Meck XB-702 restoration

[Yamamaya42]

Just making wild guesses, not having really had the chance to ever work on one of these yet...

The HV circuit is clearly a free running tuned RC circuit run from main bias with a tie to the AC line, assuming the 2 HV caps were replaced, (500pf?)
And guessing that other caps / resistors in the area were also changed with newer kinds, and this all could perhaps change the resonant frequency of the HV circuit, or overall behavior which could possibly include the proximity of the CRT to the chassis, and the parts in it.
IE, starting some kind of weird feedback loop that just won't break.

But, as stated, this is just a guess!

[Tom9589]

I still say try the lantern battery for the tuner filaments. Somewhere I saw a series filament TV that had rectifiers for the tuner filaments. Wish I could find it easily....

[bandersen]

Heading out to the hardware store now. I'll be posting a video soon where I experiment.

I just discovered that pushing on one of the filter cap bases has a minor effect on the distortion.

If I lift the CRT up off the chassis, the vertical distortion diminishes and disappear altogether at about 12 inches. The horizontal distortion is always present.

The two 500pF caps are ceramic doorknobs and I did not replace them. I did replace the 12SN7 inside the box and retuned the trimmer cap. Seems to be working OK now.

[bandersen]

What I find most confusing is that installing a shield from an Admiral 19A1 did absolutely nothing. However, moving the CRT away from the chassis eliminates the vertical 60Hz interference.

[jr_tech]

If the shield is Aluminum rather than mu metal, it won’t be effective in reducing interference from magnetic fields.

jr

[old_tv_nut]

1) Are the tranformers that could cause trouble in the Admiral near the neck of the CRT, while they are closer to the front in this chassis? Maybe this set could benefit from shielding more of bell of the CRT.

2) Are the suspect transformer sources riveted in place or screwed? If screwed, you could try reorienting them, but my guess is they are in the best orientation as built.

3) All this speculation is probably superfluous because you have seen it good once - so I still think wiggling ground connections, transformer shields, etc. and putting strong pressure on them one way and another might find something.

[bandersen]

Only shielding the rear portion seems to be the norm. Usually it's aluminum, but the Admiral is more substantial. Not sure if it's mu-metal though.

Note that Admiral is the only 7" set I have that uses a power transformer which makes me lean towards mu-metal.



Emerson and Airline examples using aluminum


Hallicrafters didn't bother and National didn't bother.

[bandersen]

Back under the chassis. I'm pointing to the cap that couples the sync signal to the oscillators on the other side of the chassis. That sync signal definitely has 60Hz riding on it. Note the proximity to the filter choke. I'm also curious if fully disconnecting the selenium rectifier will have any effect.

[bandersen]

I went back over everything and found no issues. I also reviewed footage from when I first got an image on the screen and the distortion was there. So now I'm leaning towards a defective filter choke, or some insulation breakdown.

I figure I can sub a power resistor for the choke to test that theory.

[bandersen]

OK, I've put as much effort into this set as I'm going to for now.
I disconnected the filament transformer and used 6VDC from batteries.
Replaced the filter choke with a power resistor.
Added an extra 100uF cap to both B+ and B++ supplies.

All that did reduce the 60Hz distortion from both the vertical and horizontal but did not completely eliminate it.

I'm going to patch it all back together and call it done until some new insight comes up.

[bandersen]

Does anyone out there have a restored TeleTone TV-149? The chassis is nearly identical. I'm curious if it exhibits the same behavior.

[bandersen]

Answered my own question courtesy of YouTube. There are several videos of restored models working. Should have though to search sooner.

Anyway, the images looked perfect. No 60Hz ripple/distortion.

https://youtu.be/7luj-3lnYmg

[Yamamaya42]

Defiantly a head scratchier.

Could this be a case of outside foil vs non outside?
Once stuffed in the old tubes, it's hard to tell which end is which of the new caps that are in them, that's even if side even matters any more with the new ones, I have seen debate.

ALSO, could the electrolytic capacitors that were stuck in the aluminum shell of the old be causing a problem with the ends like the can not being firmly connected?

From what I saw, you just drilled a hole for the negative and stuck the lead through.
I would not expect that to be a problem, but at this point, you never know.

[bandersen]

The negative leads went through the holes and were firmly soldered to lugs that are twisted and soldered to the chassis.

That aside, I've probed the supply rails with a scope and there is now less than one volt off ripple on them all. I also added extra filter caps around the chassis. No difference.

I also recheck the sawtooth deflection waveforms this afternoon and they are clean. No ripple.

That leaves two possibilities. One, magnetic. Two, ripple on some other element in the CRT. Like the HV supply, cathode or grid.

I'm focusing on the HV supply right now which is weird to put it mildly. It is also grounded in a rather crude manner with screws on a painted surface. I'm going to tighten them all down and do some more probing.

[jr_tech]

Perhaps try running the entire heater string from an external, well filtered DC power supply

jr