View Full Version : Help wanted resurrecting 1969 Zenith 12A10C15


Freon
10-07-2017, 04:44 PM
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 (https://imgur.com/a/IwWBU) 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.

MadMan
10-07-2017, 06:24 PM
Wow look at that thing. Built like a brick shithouse.

WISCOJIM
10-07-2017, 06:32 PM
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.

.

tom.j.fla
10-07-2017, 07:29 PM
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

Freon
10-07-2017, 07:39 PM
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.

.

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.

old_tv_nut
10-07-2017, 09:14 PM
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.

old_tv_nut
10-07-2017, 09:29 PM
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.

Freon
10-07-2017, 09:52 PM
I cleaned out the HV cage a little (https://i.imgur.com/16CQpDL.jpg). 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.

old_tv_nut
10-07-2017, 10:32 PM
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.

Electronic M
10-07-2017, 10:56 PM
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.
https://farm5.staticflickr.com/4457/37561644211_74d06dc15a_z.jpg (https://flic.kr/p/Zecfcn) https://farm5.staticflickr.com/4445/37561639501_7b08126d89_z.jpg (https://flic.kr/p/ZecdNa) https://farm5.staticflickr.com/4507/37303048620_d24bf6fa59_z.jpg (https://flic.kr/p/YQkSJG) https://farm5.staticflickr.com/4504/37303043740_a89db76f8b_z.jpg (https://flic.kr/p/YQkRhy) https://farm5.staticflickr.com/4496/37303038890_cc8e67b834_z.jpg (https://flic.kr/p/YQkPQW)DSCN2718 (https://flic.kr/p/YQkPQW) by Tom Carlson (https://www.flickr.com/photos/137849736@N08/), on Flickr

Freon
10-07-2017, 10:58 PM
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... (https://i.imgur.com/EXkUzhL.jpg) Quite an improvement, no? I decided to get rid of the tape, I could always put some back on if I have to.


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?

MadMan
10-07-2017, 11:22 PM
tfw random transistor (https://i.imgur.com/9dxrr54.jpg) :S

Electronic M
10-08-2017, 01:58 AM
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 :smoke:) 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.

zeno
10-08-2017, 10:40 AM
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:smoke:
LFOD !

Kevin Kuehn
10-08-2017, 10:53 AM
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.

Freon
10-08-2017, 10:56 AM
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:smoke:
LFOD !

So, just to make sure, I should be able to pull out the tube without unscrewing anything?

Also, any special procedure for removing the cap thing on top of the tube? Is it attached by metal hooks like the anode cap or do I just pop it off?

Electronic M
10-08-2017, 11:52 AM
HV rect Tube top cap should just lift off.

Tube should just pull out of the socket...If it is very stubborn try rocking it in a circular motion while pulling up with low to moderate force.

Freon
10-10-2017, 03:16 PM
I'm finally gonna be able to take that cup out and clean it, then I'll work up the courage to turn it on with the cover off and see what happens. I held off because I couldn't find an alligator clip lead to discharge the CRT, so I had to order a new one. Better than dying.

Since the original power cord is attached to the cabinet and I don't want to rip it out, can I use a standard two-prong power cord, you know, this one (https://i5.walmartimages.com/asr/e9ae87ea-c182-41ea-9f92-d0200865b362_1.3e0558cda04421f750b9e93fdd962931.jp eg)? It does fit, but I can only assume.

old_tv_nut
10-10-2017, 07:38 PM
You can use the new cord provided it fits the pins properly and isn't an intermittent connection. On the other hand, the original cord is usually attached to the back in such a way that you can remove it and use it as a "cheater" cord (without the back). "Cheater" refers to defeating the safety aspect of forcing the the power to be disconnected when you remove the back. Important for the average customer, but just a nuisance for the service tech who knows what they're doing.

