Motorola 12T2 Restoration Restart

[Winky Dink]

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:



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?

[Winky Dink]

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.



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

Thanks.

[Electronic M]

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

[Winky Dink]

Thanks.
I'll give it a try.

[StellarTV]

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.

[Winky Dink]

{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.



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?



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?

[Reece]

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?

[Winky Dink]

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.

[old_coot88]

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.)?

[Winky Dink]

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.

[Winky Dink]

Since getting the B+ voltages back to normal, I ran the voltage chart again. V1 through V17 looked pretty good. But when I got to V18, pin 8, which previously showed the appropriate 380 volts, now overflows my 600-volt-max multimeter. I tried again with a 1000-max-volt multimeter, and that one overflows, too. In both meters, the resistance/capacitance sections are fried. But they still work for AC and DC voltage.

My first thought was to throw the set out the window. But when I calmed down, I rechecked every connection that I had recently worked on and over the next few days rechecked components, transformers, resistance everywhere, and I found nothing wrong. Nothing wrong except having over a kilovolt at V18.
resistance/capacitance section is dead. The second meter also acts funny in the resistance area.

Other observed changes are that I get no noise through the speaker and cannot hear the flyback anymore.



Here’s another observation: It’s not good to turn on the set when the HV rectifier cap is not connected.



I’m a bit frustrated, so I put a time limit on this project. If I don’t get a raster within the next ten years, than I’ll think about quitting.

Please, what in the world should I look for to resolve the too-high voltage issue.

[bandersen]

That's probably 380 volts DC but there are some big AC spikes throwing off your meter.

[old_coot88]

Surprizing the meters didn't release a gob of magic smoke. The high level spike on the the damper cathode is normal, the product of the tube's operation. The spike can be well over 1KV in a BW set, and 3-4 KV or more in a color set. Something to bookmark.
Never put a meter on any point that's directly connected to a flyback winding.

[Winky Dink]

Thanks. That's a really good thing to know. When I started working with this set five years ago I didn't have this problem--it just read 460 VDC. I would have thought that Sams would have noted this on the voltage data chart, but perhaps this wasn’t an issue with VTVMs of that era?

Does someone have time to advise me on two other issues?
1. Since I burned up the 7.5 ohm/5W resistor-fuse, should I add a 5W fuse in line with the 7.5ohm/5W replacement resistor?
2. When I discharge the CRT and electrolytics there seems to be no charge at all--no spark, no “pop!”
In previous years, I would get a healthy discharge. Should I look for a reason it’s not charging or for something that’s bleeding off the charge.

[Notimetolooz]

I'm not sure how you would calculate the CURRENT rating of a fuse that you could add to the 7.5 ohm resistor. About 0.8 A would produce 5 W in a 7.5 ohm resistor. But the fuse isn't meant to protect the resistor!

Sounds like either something is draining off the charge or the voltage isn't as high as it was or its you imagination. If whatever you are discharging with has a higher resistance, it won't pop or spark as much. It is actually better for the capacitors not to use a dead short, less of a current surge.

Do be careful of measuring the higher voltages. If your meter is only rated to 600 V, the probes may not be safe to hold above that.