On going Zenith 25MC33 roundy restoration
 

I got this set at the ETF meet last weekend and was working on it here and there during sanity breaks during finals last week. At the meet I and another collector saw the old orange drops in it and assumed it had been restored which turned out not to be the case.
http://i1095.photobucket.com/albums/...pscdc0e783.jpg
I powered it up and got good sound but no HV. Well still assuming good caps I replaced nearly all the hroizontal/HV section tubes which were mostly bad anyway. The focus rect. tube was missing which I later found was for a reason. This landed me no net improvement. I then pulled the chassis, tested and replaced several caps in the horizontal section. This got me 11-15KV of HV, and a shrunken raster with pull-in on the sides.

What caused that?, what else I had to fix?, and what is still not working?...All that in my next exciting post.
Gotta run.

Power supply, voltage doubler problem? :scratch2:

That's where I'd start looking too. Check the cans, I bet the doubler is bad. Probably a 160 or so at 400 or so volts, been a long time since I've worked on any of my Zenith sets.

Replace the doubler. Also, check all resistors in the HV regulator section. An open one can also cause low HV.

Definitely start with the power supply. i too have a 25MC33 Zenith roundie and the electrolytic caps in the power supply are old enough now to cause trouble. This set should easily be able to produce 25 KV of high voltage which is what it should be adjusted for. These sets are full of brown drops which I'm not sure if they should be considered for replacement. My set still has most of them and I suppose one day it will need a full recap. Mine is in regular service and the only issue I have is the vertical acts like it is going to fail sometimes. I have not figured out the cause yet. The picture quality on these sets is fantastic, though, well worth the effort of a good restoration. Follow all the setup procedures in the Sams after restoration and you will be amazed at the performance.

Just went throught that on my CTC10 160@450VDC went open. I'll put a buck on it.

Yup it was the doubler. One of the doubler cans was wide open I did not have a 160uF 250v on hand so I subbed a 250uf 250V in for the time being and it corrected the horizontal shrinkage.
I kind of suspected it since I had a NEW doubler cap in my Silvertone fail and the symptoms were the same. After replacing the doubler I brought it up on a variac with my typical over caution, and as I approached line voltage I started to hear BAD corona sounds from the HV cage. My over caution proved merited; there was perhaps the worst corona I've yet seen in any set coming off the HV line. It had to be over an inch long, that purple spark, and if I cranked the variac closer to line voltage it turned to an intermittent arc that caused the CRT neck to light in step.:yikes: My camera did not want to capture it, but if you can see the black spot in the red wire with a tiny purple dot in the middle where the red wire is in-front of the HV rect. plate cap that's is where the arcing was. http://i1095.photobucket.com/albums/...pscb9dbdae.jpg
Since the wire had arced through I decided it was best to just replace the wire and HV connector suction cup with one salvaged from an abandoned SS set I scrapped. I also cleaned the rectifier cup, and removed the odd blob of clear silicon that someone put on the interior of the cup for reasons beyond my comprehension.
The cup did get better with a bath in the sink, but there was waxy dirt on the interior that soap and water refused to touch. To deal with that I broke out the big guns and used 'goof off' to remove it. That stuff can dissolve PVA and make the surface of those clear plastic covers on turntables turn rough and hazy. Since the HV cup is not clear and almost never seen I decided who cares if it messes with the finish if it gets the dirt off. Some pictures of this below.
http://i1095.photobucket.com/albums/...ps0cc8bfc3.jpg
http://i1095.photobucket.com/albums/...ps74ce0ff8.jpg
http://i1095.photobucket.com/albums/...ps6da54ffc.jpg
http://i1095.photobucket.com/albums/...ps92bc55dd.jpg
http://i1095.photobucket.com/albums/...ps55d474da.jpg
http://i1095.photobucket.com/albums/...ps9df7a7d0.jpg
http://i1095.photobucket.com/albums/...pse2277322.jpg
http://i1095.photobucket.com/albums/...ps8cae01dd.jpg

I noticed while doing this that the focus rectifier tube socket was connected to nothing and that a 60's HV diode was installed. Not exactly correct but it is period and does seem to work decently so I think I'll leave it be for now.
http://i1095.photobucket.com/albums/...ps48dbd5a5.jpg
http://i1095.photobucket.com/albums/...psf42dfacb.jpg

Now I had a few problems remaining the set had not been given a proper gray scale, purity, or convergence setup in way too long. Also the vertical don't want to fill the bottom of the screen, and for some reason it seems the chassis hole marked 'vert. centering' looks to never been populated with a control...Ugh! That and I wanted to set it up before turning the color on.
It just don't want to be pure though, and the static convergence magnets don't have as much range as I'd like to see....I think I'll try a couple of spare purity rings I have stashed away next time I try to redo the set up. the results are below.
http://i1095.photobucket.com/albums/...pscb89530f.jpg
http://i1095.photobucket.com/albums/...pse2a974c6.jpg
After giving up on that I tried the color, but that was barely functioning. Most of the color stage tubes were weak or shorted so I swapped them and got almost perfect color bars....Only which one is supposed to be the blue-green bar and which is the green bar?
http://i1095.photobucket.com/albums/...ps89083870.jpg
There are also dirty tube sockets in the IF that I need to deal with, and for some reason it seems to tune channel three better with the tuner set to two.