Freon
10-10-2017, 07:44 PM
Well, Electronic M and Zeno were right on the money. The cup's got two holes on the bottom of it (but just one in practice as you'll see), and someone obviously attempted a repair.
More electrical tape under cup (https://i.imgur.com/TU0dwvH.jpg)
One of the holes filled with black goop (https://i.imgur.com/aF4dg7r.jpg)
Bottom of cup (https://i.imgur.com/X2Enesn.jpg)
Not only that, but one of the screws holding the cup to chassis is missing.
Doesn't seem like something a tech would do. I'd almost guess an enthusiast tinkered with this one not too long ago, and perhaps decided it was beyond saving... Or, more than likely, the second hole is fresh and unrepaired.
Looks like I'm gonna have to figure out how to use silicone... Or anywhere I can just buy a new one...?

Freon
10-10-2017, 08:13 PM
Oh crap. I just broke in half (https://i.imgur.com/Djw5re6.jpg) the little rubber spacer where the anode cap for the horiz. output tube comes out of the HV cage... Gonna guess that was for grounding the wire? :( I was gonna take off the cage and clean around the area a little more, I dunno what I was thinking! I thought the two halves would snap back together, but nope. Maybe just superglue will fix it, but wouldn't it burn up if it decided to arc here?

I hope I'm freaking out over nothing but... there's so many uncertainties with these sets, especially when you're a beginner...

RetroHacker
10-11-2017, 08:07 AM
It's just a little rubber grommet to keep the wire from rubbing against the metal. It's not going to arc without it. Those things always break and deteriorate from age. Don't worry about it - the insulation on the wire is what's really important, that thing was just there to protect the insulation. When you do get it all back together, you can wrap something else around the wire - like a cut scrap of insulation from larger wire - to protect that rubbing point and tuck it in there and secure with zip ties.

-Ian

Electronic M
10-11-2017, 12:00 PM
Some hardware stores stock equivalent grommets...If you wish you can buy one unsolder the lead and thread it through the new one then resolder.

Those burn holes on the bottom: best to drill them out till there is no black (conductive carbon tracking) left, then fill the hole with RTV silicone. (you can buy a small tube that does not require a caulk gun at an auto parts store.) Make sure the patch is flush with the underside bottom of the cup so it will mount properly to the chassis. It does not matter if the patch domes up inside the cup. Let the silicone cure before applying power again.

Freon
10-11-2017, 02:05 PM
Alright, good to know... It turned out the HV cage was held to the bottom by an bent metal tab that wouldn't budge, so I gave up. Didn't look like much to clean other than the flyback plastic case which still has some gunk on it.

On the other hand, I DID find and purchase a NOS replacement cup (Workman HVS-14) which ended up being cheaper than buying a drill + silicone. And I'll get one more screw to hold the cup in at ACE Hardware, assuming the NOS cup doesn't come with screws.
Almost done, then! Let's hope this will be the fix...

Freon
10-14-2017, 04:07 PM
HAHAHAHHAHA! YEEESS!!! IT'S ALIIIIIIIIVE!!!!!

https://i.imgur.com/0FVYXkYl.jpg


Album link! (https://imgur.com/a/zhJhX)

AAAAAAaahhhh I'm so freaking overjoyed! I can't believe it! Thank you, THANK YOU Videokarma!!!! One more American crafted master work of engineering saved from destruction!
All tubes seem to be lighting up, even that damaged looking 6U10.

Now, a quick glance of those pics will tell you there's still work to be done. Most obviously, it's incredibly blurry. (I know this is a low voltage focus set, but it's really an abnormal level.) There's a slight purple tint (that clears up a bit the longer it's on) and retrace lines are visible. A TON of pot adjustments are in order. I'll take the opportunity to ask, what is the general procedure for re-adjusting a set like this? What should I do first and last? Will the SAMS Photofact 1067-2 (which I can't find anywhere) be of any help here?

