If you have oxidized connections on those wafers it will cause the VFO to act a little squirrelly as well.

[k7pp]

Good stuff to check.  I've done most of this to no avail, however,  good news.....
I noticed the VR tube would "flicker" a bit when the frequency shifting was going on and traced the problem to the .005 bypass on the screen grid of the 6AU6.  It had a couple of 100k resistance out of circuit. No telling what it was doing to the regulated screen supply.
I went ahead and replaced all the other .005's I could easily get to while I had the VFO open.  Seems to work pretty well now.  Got it all put back together.
Now all I have to do is set the bias values and adjust the keyer and PA limiter circuits and I think I'm ready to operate.
One thing I did notice while I was testing was that the frequency would vary slightly as the drive control was advanced. 
The 6CL6 uses the same supply point for it's Plate and screen voltage as the plate of the Oscillator.
I measured the plate voltage of the 6AU6 during the adjustment of the drive control and could see about a 5 volt change out of 320 volts but it did make the frequency change a couple of hundred hz.
Now I'm wondering if it might be productive to add a regulator to the osc plate to keep this from happening or perhaps I still have a problem.....??
As long as I don't change the drive once I'm on frequency it seems pretty stable.


You guys are the best.....I really appreciate the support and interest.

73s

Pete, K7PP

[k7pp]

Well,  one more development.....  Mentioned that the frequency would change slightly when I changed the drive control.  It kept buggin' me so I went back in and changed out the 43 pf cap (C18) on the VFO control grid.
Turns out it had 55K resistance.  Replaced it and now I can adjust the drive anywhere I want without the frequency shift.
Now I'm really worried as I noticed some water damage inside the VFO compartment.
I'm not gonna take that thing apart again so I've ordered up the postage stamp caps and all the bypasses for the VFO. 
Oh,  I also took the advice offered and cleaned and lubed the variable caps as well.

Pete, K7PP