On going Zenith 25MC33 roundy restoration - Page 2 - Videokarma.org TV - Video

Electronic M · #16 05-30-2013, 02:31 AM

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.

compucat · #17 05-30-2013, 11:15 AM

Quote:

Originally Posted by Electronic M

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.

*zenithfan1* · #18 05-30-2013, 11:11 PM

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.

Electronic M · #19 05-31-2013, 02:08 AM

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.

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.

dieseljeep · #20 05-31-2013, 09:30 AM

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.

Electronic M · #21 05-31-2013, 01:28 PM

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.

Username1 · #22 05-31-2013, 02:43 PM

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

Electronic M · #23 05-31-2013, 07:53 PM

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.

Electronic M · #24 06-02-2013, 12:30 AM

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.

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.

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.

compucat · #25 06-02-2013, 12:18 PM

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

Electronic M · #26 06-02-2013, 12:49 PM

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.

BigDavesTV · #27 06-02-2013, 01:17 PM

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.

Tomcomm · #28 06-02-2013, 04:18 PM

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

Electronic M · #29 06-02-2013, 06:31 PM

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.

BigDavesTV · #30 06-03-2013, 01:09 PM

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