Here's a quick quiet video (https://photos.google.com/share/AF1QipMiosPuTxPBaEbSptEQx_10PKg3zszS3CwZzzYXTc0DDH e6vpDND3D4PpH58V-Qag?key=TW9wQ0RXMzV3ZjZPUkhmRFg5ZUVVYXo1S2FhaGN3) of my baby in action. The screen has a tendency to wobble like Jello, and there is a slow horizontal wave of sorts moving from the bottom to top. That could be almost anything, but aged capacitors are likely. I am using my Sega Genesis as a makeshift pattern generator with a custom ROM, by the way.

Quick update: I'm already seeing picture improvement just by leaving the set on. No more Jello wobble. The retrace lines are coming in and out and it's getting gradually sharper. Mr. Zenith appears to still be waking up from his long slumber.

Electronic M
10-14-2017, 04:50 PM
First off reduce brightness till the blacks are not lighting the screen or are to the minimum amount perceptible. The dimmer the pic settings the longer the CRT eill live and often the better the focus. Second adjust the fine tuning knob (outer ring of the VHF channel knob for best compromise of detail, sound and color). Some sets you need to push in and turn the knob to engage the gearing....The fine tune should go from static to monochrome pic to normal pic to pic with color squigelies back to statis through it's range...If you can't hit static in both directions then something is wrong. lack of correct fine tune can look like a focus issue...

If it is a focus issue look at the focus adjustments available and see if changing them will have an effect. IIRC this one adjusts focus by swapping which terminal the focus wire goes to. Focus can change with HV level if good focus not achievable, suspect wrong HV setting.


The set may benefit from Grayscale, purity and convergence adjustments. (all searchable here, and should be covered in sam's too). Get focus, sweep size, centering and linearity right before messing with purity or convergence (they interact).

Freon
10-14-2017, 05:29 PM
Fine tuning works as intended. I got rid of the purple tint by turning up green G2.

I cannot get it to show total blackness, i.e. the picture is washed out and hence the retrace lines. I probably need to bring the master G2 down. Which, on this set, is just the HV level control, right? The picture is MUCH sharper during the first few seconds of screen on time, then blurrifies shortly after, so I think it's HV related. Too high? I wish I had a high-voltage probe, but I will have to eyeball it...

Electronic M
10-14-2017, 05:32 PM
The master HV control has nothing to do with the G2...If you don't have an HV probe don't touch it.

There is a brightness range pot wired to the brightness pot that you may wish to adjust.
The greyscale adjustment may also be worth doing (and a better move) to shift brightness level/remove tint from monochrome picture. To do greyscale with no signal input, turn brightness to min, flip service switch to service position (vertical will collapse), turn all G2 controls to min quickly (screen should be totally dark*), turn red up until line just becomes visible increase green till the two mix in proportion to give pure yellow, turn blue up to get pure white, flip service switch to 'normal' turn brightness up for normal picture, with monochrome image check that blacks are not tinted and whites are not tinted, if black is tinted start again, if black is good but white is tinted tweak individual colors of video drive controls, and recheck. When monochrome is not tinted your done with greyscale.

*If one or more guns won't extinguish turn sub bright and or video drive for that color down till it will at min G2 setting.


If only a portion of the screen is tinted on what should be an all white screen, or an all red, all blue or all green screen your sets purity is off and or the shadow mask is magnetized. Demagnetize (degauss) it and preform the purity adjustment proceedure.

Freon
10-14-2017, 05:34 PM
Got it. That could've been bad. Well other than that I don't see any master G2 control, only the individual color ones.

Electronic M
10-14-2017, 05:50 PM
I edited above.

zeno
10-14-2017, 08:02 PM
Very nice work Mr. Freon !
BTW I sure miss R12 & R22 !

Tom C. gives good advice. I will look for the Sams & PM you.
The bend going through the picture is classic bad filter caps.
IIRC these use a voltage doubler & one bar is 60 cycle getting
on the sets B+ line. When it hits the top the set tries to sync
the vert ( also 60 C ) & makes it roll. Sets with full wave bridges
will show 2 bars. Bottom line is it will probably come back.

Lastly keep in mind these sets are NOT bright or super sharp
like newer ones. After its properly set up it should look very
good.

