Zenith 25EC58 Restoration Thread

[vol.2]

I wouldn't go putting magnets around the place, sounds like a bad idea to me.

You can get pieces of steel and double bag them inside of ziplock freezer bags to prevent them from shorting out on anything. Good enough for a test.

The idea is to completely cover the parts that are making the oscillation you're seeing. If it's coupling into the video in that way, you should notice some difference. Might not cure it, but it would at least tell you that you're on the wrong (or right) path.

[TinCanAlley]

Originally Posted by vol.2

I wouldn't go putting magnets around the place, sounds like a bad idea to me.

You can get pieces of steel and double bag them inside of ziplock freezer bags to prevent them from shorting out on anything. Good enough for a test.

The idea is to completely cover the parts that are making the oscillation you're seeing. If it's coupling into the video in that way, you should notice some difference. Might not cure it, but it would at least tell you that you're on the wrong (or right) path.

What if I put several layers of foil inside freezer bags? Would that provide good/decent shielding? I have some 1 and 2 gallon bags. The 2 gallon would be large enough to wrap over the top and sides of the flyback and the 1 gallon over the tripler. I could also lay it on top of the chassis between it and the CRT.

[TinCanAlley]

Well I'm just about finished with all the connections this set is going to have. I do need to finish the trim on the table next to it, but my access to a brad nailer is gone, so I might just use wood glue. Also need to redo the carpet on the riser.

[TinCanAlley]

Oh, and I'm still confused about adjusting detector output level. I posted earlier the instructions. So I've figured out that I'm supposed to be on the highest channel without signal. I'm then supposed to connect 7.2V to E and ground, then a DMM from C1 to ground. The set is supposed to be one for this connection and adjustment and this is where I am confused. The circuit already has DC and and AC signal on it. Is me just connecting my power supply and injecting voltage all there is to it? Is the AC signal bad for my supply? What is my injecting 7.2V going to do to the DC already on it? And lastly, once this is done and I disconnect, the DC voltage on E will drop back to what it was before, so won't the adjustment I made change as well? As it stand s now, the voltage on C1 is around 7V.

Since there were untrained hands in this set before I got it, I'm checking all I can and this one is off, but not sure if the amount matters in the end.

[vol.2]

Originally Posted by TinCanAlley

What if I put several layers of foil inside freezer bags? Would that provide good/decent shielding? I have some 1 and 2 gallon bags. The 2 gallon would be large enough to wrap over the top and sides of the flyback and the 1 gallon over the tripler. I could also lay it on top of the chassis between it and the CRT.

It has to be pretty thick steel I think, not foil. There are calculations you can do, and if you wish to explore that, you can read up on Lenz's law and how it is contained in the equations of Faraday.

There are other kinds of metal that have been used by TV manufacturers that work better, the most well known of which is mu-metal.

[TinCanAlley]

What determines horizontal position in this set? I'm assuming it is something on the horizontal module as after swapping back to the original module after the failure of the upgraded module, the image is now off center horizontally. It was never centered perfectly with the newer module, but it was about 1/2 to the right. With the original module reinstalled, the image has move another 3/4 to 1" to the right. The horizontal control is all the way to the stop. If I rotate it away from the stop, the image moves more towards the right.

If a module swap is enough to mess with horizontal centering, is that because of something on the board out of spec, or the way it is interacting with the horizontal control? Really would like it at least back to where it was previously.

[TinCanAlley]

So, is my tripler 53 years old? Is the date code 7314? Other than part numbers and "YARO" or "VARO", the only thing that looks like it might be a date code comes out as 14th week of 1973. Does that seem correct?

[TinCanAlley]

I'm going to install the NTE526A in the set Friday. It's another test to find the source of the jailbars. When I replaced the focus divider years ago, the wires were long and I cut them to length. I have enough of the black to go from the new tripler to the focus connection on the flyback, but not enough of the white to reach the pot. I found some 18AWG 30KV wire in white and it should be here tomorrow. I really didn't want to cut the wires off the divider as I might end up keeping it and the tripler if the new one doesn't fix the bars. This way I have the OEM back and the NTE as a spare. If it fixes the bars, is there some kind of tests I can do on the OEM? Resistance? Diode check?

And of course a question. Is it okay to run the new tripler without RTV on the connections for testing? I want to be able to quickly swap it if it doesn't solve the bar issue. If not, is there something else less permanent I can do for testing to protect from arcing?

[TinCanAlley]

I got my hands on the plans to gut a 9-57 horizontal board and piggyback a 9-90 so I can use my old RCA VCR without the waving at the top of the screen. Only issue is I don't know how the B+ 125V is going to like/not like it. Since the B+ is passed module to module and the horizontal module is the last in line, does the B+ line care if it is cut at that point? With the 57 board, it goes in and out, but isn't used by anything on the board. If I gut the board so I can just use it as a base for the 90, the return path for the B+ will be severed.

