Viking II lack of grid drive on 20-15-10 meters

[WA2ROC]

First off, thanks to all the technical gurus out there for the suggestions to get the Apache back up and running, and not wandering all over the band.

Now my Viking II has little or no grid drive on 20-15-10 meters.  160, 40, and 80 have plenty and on 20-15-10 the buffer current is as high as the other bands.  This leads me to believe that the buffer circuit and everything ahead of it is OK.

Since the 122 VFO has output enough to drive the VII on its 40 meter setting, I can probably eliminate that too.   

I plan to swap out the buffer tube with a known good one first because it's easy to do.  I already cleaned the bandswitch contacts, to no avail.

Also swapped in a set of 6146's also to no avail.

Next will be component replacement, including the coupling capacitors between the buffer output and the final grids, all the bypass caps in the buffer and final stage and maybe those resistors across the buffer in and out coils (what are they used for?)

Any other suggestions would be greatly appreciated.

[WQ9E]

Dick,

One difference is the buffer stage operates as a multiplier on 20-10 instead of straight through like it does on the lower bands so it does require considerably more drive to operate efficiently as a multiplier.  Thus the oscillator/VFO buffer tube and VFO aren't entirely off the hook.  But I would try a new 6AQ5 before going any further with troubleshooting.

[WA2ROC]

New tubes, resistors and caps on the way.

I did try another 6AU6 and 6AQ5 but same results.  Seems like the 6AQ5 is running very hot on 20 meter position, possibly because the drive pot is set almost to the max. 

I have the 122 VFO set to the 40-20-15-10 meter position and there is plenty of drive and 6146 grid current when I tune up and operate on 40 meters.  There's lots of 6AQ5 cathode current when I try tuning up on 20 meters,  but not much grid drive.  I get less than 3 ma grid current in "tune" (HV off) but just about 0 ma grid current when I turn on the HV. 

I do get RF out on 20 meters, close to 100 watts, but I can't figure out where the drive is coming from.

The bottom panel comes off tomorrow , the scope and DVM come off the shelf and in we go....

[WQ9E]

Do your oscillator and buffer dial positions match those in the manual when tuned for 20 meters?

[WA2ROC]

Yes, pretty close. 

It's been working on 20 for years and just quit a few days ago at same dial positions.

[w4bfs]

sounds electro-mechanical .... switch continuity and tuning capacitor rotor grounding

[WA2ROC]

I'll get out the cleaners and scrubbers and look for cold solder joints while in the neighborhood.

When the parts arrive I'll replace them too even if cleaning and scrubbing fixes the problem.

[AB2EZ]

Check the value of the fixed grid bias on the 6AQ5 buffer / multiplier. [The 6AQ5 buffer / multiplier also has some self bias that results from the average 6AQ5 grid current that flows through R23]. According to the manual, the fixed bias on the grid of the 6AQ5 should be about -20V. It should be measured at the bottom of the 47k ohm resistor R23 (at the point where RF is bypassed to ground via C24). If the fixed 6AQ5 grid bias is not negative enough, then the stage will not work properly as a multiplier... although it will work fine as a buffer (with more plate current than normal)

Therefore, if the fixed 6AQ5 grid bias isn't negative enough, the rig will not produce sufficient grid drive to the 6146 finals on bands above 40m.

Stu

[WD5JKO]

Therefore, if the 6AQ5 grid bias isn't negative enough, the rig will not produce sufficient grid drive to the 6146 finals on bands above 40m.

In the mid 1980's, myself and a friend both obtained Viking I's along with the 122 VFO. We both had grid drive issues on the upper bands where 10M was the worst. Neither of us could find a satisfactory solution that gave good grid drive on 10M such that the modulation peaks were good.

We both made changes, and while working independently we came up with similar solutions. He replaced the 6AQ5 with a 5763, so a socket change, and some other circuit tweaks. This worked well. I was playing with Super Modulation at the time using a 600W audio amplifier as a modulator. I needed more grid drive for 200% positive peaks. So I elected to replace the 6AQ5 RF driver with a 2E26. Boy did I have grid drive!

I am not saying with a Viking II that major surgery is necessary. What I am saying is the stock design, on a good day, with optimum parts is just barely enough. If I wanted to keep the rig close to stock, I might replace the 6AU6 crystal oscillator / VFO buffer with a 6AH6 and see what that does.

Jim
WD5JKO

[WA2ROC]

Thanks for the info Stu.  I measured -23 volts on the 6au6 grid at full rf out and I also got the grid drive up to more than 5 ma.
How, you ask?
Lots of contact cleaner with a stiff bristle brush on tube sockets, band switch and variable cap contacts.  I also reset the clamp adjustment and things look good.
I will use the new tubes and coupling  and bypass caps when they get here.
60 years of crud takes its toll!

[WQ9E]

Dick,

My preferred method of adjusting the clamp circuit is to first make sure that there is proper grid drive and temporarily set the clamp pot to the extreme end of the range where it has no effect.  Then load the transmitter to rated input with the proper grid drive.  Now with the transmitter producing full RF output adjust the clamp circuit to the point where you just see the output level wiggle and return it to just before the point where the output dropped.

Using the method typically specified in the Johnson and Heathkit manuals it is easy to end up with the clamp circuit holding the output level down when  it should not.     

[WA2ROC]

Voila!   The tubes I ordered arrived and I replaced the vfo, rf amp/crystal oscillator and driver.  I also found the original vfo tube was a 6ba6, not a 6au6.  Not a good thing. 

New tubes in place and we now have more than enough grid drive and plenty on 15 and 10 too.

Vfo was aligned and we're off and generating lots of rf again.

Tnx for all your help

[Jim, W5JO]

6BA6 will sub for the 6AU6A. 

[w1vtp]

Dick,

My preferred method of adjusting the clamp circuit   <snip>


Using the method typically specified in the Johnson and Heathkit manuals it is easy to end up with the clamp circuit holding the output level down when  it should not.     

Rodger

Thanks for the tip.  I'm printing this and inserting it into my V2 manual

Al

[ka4koe]

Rodger...need to check my Valiant for this condition with the clamper. This may explain the low grid drive (now that I have some to play with) on the upper bands.

Philip

[WA2ROC]

FYI, I followed that procedure and my V2 made a lot more grid drive on 20 and 15.

10, not so much.

I may swap the 6AU6's to 6AH6's and see what happens.

I hear that the calibration on the VFO will change when using the 6AH6 as a VFO tube, but that's easy to correct.