Meck XB-702 restoration

[vortalexfan]

I know with my Meck TV it gave me some interesting issues, but nothing like the issues you're having with yours.

Its kind of strange that you're having issues with 60 Hz hum in the video stages on your TV, I don't think I had that issue with mine, although mine is about a couple years newer than yours though, and a little different circuit design.

[bandersen]

Yeah, 60Hz all over. It's crazy to see the test pattern snap into perfection for a brief moment when power is turned off.

It probably doesn't help that none of the IF transformers have shields.

Here are the 3 video IF stages. I've been checking that all the ground connections are sound.

I have not checked the ceramic or mica caps yet.

[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

[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.

[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.

[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 ?

[jr_tech]

Originally Posted by bandersen

Yeah, 60Hz all over. It's crazy to see the test pattern snap into perfection for a brief moment when power is turned off.

Perhaps you could turn off power to the DC supplies while leaving the heaters on
and vice versa, to narrow down the source of the 60 hz interference.

jr

[Kevin Kuehn]

Whenever I see 60hz all over a chassis I think ground loop, which can be as simple as how you're hooking the scope into the circuit. For instance not having a good ground very near the end of the probe in the near vicinity of where your taking the measurement. I'm just making a general comment for those following along that may not be aware of that situation. It's more important when the piece of equipment is floating on an isolation transformer.

[old_coot88]

Originally Posted by jr_tech

Perhaps you could turn off power to the DC supplies while leaving the heaters on
and vice versa, to narrow down the source of the 60 hz interference.

jr

Seems like the reverse would be preferable; kill the heaters, so there'd be several seconds of watchable display while they cooled

(EDIT.) OOPs. You aready said it. Dumb me.

[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 ?

[bandersen]

Originally Posted by 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

It sure does. That's a series string, floating chassis if I recall correctly.

Originally Posted by 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 ?

The problem does not go away when I move the CRT away. At least not horizontally. Perhaps it's a little better vertically.

The hum moves slowly with respect to the sync. If I trigger my scope from the line, the hum locks in place.

[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.