Motorola 12T2 Restoration Restart
 

This Motorola 12T2 is my family's first television set. I spent many hours sitting in front of this set, learning about life from Howdy Doody and Captain Video.

https://lh6.googleusercontent.com/-i...scued%2521.jpg

A few years ago, starting at electronic ground zero, this was my first project. I had a pair of dull wire cutters that could only crush wire; an analog volt-ohm meter that didn't quite work; a soldering iron that couldn't melt butter; and a lot of guidance from Phil Nelson. After several months of work, I was stopped by a bad deflection yoke.

Now, with a little more experience and a great deal more equipment, I've finally picked up a replacement yoke and I want to restart the project. The first issue is, "Do I have the right yoke?" The only difference I can see is the resistance across the coils.

https://lh5.googleusercontent.com/-w...%2520Yokes.jpg

Can I go ahead with this yoke?

According to my Thordarson replacement guide Y18 does appear to be the correct sub. And I don't see a problem with the DC resistance being off a bit. I think impedance is the more critical value, and I'm sure Thordarson took that into consideration. Looks like your TV was taken very good care of over the years.

Thanks, Kevin. Electromagnetic fields are still a mystery to me. I've done a bit of studying, but I just can't buy this "electron theory."

I've subbed yokes of different makes on similar tubes blindly, and had it work.
If more than one cross reference says it's good, then as long as the replacement is not a dud you have nothing to worry about.

TRUST me you do not want to learn Maxwell's equations (which completely describe/explain electromagnetic and electrostatic fields). The calculus involved in properly understanding/applying them will fry your brain!

As long as your new yoke is a 52 degree deflection, it should be fine. Thordarson always made the best replacement parts. :thmbsp:

I always though calculus was a fancy name for an educated guess. ;)

Yeah, that pretty much sums up how I do calculus...

Cross Reference for Yet Another Part?
 

Focus coil for this Motorola 12T2, chassis T-53
The coil was open. I cut open the case and repaired 3 breaks, got 700 ohms through the coil, and rewound it. I’d like to find a good NOS or used part in case this one doesn‘t work.

https://lh5.googleusercontent.com/--...%2520break.jpg

I only have Motorola part numbers:
Original part is 24B792581 I.T.E
Sams (part T6), same number 24B792581 (650 ohms)
Rider (part L24) number is 24K700655 (670 ohms)

Can someone help me with a cross reference for this item?

Still hoping someone has the time to look up other part numbers for this focus coil:
original part 24B792581 I.T.E
or 24K700655 (from Riders)
Thanks

Unfortunately my Thordarson reference doesn't cross focus coils. And I think Thordarson only ever made 4 types. Probably because manufactures quickly transitioned to permanent magnet focus.

Thanks for trying.

Have you tried contacting Moyer's with the part numbers you have and see if they can provide cross references

http://www.moyerelectronics.com/Depa...-TV-Parts.aspx

Meanwhile, if you rewound the coil with the same gauge of wire and same number of turns, is there a downside to trying this one out?

Phil "ever the optimist" Nelson

I did contact Moyer. They said that none of the replacement companies list it and they never had original Motorola parts.

Phil, I repaired several breaks, then rewound the original wire. The resistance of the repaired coil (700 ohms) is close enough to the original spec (670 ohms), so I will try it. But if I had a known-good coil, then I'd use that instead.

Thanks.

I'd guess that unless you reused the old wire, and scraped enough of the insulation off to cause shorts that it will work fine. Rewinding simple reasonably thick wire coils like that is usually a successful process.

BTW, is it just me or does your avatar look sadder recently?

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

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.

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.

Trying to deal with multiple suggestions.
 

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

https://lh4.googleusercontent.com/-R...d%2520Yoke.jpg

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:”

https://lh4.googleusercontent.com/-b...%2520aglow.jpg

There is high voltage:

https://lh6.googleusercontent.com/-k...0Kilovolts.jpg

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.

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.

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

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.

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?

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.

Picture of the Meter(s)
 

https://lh3.googleusercontent.com/-T...0A%2520405.jpg

“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.

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).

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

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).

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.

Update & Wire Question
 

I redid almost everything I had finished four years ago. Basically I tested and confirmed values of components and made everything cleaner and neater. I did find a couple of cold solder joints, one cap of wrong value, and replaced several more of the original resistors. It took several days just to redo the electrolytics, replacing three separate bundles of capacitors with this setup, which fits nicely where the selenium rectifiers used to be:

https://lh3.googleusercontent.com/-H...2520Redone.jpg

One of the things I hadn’t noticed previously is the need to replace three shielded cables. Since I started this project, the insulation of the conductor has crumbled to dust. The photos below show the condition of the cable five years ago--and the condition today below that.

