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  #1  
Old 10-07-2017, 04:44 PM
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Help wanted resurrecting 1969 Zenith 12A10C15

Long story short, this Thursday I rescued a 1969 Zenith A3520, chassis 12A10C15, from my college town's electronics recycling drop-off point. A knob-tuned Zenith color set was my Holy Grail of CRTs and my long search for one was finally over. I assumed it was a mid-70s set, probably solid-state.

I got it home, realized what I had was a genuine tube set, what appears to be an early Chromacolor, right when they started using plastic cabinets. I had class soon so in the nick of time I foolishly did a test power-up. Things actually looked pretty promising, I saw the orange glow of tubes, and in 20 seconds the CRT began to light up white and was suddenly halted by a loud screeching noise of varying pitch, which turned out to be arcing somewhere on or below the bottom middle of the CRT body. Some split-second sputters of snow-like lines blinked on the screen as the screeching continued. After 25 seconds with no signs of improvement I figured enough was enough and I didn't want to cause more damage than I potentially already have. I shut off the power, the screen blinked out like it's supposed to. And no, I'll NEVER do something that stupid ever again. I'm very sorry.

I decided this set would be better off going to a tube TV enthusiast with more experience and equipment so I listed it on the classifieds of this forum. I knew about this place from some months of lurking and had made an account prior. I actually got enough encouragement from other members that I decided to change my stance. I'm definitely going to need your help here. I'm learning everything here as I go, and I'm slowly starting to get aquainted to tube equipment. Videos from Shango066 and Mr. Carlson's Lab on YouTube, in addition to reading through repair threads on this very forum, have been a big help.

Yesterday, I figured out how to get the set apart and gave everything a much needed dusting on the inside. Here is a photo album with more info, remarks, including where I believe I saw the arcing.

What I can tell by now is that the CRT is probably okay, and so is the horizontal and vertical deflection as I saw a picture fill the entire screen. I still haven't nailed down the cause of the arcing; that is my primary goal.

Someone on the classifieds thread mentioned that the focus voltage divider may be arcing. I have no idea where that is.

Last edited by Freon; 10-07-2017 at 07:50 PM.
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Old 10-07-2017, 06:24 PM
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Wow look at that thing. Built like a brick shithouse.
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Old 10-07-2017, 06:32 PM
WISCOJIM WISCOJIM is offline
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That black electrical tape on the inside of the HV cage door should give you an idea of where someone in the past may have experienced some arcing.

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Old 10-07-2017, 07:29 PM
tom.j.fla tom.j.fla is offline
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What i see are some very crunchy high voltage filament wire. Tape is at the same place as the burnt spots on the filament wire, since there is H.V. on that wire you will have arching. Just my two cents. All the best, Tom.J
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Old 10-07-2017, 07:39 PM
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Quote:
Originally Posted by WISCOJIM View Post
That black electrical tape on the inside of the HV cage door should give you an idea of where someone in the past may have experienced some arcing.

.
Quote:
Originally Posted by tom.j.fla View Post
What i see are some very crunchy high voltage filament wire. Tape is at the same place as the burnt spots on the filament wire, since there is H.V. on that wire you will have arching. Just my two cents. All the best, Tom.J
So it looks like the HV rectifier could be our culprit? It seems to be screwed on so I can't really get a good look at it. I barely even noticed that tape - I just figured it was put there in the factory or something...

I'm just wondering if it would be safe to blow out all that dust in the cage. Maybe THAT is causing the arcing? Just being cautious, I don't want to screw up again.

The fact I saw it so far away is still weird though... I distinctly recall it started and stopped along with the screeching. It couldn't have been anything else.

Last edited by Freon; 10-07-2017 at 07:48 PM.
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Old 10-07-2017, 09:14 PM
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Always good to get the dust out.

If it has arced before, it may have developed a carbon track someplace that keeps it arcing. Investigate the crunchy looking wire for sure.

It may be arcing in more than one place.

Arcing somewhere on the CRT could be a bad / dirty ground contact, which may be the purpose of that spring.
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Old 10-07-2017, 09:29 PM
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The HV rectifier is screwed on the chassis, not the rectifier tube itself. There could be some kind of spring clip in the socket cup that holds the tube and has to be pressed down to release the tube.

See if all that black around the cup wipes off, or if the cup is deteriorated (carbonized).

I can't identify a focus block in your pictures. If it has one, I would expect it to be wired to the high voltage rectifier and look like a brick of plastic.

Not familiar with this chassis, so take anything I say with a grain of salt. Others here can probably get much more specific.
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Old 10-07-2017, 09:52 PM
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I cleaned out the HV cage a little. The crunchy stuff on the wire was just dust, thankfully.

Which part is the cup? I'm guessing the thing surrounding the tube? In any case, I'm able to scrape that brownish stuff off both big plastic pieces. You can see where I scraped off in the picture.
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Old 10-07-2017, 10:32 PM
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Yes, the cup is the white plastic, well, cup that the tube is in. Don't be afraid to wash things, with a damp rag and dish soap for example. Rinse with a damp cloth and air dry or use a hair dryer.
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Old 10-07-2017, 10:56 PM
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Rubbing alcohol or Goof Off is best for cleaning HV cages. There are thin wires exiting the side of the flyback be careful not to disturb them too much.

