Zenith 25EC58 Restoration Thread

[old_tv_nut]

"Guess they figure since the supply line going to the diode has one of those capacitors across it, thing the supply side of it to ground isn't necessary?"
A cap across a diode and a cap from one end to somewhere else serve different purposes. One place doesn't sub for the other. There must be something different about the flyback transformer itself to require removing the cap.

Re clip leads: keep leads short and be very careful about accidental shorts, as the diodes would not like that.

[TinCanAlley]

Originally Posted by old_tv_nut

"Guess they figure since the supply line going to the diode has one of those capacitors across it, thing the supply side of it to ground isn't necessary?"
A cap across a diode and a cap from one end to somewhere else serve different purposes. One place doesn't sub for the other. There must be something different about the flyback transformer itself to require removing the cap.

Re clip leads: keep leads short and be very careful about accidental shorts, as the diodes would not like that.

If there is something different with the flyback, I can't find it (visually). The capacitor only stays in if you're replacing the -02 with this -02. The original -00, -01 and -03 deletes the capacitor. Not going to put anymore thought into it. Just glad I now have a 2KV capacitor. If I do find that one of the current ones is not up to spec, I have a replacement.

The leads I have are 10" and rated for 600V. Clips are insulated except for the bite section.

[old_tv_nut]

Voltages in some parts of the horizontal section can exceed 600V, so i would play it safe.

[TinCanAlley]

Originally Posted by old_tv_nut

Voltages in some parts of the horizontal section can exceed 600V, so i would play it safe.

I only plan on testing on 240V boost and the damper diode. According to the SM, the collector on the HOT is 125V. And of course no connecting/disconnecting while set is on.

[TinCanAlley]

Okay, curious results of parallel capacitor on 240V Boost.

I remembered I had a 560p 2KV capacitor, so I decided to use that in parallel to the 600p 2KV across the boost diode. The results were noticeable, but didn't eliminate the bars, but did reduce them. Don't know if it reduced the ringing or the overall brightness of screen. I'm assuming it hasn't done anything to eliminate the bars, just made them dimmer, which might mean an overall dimmer screen? If the original cap is in spec, then I upped it to around 1160.

Anyway, I also measured voltage. On the supply side from the flyback, it's 131V. On the other side of the diode, it's 249V. I'm assuming that is within specs for the 240V boost circuit.

I've attached a couple of pics. They look similar, but the one with cap is dimmer. I figure the bars should have changed in other ways, narrower, less of them, etc.

Tomorrow I'll try across the damper diode. Need to put the set on its side and remove the base and access panel for that.

[vol.2]

Originally Posted by TinCanAlley

The results were noticeable, but didn't eliminate the bars, but did reduce them.

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.

[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?

[TinCanAlley]

Well paralleling the damper diode didn't get rid of the bars, but it did something I didn't think would happen. It reduced the overall brightness. I guess the added capacitance did that?

It's all buttoned up and proudly showing off her amazing image. Jailbars don't show up on dark scenes where I'm assuming it's below the level of black. For instance, I put on Buck Rogers TV show from '79 and all the space scenes were perfectly black. The only time the bars showed was during scenes with low light, not the absence of it.

[TinCanAlley]

One more part of the video chain eliminated.

The IF module is not the source of the bars. I installed the new IF and after adjusting the detector output to proper level, the bars are still there. Put back the original IF.

Next in the chain are the ABL, Blankers, 3rd Video Amp and then 4th Video Amp.

So far I've eliminated the tuners, IF, horizontal module, subcarrier module, B+, damper diode/capacitor and 240V Boost diode/capacitor. With all the electrolytics replaced with exact parts noted in SM as well. Transistors on the chassis and horizontal module have been replaced with factory correct.

There is a zener diode on the 4th video amp (emitter to collector/ground). Since this is acting as a voltage regulator, I don't think it can create the kind of noise to cause bars.

I also have checked all soldered ground points on the terminal strips to chassis, shielded cables and on the boards I've tested. I have tried a number of different routings of leads and isolated the yoke leads from all others.

If I never get rid of the bars, I'll still be very please with the overall image quality I've been able to achieve. The bars will be a very small price to pay to have an amazing set.

[TinCanAlley]

Best compromise between no jailbars and light jailbars. The overall level of brightness and contrast is too low when set to remove the bars. Then they are way too distracting when set to proper levels. So I have set it below where I where I normally view, but above where the bars are too distracting.

[TinCanAlley]

A few pics of the journey so far.

[old_tv_nut]

Looking at these pics on my laptop, I can't see jailbars in any of them. Have to try on the desktop monitor, I guess.

[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 old_tv_nut

Looking at these pics on my laptop, I can't see jailbars in any of them. Have to try on the desktop monitor, I guess.

They are there, but the settings I use on my camera to get decent pictures seems to diminish them at bit.

[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.