I got this far without a schematic or any service data beyond the tube layout inside the cabinet, and I'm proud of that. But there are only a few tricks I've got left before I need to grab a reference to see exactly where it is off speck.

zenith
 

looks like its coming along nicely, if memory serves the HV rectifier is a 3at2? Ive got a bunch, being an RCA guy ,that I will never use, you are welcome to them, Bruce

If I ever need them I'll contact you. I have a fairly good stock of HV rectifier tubes already though. Whenever I go to a radio club meet and TV tubes appear in the donation auction they usually end up coming home with me.

I hear you, it didn't take me too long to have a stash like an old time electronics store, have fun

With the Service/Normal switch in the service position, you should be able to verify vertical centering. If it's off-center there should be some nylon/fishline "loops" that are hanging out of the yoke. Pulling on one will affect the horzontal, the other loop the vertical. Most likely linearity and height will be difficult to achieve. See if the cathode capacitor is still the original and check B+ and load resistors.

You should have that set playing in no time!

Ohh, so that is what those strings are! I thought they were junk (like the pieces of HV wire some tech left) and tried to yank them out....Good thing I failed to yank them off. I probably messed up the settings when I pulled on them. If they are all the way back is there a way to get them forward?

Service switch kills ALL light on the screen so I did the screen adjustments by pulling one of the vertical wires to the yoke. It was off center(too high) when I did that so I guess I shouldn't have yanked those strings.

I'll have to look up the datasheet for the vertical output tube or get the sam's to find the cathode pin. By "B+ and load resistors" do you mean in the PS or vertical circuit?

Quote: By "B+ and load resistors" do you mean in the PS or vertical circuit?

Just referring to the vertical circuit at the moment!

Thanks for posting such detail on that set. I also have a 25MC33 Horiz output tube was cracked. I figured it had no osc, and overheated and cracked. I got a new tube but then didn't have the time to do any more this winter. Actually I still didn't turn it on, I found the bad tube just eyeballing around in the set before thinking about turning it on... I'm kinda inspired now to get busy on it....

That was really a hack that worked on that set sometime in it's past.
I'm guessing that the HV rectifier filament winding arced through at one time, so some clod used the focus rectifier winding instead.
It took some ingenuity to do it in that manner, but still hack at the same time. Plus poor lead dress.
When I found the same problem, I would cut out the filament winding and use a solid state replacement for the HV rectifier tube.
I still prefer a focus rectifier tube instead of that lousy stick. :thmbsp:

I'll try to find time in the afternoon to look at the vertical circuit.

I tried the upper yoke string today and it seemed to affect the vertical centering, but the only effect it seemed to have was to allow me to either get it close to centered or WAY too high.
The way the static static magnets for all functions on the CRT behave makes me feel like someone screwed the magnets up on purpose.

Mr Squirrel, reminded me that my set had the wrong HOT as found. It had a 6LB6 where a 6BS6 IIRC was supposed to be used.

I have no clue what the exact original HV configuration was, but the wire on the HV filament is a different color than the stubs on the focus socket so I doubt the focus wire was reused on the HV tube.

If I get a schematic for it I might revert the focus circuit, but aside from my CTC-4 project set my other roundys are SS focus and do fine that way. I will at least try to tweak the focus circuit to give me more range as I have it set to min on the pot to get best focus, and would prefer to find a peak somewhere in the range.

My 25MC33 has the correct focus circuitry and even after replacing the focus rectifier tube and the resistor string it still requires the pot to be at one end of the range for correct focus. That might just be the way these sets are. I have seen many old sets that require the focus control to be at one end of the range or the other to focus correctly.

It's not the way they are supposed to be, check ALL resistors in the focus circuit. Make sure they are correct, I mean like, spot on. I'll bet you find a bad one...
Edit: Also the POT can go to crap, I've replaced all of the above and was able to get the focus nice and sharp with the control set somewhere in the middle. Drifty resistors in related circuits can cause it to be off too.