One thing I often to do is remove the HV rect insulator cup and clean the bleep out of it...You clean it by discharging the HV (lots of searchable info here on that), removing the HV rect tube, unscrewing it's socket from the insulator and unscrewing the insulator from the chassis. Clean that insulator spotless, and if there are any black spots that look like they go through they will need special treatment like on the previously mentioned similar Zenith I serviced.

Once you've cleaned or perhaps before you clean it is wise to run the set briefly to find the exact spot it is arcing, and or better localize it so you can fix that easier/better.

Here is the schematic. Pics are a bit blury so if you need to see an area I can take a closeup. There is NO focus divider off the HV in this set. It looks to be a low voltage focus tube the highest focus potential is the 720V boost line (which is low compared to 3-4KV focus tubes found in consoles of the time).
click on pics, then click flickr's full size button to get bigger versions.
DSCN2718 by Tom Carlson, on Flickr
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Old 10-07-2017, 10:58 PM
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Quote:
Originally Posted by old_tv_nut View Post
Yes, the cup is the white plastic, well, cup that the tube is in. Don't be afraid to wash things, with a damp rag and dish soap for example. Rinse with a damp cloth and air dry or use a hair dryer.
Well, if you say so... Quite an improvement, no? I decided to get rid of the tape, I could always put some back on if I have to.

Quote:
Originally Posted by Electronic M View Post
Once you've cleaned or perhaps before you clean it is wise to run the set briefly to find the exact spot it is arcing, and or better localize it so you can fix that easier/better.
Do you suggest I try running the set with the cabinet off and maybe even the HV cage open? Definitely too afraid to do that, especially on a carpet floor. I could clear up a table for it, though.

Does anyone know if that 6LF6 I mentioned will be alright? I hope so, it's an expensive replacement. Speaking of tubes, the 6U10 looks like it is barely holding on... looks like almost all of the getter coating is burnt off. Thankfully THAT's a dirt cheap replacement. I will check the 3DB3/3DC3 tomorrow. It won't budge so Im guessing I will have to unscrew the cup first?

Last edited by Freon; 10-07-2017 at 11:14 PM.
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Old 10-07-2017, 11:22 PM
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tfw random transistor :S
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Old 10-08-2017, 01:58 AM
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Quote:
Originally Posted by Freon View Post
Do you suggest I try running the set with the cabinet off and maybe even the HV cage open? Definitely too afraid to do that, especially on a carpet floor. I could clear up a table for it, though.

Does anyone know if that 6LF6 I mentioned will be alright? I hope so, it's an expensive replacement. Speaking of tubes, the 6U10 looks like it is barely holding on... looks like almost all of the getter coating is burnt off. Thankfully THAT's a dirt cheap replacement. I will check the 3DB3/3DC3 tomorrow. It won't budge so Im guessing I will have to unscrew the cup first?
That is exactly what I suggest. Once you loose count of the times HV has bit you (as I have ) you'll still avoid it and respect it, but the fear will fade away.

Unless the plate (large outer element) starts to glow in the dark or the tube arcs internally, if you still have healthy amounts of HV and decent raster width the tube is fine. It can handle the extra load of HV arcing reasonably well...The set that I had to drill out a carbon track from the HV cup on was arcing so bad the raster would disappear from the HV loading down, but the H output tube was fine....Even if your set was worse under 30 seconds of active arcing should not hurt it appreciably....When the going gets tough transistors die instantly, but tubes begrudgingly muddle on for a few minutes before biting it.

Madman: welcome to the world of late 60's-mid 70's hybrid TV chassis. Ever work on a set where there are ~12 tubes about as many discrete transistors, and a DIP IC....All chassis mounted?...I have. The makers had some STRANGE component combos in that transitional era. I know of an instant on knob tuned remote controlled set with tube based sweep and an OSD generated by a large DIP chip. Part of the fun of working on the later sets that the big money collectors scoff at is that you get to see some unique things that you'd never expect to see paired up.
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Old 10-08-2017, 10:40 AM
zeno zeno is offline
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The HV cups were high failure rate on these & the 16"
cousin. Would get pin holes. Also filament windings would
arc. Fire it up with the door opened to see where its arcing.

Disassembly:
First discharge the HV several times. If the tube wont
budge it may be rusted in. Remove the separate plastic
part of the socket ( pull up) then the 2 screws holding in the socket
then the cup. Clean up & fix the arc.
Remember that all the HV & wiring is sitting as apx 22KV so
any rewire needs high breakdown wire.

73 Zeno
LFOD !
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Old 10-08-2017, 10:53 AM
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Kevin Kuehn Kevin Kuehn is offline
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That's a really interesting TV. I believe the tubes remained so late in the game because Zenith engineers felt they were still considered the most reliable components for their specific circuit functions.
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