73 Zeno:smoke:
LFOD !

Freon
10-15-2017, 05:00 PM
Well I spent all of last night messing with the back pots and I've gotten it as best as I possibly can with what I currently know. See here (https://imgur.com/a/X20Wg). Greyscale is good, but overall picture is still washed out and everything has this weird bright halo to it. Sharpness still varies quite a bit, I caught these pics when it was on the very low end. Typically it looks good on power on and gets gradually worse. I've found the only way to get a correct contrast image without overbright is to turn every color G2 to its minimum, and user brightness to maximum. But this produces a lot of black crush.

I still believe there is a master G2 somewhere, and it is WAY too high. But I can't find it anywhere, or the focus terminal for that matter, and hoping at this point that someone else will know where they are..

Bill R
10-15-2017, 06:44 PM
Master G2 is often labeled CRT Bias. Run it as low as possible.

Electronic M
10-15-2017, 06:46 PM
I saw your PM and am working on the schematic. My box is close to PM capacity and I'd prefer to limit it's use to transactions and confidential info. All you gotta do is ask here and I'll post better pics as soon as I can. [edit: here are the pics]
https://farm5.staticflickr.com/4493/37011714264_a5b411ce95_z.jpg (https://flic.kr/p/YoAHby) https://farm5.staticflickr.com/4463/37051116613_c5dafd42d5_z.jpg (https://flic.kr/p/Ys5E8p) https://farm5.staticflickr.com/4443/37011704444_7b572e4cc7_z.jpg (https://flic.kr/p/YoAEgf) https://farm5.staticflickr.com/4512/37011718794_bd744d3a00_z.jpg (https://flic.kr/p/YoAJwE)
In the top right hand corner of my third pic you can see the complete G2/focus control circuit. Each color gun has it's own G2 pot the pots connect to the 270V B+ rail and the 720V B+ Boost rail there is no master G2 or anything that would create that effect electrically. The focus is controlled by connecting the black wire from pin 9 of the CRT to one of three points 720V B+ Boost (through R206 for safety), 270V B+ (through R226 also for safety), or directly to chassis ground...You try each connection for focus and leave it set to the one that looks best....You could add a 5M ohm pot between any of the two taps and connect the focus grid of the CRT to the pot's wiper.

I have over 20 deltagun color TVs and have had many more. I can tell you that almost* no deltagun CRT based set ever had a master G2, and yours definitely does not. Master G2 controls did not become standard until inline-gun tubes took over.

* I have a late 60's Admiral that has only a single (master) G2 control...The rest of it's grey scale control system is completely idiotic and ridiculously hard to set up correctly.

Bill R
10-15-2017, 06:47 PM
Have you tested the CRT? The blue looks like possible low emission. Focus is deffinately off. You will need to get a HV probe to measure high voltage at the CRT. It looks low. That could cause the focus problem as well.

Electronic M
10-15-2017, 07:12 PM
Have you tested the CRT? The blue looks like possible low emission. Focus is deffinately off. You will need to get a HV probe to measure high voltage at the CRT. It looks low. That could cause the focus problem as well.

Yes, If the HV is low it can ruin the focus. If the CRT has good enough emission HV can be VERY low and reasonable normal brightness can still be achieved at the cost of terrible focus.

zeno
10-17-2017, 11:50 AM
There is a brightness range on rear near the power
transformer. When you are done you should be able to
turn the brite, contrast & color level all the way DOWN
and not & not get a pix. Also should have no thin white lines.
Recommend you also test the HV.

73 Zeno:smoke:
LFOD !

Freon
10-21-2017, 04:32 PM
Belated announcement time: As I don't have the tools or experience to move on with restoring this set, and I am ridiculously short on space as mentioned before, I have decided to sell this set once again. It was a tough choice to make, in part because I didn't want to disappoint y'all by giving up right after raising this set from the dead. But, I've got a Sylvania Superset doing me service with no sign of failing anytime soon, and I have no need for another CRT right now.

So, new sale thread here (http://videokarma.org/showthread.php?p=3191097).