[TinCanAlley]

About to install the new setup switch. I got the old one out and opened it after testing. Testing on the new switch showed no resistance. On the old switch, it showed resistance, mainly in the "setup" position. Upon inspection of the contact pads, I can see where the resistance is coming from.

Took plenty of pics to make sure I get the wiring on the new switch correct. Sure wish there was more play in the wires for easier install and soldering.

I've attached a pic. Can you guess which pads are for "setup"?

[TinCanAlley]

The tripler has a 500ohm 7W resistor that is one of the cement (sandbox) type. Since I'm replacing the tripler I figured I'd go ahead and replace the resistor as well. Finding the cement ones is not easy, so I just ordered a WW in the proper resistance and wattage that is a 1%. It also has a temp of up to 250c.

[TinCanAlley]

Setup switch and tripler are installed. Just need for the RTV to set and I'll reinstall chassis and test.

I don't think I did too bad with the wiring and soldering. The resistor was a bit longer than the original, so I had to set it a bit higher up with a longer lead on the tripler end. A bigger blob of RTV covered the longer lead.

Always wondered about the focus control lead. It pretty much just runs along the top of all the circuits on its way to the pot. Wondering if it's a noisy lead or not enough to interfere with other components.

[TinCanAlley]

Well I'm 0 for 2, but there is a side benefit for the work I did.

The tripler is in and set is running, but the bars are still there. The only plus is I'm now able to fully focus to the sharpest image without the moire effect on the sides of the screen. Some of the pics I posted show it. Image is as sharp as can be and no signs of the moire effect.

The setup switch is working just like before, so I the switch isn't the issue. Of course I didn't touch the controls as I'm quite happy with the grayscale I have and I figure with the brightness I currently have, I should have seen some kind of line flipping the switch.

So now I'm pretty much at the point where the only things I haven't replaced are the yoke and flyback. Guess it's time to just button her up and enjoy watching while learning to ignore the bars when they appear in darker scenes.

[vol.2]

Originally Posted by TinCanAlley

So now I'm pretty much at the point where the only things I haven't replaced are the yoke and flyback. Guess it's time to just button her up and enjoy watching while learning to ignore the bars when they appear in darker scenes.

I don't think that replacing parts will get rid of the jailbars. I don't think it would in general

The way I understand it, jailbars can end up in older TVs because the manufacturing tolerances are much better now and newer parts are so good that they show the bars. The bars were always there, but the older parts had added impedance or more efficient transistor junctions, and thus do not inhibit them.

Finding the source of jailbars, as I understand it, it usually a process of putting blocking capacitors in different places to see if they change, or placing magnetic shielding around reactive components

Perhaps I'm mistaken though. There's a lot of older, more experienced people on the forum who would have a more reliable opinion

[TinCanAlley]

Originally Posted by vol.2

I don't think that replacing parts will get rid of the jailbars. I don't think it would in general

The way I understand it, jailbars can end up in older TVs because the manufacturing tolerances are much better now and newer parts are so good that they show the bars. The bars were always there, but the older parts had added impedance or more efficient transistor junctions, and thus do not inhibit them.

Finding the source of jailbars, as I understand it, it usually a process of putting blocking capacitors in different places to see if they change, or placing magnetic shielding around reactive components

Perhaps I'm mistaken though. There's a lot of older, more experienced people on the forum who would have a more reliable opinion

I was hopeful, but didn't get my hopes up. I read a few sites where someone fixed their jailbars by cleaning their CRT socket and replacing the tripler. It was the same chassis as mine. Also read where someone replaced their tripler and got jailbars. They went back to original and they were gone.

For the life of me I can't remember if the set had jailbars when I first got it. I remember it was in bad shape, calibration wise and I was told that before I do anything I needed to replace all the safety caps and then the electrolytics. If the jailbars were there to begin with, they might have been masked by all the bad calibration settings.

I've been watching a youtube channel where they show the results of tantalum caps across zener diodes and uses a device to "listen" to the noise from the diode before and after. They also show using other capacitors across other diodes. Doubt that's where my issue is, but interesting learning about controlling noise in circuits. I thinking that my issue might be with a decoupling cap. I've noted a couple on the schematics that are in the boost (750V and 240V). I figure I can note the solder pot from pics and then mark their topside connection and try adding another bypass cap and note the affect, if any, on the bars.

I going to make an uneducated guess here and say the new tripler has had some impact on HV. My static convergence was off after the swap. I had to readjust it and then a small dynamic adjustment. It's back to what it was, if not slightly better.

I'm going to pause my work on this set and move on to a 13" Hitachi that I'm using as my monitor for a TI-99/4A computer. Believe it or not, but it also has jailbars on the left side of the screen. I order new capacitors and am going to test the CRT. Beyond that, it isn't worth putting money into. Following that, I have a B&W RCA from '75. It blew the fuse a few years ago, but not since. I'm assuming the old fuse was tired. I figure I'll hook it up and let it run for a while and see if it acts up.