Motorola 12T2 Restoration Restart

[RDusel]

WinkDink,
I have a parts chassis in my garage from a similar Motorola set with a 10BP4 CRT. If you will pay the shipping from NY it's yours.
Weather is getting warmer here so I could try and pull the yoke this weekend.
Drop me a PM if interested.
Rob

[Winky Dink]

The avatar was sad because he lost his Boise State Broncos lucky cap. He feels better now.
The reason that I started looking for a focus coil was that I read a post by you (Electronic M) in another thread referring to non-working vertical and horizontal yoke would result in a pinpoint rather than a line. When I stopped working on this set four years ago the only problem I had was the dead yoke--no continuity through either vert or horizontal. All the other components tested good, voltage data was good, resistance data was good, and the flyback was whining. Still, I didn't even have a pinpoint on the screen, so I thought maybe my repaired coil wasn't working.

The CRT was tested by a local TV repair shop four years ago. I don't have exactly total confidence in them, so I'm refurbishing a Beltron 8080A right now--hoping to do a reliable test if I can figure the thing out.

[Electronic M]

The focus coil will not necessarily cause the horizontal or vertical to not work. The focus coil in most sets is used as a B+ choke. So deflection should continue to work, unless it goes open and removes power from the correct B+ lines to kill the deflection circuit.

If the focus coil is radically out of spec or not receiving drive current it may be possible to have deflection and high voltage yet see nothing on the screen.

Sets with ion traps, and or focus coils tend to be finicky about the placement there of....If it is not all lined up right you can have HV but nothing on screen. You need a HV probe (a neon bulb could be used to for a crude indication in a pinch), and a test CRT. A test CRT can be used with just the yoke on it's neck to check if the chassis is working properly.

[Winky Dink]

As you can see, the original deflection circuit was a non-player.



Fortunately, before I changed any moveable part connected to the CRT I indexed the position on the chassis or on the CRT and took many photos. In the original configuration it was working properly (I was watching the set in 1969 when the picture went away) and it hadn’t been touched since.

These are photos from 5 years ago.
The CRT “works:”



There is high voltage:



I’ll have to look into fabricating a high voltage probe.

I would like to run the CRT test with the Beltron. I have no other CRT to play with at this time. The Beltron is putting out 15 filament volts with no load. I think that, if I hook up the CRT, the worst thing that can happen is failure of the voltage rheostat which would put 15 volts across the 6.3V tube. Is 15 filament volts likely to damage the CRT? If not, then I’ll try the test and then start reassembling the set.

[Electronic M]

15V on a 6.3V CRT is not a safe voltage.
The CRT would likely load the tester down to the correct specs, but to be safe here is what I'd do. Google the number of your CRT and add 'tube data' to the search string. You should find a data sheet. That sheet will give you heater voltage (likely 6.3Vac) and current (likely 600mA or 450mA). Dig around in in your signal tube stash for one you can live with ruining that also has the same heater ratings as the CRT and use it's heater as a dummy test load for the tester. If the tester puts out around 6.3V your good to test the CRT, but if it is over 9V there is some further tester repair to be done.

[jr_tech]

A 12LP4 draws 600 ma (0.6A) at 6.3V I would connect a 0.6A load to the heater pins and measure the voltage... do not apply 15V to the heater if the tube!

jr

PS: A 10 ohm resistor 5W should serve well
Edit add: I see that E-M had a good idea about using a small tube as a load

[Winky Dink]

Thanks. I can do the suggested resistor load easily.
But (I don't know the jargon) what/which are "signal tubes?"

[Electronic M]

Originally Posted by Winky Dink

Thanks. I can do the suggested resistor load easily.
But (I don't know the jargon) what/which are "signal tubes?"

Anything that ain't a CRT. Find a tube with the first digit of it's ID number being a 6. Look up it's data on the net, and if it is a 600mA heater you can use it's heater as a dummy heater load.

[Winky Dink]

OK. Used a 6.3V tube for the load, and the 8080A performed like it's supposed to perform.

Ran the "position 1" emissions test on the 12LP4. With 6.3 volts on the heater, the meter takes about 8 seconds to peak at .75 milliamps. Press the interrupt button and almost immediately the needle starts dropping and takes about 8 seconds to reach the neighborhood of zero milliamps. I repeated the test two more times with the same result.

By the criteria in the Beltron instructions, this would be a hopeless CRT--but I don't know if those criteria are relevant to a 65-year-old CRT. Video dudes, what do you think of this CRT?

[Electronic M]

What does .75mA correspond to on the emissions good bad scale?

The quick drop is a life test. A life test is supposed to predict how long into the future the tube will last in regular usage, but tells nothing about it's present condition. The emission test tells about it's present condition, but tells nothing about how long the tube has left.

A CRT can have a lousy life test and still be usable as long as you don't put a ton of hours on the set. Low emissions (assuming some still exists) can also be compensated for by adding a brightener or increasing HV. Also CRTs that have been dormant for over a decade often are asleep...As in the cathodes have been contaminated with gas, and the tube needs to be ran at 1.4X the heater voltage for 5 minutes-5 hours to boil off the gas contamination and restore normal emissions (if emissions are low run it at higher heater voltage for a while on the tester, and see if it improves, and be sure to let it sit at high heater for at least a good 10 minutes after the emissions improves).

A picture of the meter would be helpful to those of us with different tube testers with different scales.

[Winky Dink]

“Good” is .85 milliamps. B/W CRTs register on the green gun meter. I’ll follow the Beltron cleaning instructions first, just to see if the tests improve. In the meantime, I’m reassembling the set.

Thanks.

[old_coot88]

The initial reading you got would equate to the green/'good' indication on many testers (as opposed to the yellow or red zones indicating 'borderline' or 'weak' emission).

Please wait and see the raster before thinking of zapping the tube (assuming you haven't zapped it already).

[bandersen]

Those test results sounds pretty good to me. No way I would try doing anything to it.

[Electronic M]

I'd get the chassis working right, and then maybe consider running the heaters high for a few minutes if it was too dim, but you can likely get good use out of it.

I've gotten good results from tubes that test squarely in the bad range (at about 1/4 full scale deflection) with a good chassis adjusted to drive the tube as best they could....It may not be 'watch it with direct sun shining on the screen' bright, but in subdued lighting it should be capable of producing a good picture (assuming all the drive circuitry, and magnetics in the set are working right).

[Winky Dink]

Actually, I just sat down to read the cleaning instructions, then thought, "Gee, I should check the forum to see if someone advises me to hold off until I have the set operating." I'll wait. Thanks.