The gray-insulated copper cable in the lower photo is what I hope to use. I don’t know what gauge the conductor is, but it is seven-stranded copper like the original. Can you see any problem with using this shielded audio/video cable to replace the original?


https://lh3.googleusercontent.com/-B...lace%25202.jpg

Next Problem: Broken IF Coil Lead
 

As pictured, it looks like I'll have to remove the hardened tape to uncover the broken lead. The tape is quite hard and doesn't chip away easily.

https://lh3.googleusercontent.com/-Q...2%2520lead.JPG

Any advice on how to soften and remove the tape without damaging the coil wire insulation?

Thanks.

I'd try soaking it in water first, then maybe rubbing alcohol if the water don't work.

Thanks.
I'll give it a try.

Re your Beltron, that non-load filament reading will definitely throw you- I thought there was something wrong with mine when I first used it too, until I plugged in a CRT and the reading dropped.

I've never gone wrong running a clean cycle on a CRT with the Beltron, it's quite gentle. The machine itself is a miracle- it's brought back CRTs which were DEAD that my B&K or Sencore wouldn't touch. The restore function on these is purely manual though and can heat up a grid red hot in seconds if used improperly.

Blunder of the Month
 

{Re: Beltron 8080: I just got lucky with this purchase. Saw it in the classified section of a ham website a few years ago. I had no clue whether it was good or bad.}

BLUNDER:

I checked the resistance data and it was all good, so I gradually turned up the power to 117 volts. All the tubes were glowing nicely, so I started to go through the voltage data table. It seemed that I wasn’t getting any B+ voltage, so I leaned over the set to look at the tubes again and I noticed an awful lot of heat coming off of R97, a 7.5 ohm 5W resistor/fuse.

Yes, I killed R97.

https://lh3.googleusercontent.com/-O...istor-Fuse.JPG

It didn’t take long to figure out that I had miswired electrolytic C1. Instead of connecting C1 negative to R97, I connected it to the common ground with the other electrolytics. I replaced R97 with a normal 7.5 ohm 5W power resistor and starting checking voltages again.

Now all of the filament voltages are good, but the B voltages are way low. When I last worked on the set five years ago, all the voltages were good. The red numbers on the diagram are my current readings. So, what did I break?

https://lh3.googleusercontent.com/-o...ansformers.jpg

I’ve checked everything I know how to check. In all transformers my reading of DC resistance matches the specs (except I have no resistance data for T2, so I noted my readings on the diagram).

Link to full schematic:

https://picasaweb.google.com/lh/phot...eat=directlink

My knowledge only takes me as far as, “Maybe it’s T2?” Incidentally, the Stancor replacement, part P-6134, seems to be readily available.

What else should I be looking at?

Re: IF coil tape removal -- maybe try heating it gingerly with a hair dryer.

T2 is just the heater transformer for the 6W4. The tube is rated 6.3 volts at 1.2 amps. If you have 6.3 volts nominal with the tube lit, T2 should be fine. Are your diodes still good? M2 could have shorted to ground?

Re, tape removal: I tried hours of soaking with several solvents including isopropyl alcohol, Goo Gone, and Goof-Off. Finally did a four-hour soak with paint thinner which did nothing to the adhesive, but softened the tape fabric enough to scrape it off.

I replaced the diodes with new 1N4007's after I fried the resistor. There are no apparent shorts, but I'll keep looking.

Thanks.

What's the voltage at the top of C2?(should be the same as pin 1 of the spkr socket)

Dumb question, but is M2 installed with correct polarity (banded end to C2/M1 jct.)?

Thanks, guys. I wasn't finding any shorts. Following Old Coot's post I started checking voltages at the plus side of all the electrolytics. Some of the readings didn't make sense--AC instead of DC, meter unable to settle on any reading. The diode polarities were right, but I had the negative side of M2 connected to the C1 negative (which, as I previously learned, does not go to ground).

I straightened out the wiring, and as soon as I turned on the power I could hear the flyback. I checked just one voltage: V18 now has the appropriate 380volts versus the previous 200volts.

Thanks a million. I'll pick up on this later tonight.