I figured out how to get the vertical centered. Apparently neither of the strings is solely for horizontal or vertical centering. They each affect both axises in the same way centering rings did on monochrome sets.
http://i1095.photobucket.com/albums/...ps44fde2d6.jpg
http://i1095.photobucket.com/albums/...ps02ec05bb.jpg

I have the height pot maxed out and the linearity as high as it needs to be to fill the mask yet be as linear as I can get it. Tomorrow I'll try to pull the chassis to check for off value vertical resistors.

As a bonus the issue with the blue-green bar being green has gone away on it's own, and I now have good color rendition.

Tom! I'd still be tempted to restore the focus circuit to the original scheme.
Those focus sticks are selenium, not silicon! They are subject to getting weak and causing focus problems.
Of course, Mark is right, regarding those high value resistors, drastically changing value.
One of the first things I check is the electrolytic in the cathode circuit, of the vertical output tube when dealing with vertical size issues. :yes:

With out a schematic I don't feel like changing the focus circuit. I'd be more likely to just change the flavor of incorrectness on my own without data on the original(plus I don't think I have enough HV wire). It focuses DAMN sharp at the bottom of the controls range right now...Probably better than most color sets I own.
Thanks for the tip on the vertical lytic. I may have tested that one already. I'll need to trace it down to see if I did.

That looks great ! I assume your not going to fix the cataracts right now..........

Actually Before I read your post I pulled and prepped the CRT for cataract removal.... If the forecast of sun tomorrow is accurate I should have the jug back in by Sunday.

I'll get to the chassis tonight if I can.

Well I did the cataract removal today(well it was today before it hit midnight).
The tube is a Silver-Glo rebuild(I assume). The PVA was really weird...The outside edge was calcified for lack of a better word, and the center stuck to the face of the tube like Zenith type PVA.
http://i1095.photobucket.com/albums/...ps813435fd.jpg

The sun was a tad patchy, and between it 16 cloths pins as wedges around the perimeter and a hair drier it barely wanted to come unglued around the edge. I went a bit far with the wedges and the safety glass cracked. I ended up chipping off the safety glass which was not fun.
http://i1095.photobucket.com/albums/...ps2a3464c3.jpg
http://i1095.photobucket.com/albums/...ps0f9964e0.jpg
All in all aside from the loss of the safety glass it seems to have turned out well.
Tomorrow I'll finish remounting it and resume work on the electronics.

Is it safe to use that tube without the safety glass? I would expect now you have no implosion protection.

I seem to recall someone saying that 70's "loss leader" roundys did not usually have safety glass installed when they were built so I'd imagine if it was okay then it should be okay now. I've never had a roundy CRT implode (knock on wood) and I've got 4 sets and 6 total CRTs, all of which I've handled at some point. The one I cataracted above even got tapped with a hammer on the screen a few times and is fine so I'm not all that worried.

If it is really that dangerous(or if I can't achieve purity on this tube) I have a spare Zenith CRT that was already cataracted(by me) with the safety glass siliconed back on. There is a tiny part of me that wants to put the Zenith tube in anyway for better authenticity.

Hi Tom, Great job on that nice Zenith roundie from the ETF MEET! By the way, nice GE Porta Color on top in the first picture! :-) :-) I'll check this afternoon and see if I have the Sams for your Zenith. Dave.

CRT Conversion
 

Hi Tom......You now have a 21FBP22A with the light gray face plate like my 21CT55's. The "A" signifies a rare-earth phosphored screen which should produce outstanding reds for you. Question: Does your rebuild have a noticiable glass weld on its neck where the new base is attached? Do all rebuilds have this tell-tail, anyone?..........Tom

If you could find the schematic that would be a big help. Flying blind is a good confidence builder until one starts to hit the sort of odd little bugs I'm dealing with. I'm enjoying the portacolor. There are a few tiny bugs I need to address(I think a couple controls need cleaning), but otherwise it works a lot better than what I'd expect from what I've read about portacolors.

Tom: Thanks for the info. It did have a dark tinted safety glass, but it got destroyed in the cataract removal. I don't know if this tube has a neck weld as I have never removed the neck hardware. My standard operating procedure when decating a tube that has already had the neck hardware adjusted is to pull the tube with all neck hardware on and plop the whole thing in the bucket I use as a stand during decating(I did have to hang the convergence adjustment chassis outside the bucket on this one though). This method has resulted in not having to re-setup the tube in several sets or to only have to make minimal touch-ups.
I may be able to answer your question soon. If a couple of things I'm going to try fail to get this tube to have acceptable purity. I'm going to assume the Silver-Glo that is in there now is defective and swap in my spare Zenith 21FBP22 CRT. I've degaussed the living bleep out of it, so about all I can think of doing now is to swap the purity rings and possibly the CRT.

