Zenith 25EC58 Restoration Thread

[TinCanAlley]

Originally Posted by vol.2

In my experience with fighting jailbars (so far), eliminating the bars is often related to smoothing out them out. Doing so tends to make the picture a little softer in the process.

After duplicating the test, I determined that it was the bright logo from the DVD player's screen saver that created the difference it bar intensity, not the capacitor. So the boost section capacitor is doing what it's supposed to be doing.

I'm about the flip the set on its side and remove the bottom and parallel the damper diode.

You wouldn't happen to know if removing the Automatic Brightness Limiter's transistor would disable the function and leave the set running without the function? The reason I ask is that of all the places I've scoped, the ringing is most visible on that circuit. I also remember last time I worked on it, just getting the scope's probe near the focus lead gave a ringing pattern. Is it possible for jailbars to come from the tripler?

[vol.2]

Originally Posted by TinCanAlley

The reason I ask is that of all the places I've scoped, the ringing is most visible on that circuit. I also remember last time I worked on it, just getting the scope's probe near the focus lead gave a ringing pattern. Is it possible for jailbars to come from the tripler?

Yes. The tripler is part of the flyback circuit, which can be the source of the bars. When this journey started, I recommended shielding all of that to see if it went away. Did you ever try that?

Sometimes the bars can couple into the video just from proximity. One test I've done in the past is to make long extension leads between my flyback and the PCB and physically move the whole circuit away from the set and put it inside of a big steel can covered in thick plastic to prevent anything from shorting together. This is really the only foolproof way to eliminate proximity as a source for the bars coupling into the signal. It's dangerous though, so be very careful if you actually try it. I don't know how having an external tripler would effect the situation though. That's lots of high voltage parts to juggle, and the risk of arcing would be high

The important distinction here is that the bars can be one of two things. Either they are a function of the deflection, or they are a function of the video signal (ie they are modifying the actual video information before it gets draw by the deflection system).

The way you see the pulses on the probe is normal in a TV, and they will always be there. The problem is only when they are either coupled into the video signal, or when they are present in the deflection process during scan time. The bars will always be in the deflection during the blanking period and that is normal. Seeing those pulses on a probe is expected and doesn't tell you anything about where the bars are coming from.

Yes, it's normal that adding capacitance across the damper diode could cause a change in brightness. That was only to see if the bars went away, not a permanent fix.

[TinCanAlley]

Originally Posted by vol.2

Yes. The tripler is part of the flyback circuit, which can be the source of the bars. When this journey started, I recommended shielding all of that to see if it went away. Did you ever try that?

Sometimes the bars can couple into the video just from proximity. One test I've done in the past is to make long extension leads between my flyback and the PCB and physically move the whole circuit away from the set and put it inside of a big steel can covered in thick plastic to prevent anything from shorting together. This is really the only foolproof way to eliminate proximity as a source for the bars coupling into the signal. It's dangerous though, so be very careful if you actually try it. I don't know how having an external tripler would effect the situation though. That's lots of high voltage parts to juggle, and the risk of arcing would be high

The important distinction here is that the bars can be one of two things. Either they are a function of the deflection, or they are a function of the video signal (ie they are modifying the actual video information before it gets draw by the deflection system).

The way you see the pulses on the probe is normal in a TV, and they will always be there. The problem is only when they are either coupled into the video signal, or when they are present in the deflection process during scan time. The bars will always be in the deflection during the blanking period and that is normal. Seeing those pulses on a probe is expected and doesn't tell you anything about where the bars are coming from.

Yes, it's normal that adding capacitance across the damper diode could cause a change in brightness. That was only to see if the bars went away, not a permanent fix.

I don't know if it's the way I've adjusted things, but I swear the bars have a more defined edge to them. Almost perfectly smooth and uniformed. I'll have to do more low light scenes and make sure.

I have not figured out a way to shield things without shorting something out. Unless there's a material that can be used as shielding that doesn't conduct, I just know I'll kill something. Is it safe to wrap the anode lead in foil all the way down covering the tripler? The connections on the tripler have black RTV on them, so I don't see an issue on that end.

Would ferrite magnets be of any use? I need one years ago and had to buy 20 of them. They are small and would fit up to 10 ga wire.

As for the tripler, I have an NTE replacement, but that would require ditching the divider and I'm not sure if the NTE is better used as an emergency replacement. I haven't been able to find and OEM one.

Tomorrow I will be scoping the 3rd and 4th video amps. Now I know the settings for the test points from the waveforms in the SM, but if I'm looking for something outside of that, what should I be looking for? AC? DC? Certain time/div? The reason I ask is I've noticed a couple test points that have two different waveforms and each one has a different time/div and produces different forms. I figure it safer to scope all leads on the transistors, but not sure if that's necessary.

[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.

[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.