Meck XB-702 restoration

[Kevin Kuehn]

Here's a set with a similar problem at about 1:00 in the first video.

https://www.youtube.com/watch?v=Ygpw87pqTz0

https://www.youtube.com/watch?v=aSEMVpJ1hzk

[Notimetolooz]

Another thing is that the problem seems to go away when you move the CRT away from the chassis.
I just had a thought.
When you look at some of the signals with the scope, like the sync, does the hum look stationary with respect to the sync pulses or does the position slowly move ?

[Yamamaya42]

could by any chance, one or more of those .005 disc caps on the heater string, or on the filament transformer be leaking?

they rarely go bad, but it's NOT 100% unheard of, and would explain a lot

[Kevin Kuehn]

I think I would go back to the B&K Analyst and see if there's something obviously different about the way the 60hz is riding on the video and sync. The fact that the wave clears up with the vertical synced to the AC line is a significant clue to finding the cause.

[bandersen]

Would it though? The chassis is hot so there's already AC on it. A leaky cap would be partially shorting out the AC supply.

[Yamamaya42]

Originally Posted by bandersen

Would it though? The chassis is hot so there's already AC on it. A leaky cap would be partially shorting out the AC supply.

perhaps so but it does not have to be much to cause a problem, it's kind of odd that it goes away the sec you switch it off, and the heaters are still warm enough to work and there it still enough power in the caps to run for a brief sec or two.

It's less likely to be a bad rectifier or something like that. cause i'm sure you checked.

I'm just trying to think of a possible path, this is a VERY odd problem. :O

[bandersen]

Originally Posted by Kevin Kuehn

I think I would go back to the B&K Analyst and see if there's something obviously different about the way the 60hz is riding on the video and sync. The fact that the wave clears up with the vertical synced to the AC line is a significant clue to finding the cause.

It's not cleared up, it's just stationary.

Originally Posted by Yamamaya42

perhaps so but it does not have to be much to cause a problem, it's kind of odd that it goes away the sec you switch it off, and the heaters are still warm enough to work and there it still enough power in the caps to run for a brief sec or two.

It's less likely to be a bad rectifier or something like that. cause i'm sure you checked.

I'm just trying to think of a possible path, this is a VERY odd problem. :O

Well, the instant power is turned off, there's no more 60Hz AC coursing through everything. So I would expect it to go away.

What's killing me is that nothing I've tried has had any significant impact. H-K leakage on the video amp or sync tube seems the most likely to me. So would a bad ground connection. Or could it be something stupid like the big wirewound filament dropper resistor radiating 60Hz ?

[bandersen]

This is turning out to be quite the adventure and I hope to learn something from it so I'm not giving up.

Powering the filaments from DC seems like a good place to start. I will have to be careful about the HV box though. A wire comes right off the AC plug, into the box and goes to both the tube filament and the turn for the oscillator. I'll have to separate the two and run a new wire.

[dtvmcdonald]

I had similar problems with my Pilot 3" set. It had hum bars in the picture, and would lose sync at the same phase of the 60 Hz.

It turned out to be mostly heater-cathode leakage and/or pickup from the filament line in either the tuner or IF amplifier stages.

Getting rid of it required buying three or more of each tube type, NOS, and trying tubes. If a new tube was even a little
better, I left it in and tried replacing other tubes. Repeat until every possible
change makes things worse.

It was worse at high signal levels. So:
Try a variable attenuator in the RF line (in the coax part of the line.)
Use an isolated transformer not a DC-connected balun to go from 75 to 300 ohms.
DIP-package counterfit Mini-Circuilts ones are available cheap in bulk from China and work fine.

[bandersen]

Thanks for the suggestions. I have many 6AU6s so that's easy to try. I'll go over my signal hookup and try some other baluns

[bandersen]

I did do some experiments with moving the CRT around. It does seem the vertical distortion is less when I lift it up but it's not a dramatic difference. The horizontal is not effected.

I tried waving some magnets around to see how strong a field is needed to cause distortion. The result surprised me. A refrigerator magnet had no effect. It took a strong speaker magnet to deflect the beam.

Removing the only two major sources of magnetic field also had little effect. That would the B+ filter choke and tuner filament transformer. I did that by running the tuner tubes on 6VDC from a battery.

That's how I concluded the issue was electrical in nature rather than magnetic.

[old_tv_nut]

Wild random thought: what happens if the signal source is running off the same isolation transformer as the set? (and try reversing one of the plugs)

[bandersen]

Sencore TVA92 just arrived. Will unbox tonight and try it out. It will generate a video signal up to 200 volts.

I've never used one before and am not sure how to set it up. I've heard you have to plug it into the VG91? I think there's a 15 pin connector on the back.

[bandersen]

This set is really trying my patience. I was thinking that maybe the filament voltage wasn't dividing equally among the tubes and did some testing.
The result was low readings on all the 12SN7s (10 volts) and the 25L6 (20 volts) so I went over the schematic. It makes no sense!

If you add up the string with the 50 ohm dropper, you get 119.5. That's without the 50 ohm resistor! There is no reason for it to be there unless they intentionally want all the tubes to run lean.

The Teletone version does not have this resistor.

The other string adds up to 75.4 which should have a 139 ohm resistor to get 117. 150 if close enough.

Why in the world would they want to starve one of the filament strings?!?

[Kevin Kuehn]

With the dropper resistors in series it stands to reason that the higher filament voltage tubes will be starved of proportionally more voltage than the lower voltage ones. That being said ohms law would indicate you likely have closer to 250 ma going trough the 12SN7's, 25L6 string. But yes, I doubt they are that critical.