Meck XB-702 restoration

[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

[Notimetolooz]

Originally Posted by bandersen

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.

Oh, I must be thinking of something else. I had a thought that if it did go away when you moved the CRT away that just maybe it was the sweep on the CRT plates that was appearing on the sync signal on the scope and the 60Hz sweep interference on the sync wasn't really the problem. However since you can lock on the line 60Hz the interference on the sync signal does come from that source.

[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?!?

[bandersen]

I shorted out the 50 ohm resistor and saw little difference in performance.

[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.

[bandersen]

But why have it at all? It's hacked in near the HV coupling caps like an afterthought.

[Kevin Kuehn]

Maybe it helped tame that atrocious audio hum.

[Electronic M]

If it wasn't "tacked in like an afterthought" I'd guess maybe it was a means of extending tube life assuming they weren't asking for full tilt emissions from them in the biasing design.

Barring that there's 3 thoughts that come to mind. IIRC they hadn't invented the GTB suffix tubes (controlled synchronized series warmup heaters) until the 50s... Lacking those they may have had issues with heaters on different tubes warming up at different rates and one tube getting 2-3x rated heater voltage on warmup and failing prematurely, maybe during warmup an OSC wasn't biasing an output or temporary heater voltage imbalance was causing frequent HK shorts.
It's also possible they wanted the RF to warm up first so there'd be picture and sound as soon as there was light.

But with that afterthought installation it almost HAD to be a quick fix for a post design or field issue. Any maker back then would say that you don't just tack in an expensive power resistor for sh!ts and giggles.

[Notimetolooz]

That resistor would also reduce the filament current through the 7JP4 and the 6X5.

[bandersen]

Yes, it would. That whole filament string would be running about 10% lean. The 7JP4 and 6X5 about 5% lean.

[Yamamaya42]

IIRC, I seem to remember seeing a complete working Sencore TVA92 on u-tube in the hands of Shango066, not 100% sure.
If he does have one with cable, getting him to assist with info about it is another matter. :/

[vortalexfan]

I don't know if this will help or not, but I think your TV is actually an XA-701, which is located in Sam's Set 61 Folder 16, maybe this might help you track down your issues easier if you have the correct service data for your TV.

There was a few different versions of this set cabinet wise.

See picture below, Picture Credit to the Early Television Foundation.