Hi Tom, I DO have the Schematic!! Turns out it's the same as my 1965, 25MC43 chassis, according to the index. Send me a PM with your Email, when you have the chance to do so. Glad you're enjoying the Porta Color, it does have a decent picture, even as is, one reason I didn't put a high priority on working on it, too many "irons in the fire!" Dave

Great Dave! PM sent.

I was able to achieve decent purity(on any other set the more OCD parts of my mind would compel me to make it just a bit better) by flipping the rings 180 degrees and degaussing it even more rigorously than before. It was the first time I've been nuts enough to stuff my degauss coil inside a set let alone while the set was running.
The vertical out cathode cap tests like new on my Heathkit C-3 so I'm starting to test resistors.

[QUOTE=Electronic M;3071062]I seem to recall someone saying that 70's "loss leader" roundys did not usually have safety glass installed when they were built so I'd imagine if it was okay then it should be okay now. I've never had a roundy CRT implode (knock on wood) and I've got 4 sets and 6 total CRTs, all of which I've handled at some point. The one I cataracted above even got tapped with a hammer on the screen a few times and is fine so I'm not all that worried.
QUOTE]
That someone was wrong!
Any set without either a safety glass or some kind of an integral implosion protection would never pass U/L listing standards.
All the low end roundys at the time used either a 21FJP22 or a 21FBP22 with a tempered safety glass and a rubber sealing ring.
If you notice, the bonded safety glass is not tempered.

So is this a big enough issue that I should transplant the safety glass from my decated Zenith spare or swap that CRT in?

Keep your Zenith, all Zenith. :D

I'll have to test both tubes to see which is stronger, as IIRC there was a reason that Zenith jug didn't end up in my CTC-4... If it is weaker and or has worse color tracking than the Silver-Glo I'll probably just swap the safety glass.

Hey Tom, great job on that set. I have one that is identical, but with a metal cabinet. I have posted a thread or two about this, but being on the same subject, I thought that you might know something that I need to verify and perhaps correct in my set.

Did you feel or touch the horizontal efficiency (linearity) coil while tooling around under that chassis? Mine is wobbly and supposedly these have a habit of getting brittle and cracking. Just wondering what you found. Do you happen to have an extra by chance? Most people in the know say it needs replacing if it has cracked.

My lin. coil does not seem brittle, but it looks a bit melted. The shaft to the chassis looks almost like it broke long ago and was melted together sloppily with a lighter. It is hard to describe, but I could post a picture if you want. I wonder if the slug is stuck...

I have a spare form that is currently wound with the wire to be used as the horizontal hold coil that I would offer you if there weren't potentially two sets in my collection that might need to have their wire wound on that form.

You can post a picture if it's not too much trouble. I found a source for a replacement, so I guess I may as well just replace it since the horizontal and HV sections are so critical in color sets. Most of my collection has always been 40s-50s black and white, so I am a bit lost at times with the many issues that come into play with color sets. I must say that mine came in the door with one beautiful color picture after decades of dormancy. Like yours, I had to replace a few tubes in the HV section and the plate of my H.O. tube was glowing red. Once I replaced the bad tubes it came alive like the day it came off of the assembly line. Of course after about an hour some quirks developed and the color went out. Someone here sent me a crystal that supposedly is a likely culprit with the color problem, but who knows really. Nice of them to help me though. That was over a year ago and I haven't had time to do anything else, but I'm getting the itch bad. I need a nice control door if you see one or have one. Mine is there but has a few dents albeit small.

Tubejunkie: Glad you found a new lin coil. I'll post a picture when I pull the chassis later today. The door on my set is dented too, on the handle. I may try to fix the dents, if I don't like the result I do have a spare door. If whatever door I have left at the end is presentable I'll let you know about it.

That is how it will be. The Zenith tube tests 85% on the red and 76% on all else and the Silver-Glo tests 72% across the board.
The Zenith tube is on the border line of failing the color tracking test though. So if I can't get the colors to track I may have to do a mild rejuv to balance emissions better.

I'm starting to think the ETF CRT tester is tuned to be a bit optimistic in it's readings. It tested the Silver-Glo as ".1 Excellent" which as the set was advertised for the meet is the point where the meter pegs on their tester...On my tester it tests at 72% of full scale which is the lower end of 'good'. I've tested CRTs which make that value look anemic on my tester.

Regarding the slogan about keeping your set all Zenith.
I was just quoting an old advertisement about keeping your GM car all GM, by using only their replacement parts. :yes: