Zenith 25EC58 Restoration Thread

[vol.2]

Originally Posted by TinCanAlley

I'm starting to think the jailbars are coming from some other source (IF?).

Random jailbars in the raster is one of the very most difficult things to figure out.

From what I've been able to gather in my quest to solve jailbar problems in my own sets is the following:

1. They can be caused by missing shielding. Sometimes there were big metal shields covering things like the flyback or other noisy sections of the chassis, and they go missing during the life of the set
2. Sometimes grounding problems can cause them. This can also be caused by a missing ground that was completed by a shield or a poor connection on the tube itself or elsewhere.
3. In many cases, the jailbars were always there, and the reason you are seeing them is because newer components are somehow accentuating their visibility. I have been told by people more experienced than me that this can be due to fast, better performing parts passing through more of the bars. I would assume that this is a frequency related effect where the edges of the bars would have originally been smeared together more, and now your fancy new parts are allowing more high frequency information through and it's simply showing the edges of the bars that always there more clearly.

In the last case, my approach has been to look for a place that changing a value could potentially reduce that a little bit. Basically to create a low-pass filter in the exact spot where the jailbars are being introduced.

I haven't had a lot of success with it yet though, so YMMV. I know just enough here to understand people's suggestions, but I don't have a strong enough background in electronics to do much innovating.

[TinCanAlley]

Originally Posted by vol.2

Random jailbars in the raster is one of the very most difficult things to figure out.

From what I've been able to gather in my quest to solve jailbar problems in my own sets is the following:

1. They can be caused by missing shielding. Sometimes there were big metal shields covering things like the flyback or other noisy sections of the chassis, and they go missing during the life of the set
2. Sometimes grounding problems can cause them. This can also be caused by a missing ground that was completed by a shield or a poor connection on the tube itself or elsewhere.
3. In many cases, the jailbars were always there, and the reason you are seeing them is because newer components are somehow accentuating their visibility. I have been told by people more experienced than me that this can be due to fast, better performing parts passing through more of the bars. I would assume that this is a frequency related effect where the edges of the bars would have originally been smeared together more, and now your fancy new parts are allowing more high frequency information through and it's simply showing the edges of the bars that always there more clearly.

In the last case, my approach has been to look for a place that changing a value could potentially reduce that a little bit. Basically to create a low-pass filter in the exact spot where the jailbars are being introduced.

I haven't had a lot of success with it yet though, so YMMV. I know just enough here to understand people's suggestions, but I don't have a strong enough background in electronics to do much innovating.

I don't know if my way of thinking about it is correct, but I feel that if none of the changes have even affected the bars, it must be somewhere else. I've changed every diode, the related ceramic capacitors and resistors. I changed the damper diode. Rerouted harnesses. Cleaned contacts. Checked all ground points on terminal strips. Replaced every electrolytic with part numbers from SM. Checked IF cables for damage and proper grounding. Not once have I seen any change in the jailbars. The only sections I haven't dealt with is the flyback, IF and yoke. The only missing shield is from the subcarrier module and that's due to the new module not needing it as it has its own shielding.

I found an NOS IF module that is bright and shiny and the box doesn't say "remanufactured" on it. The only thing I don't know is if the module comes pre-aligned. If not, then I'm short on equipment to do so. I also found an NOS flyback. I can easily test the IF module, but the flyback isn't plug and play.

I am dedicated to getting this TV back to as new a condition as I can and have been happy with all the work I've done that has increased the image quality and audio, but the jailbars are way too noticeable. They cover the entire screen, albeit less as they go from left to right, but they're still there.

[vol.2]

Originally Posted by TinCanAlley

I don't know if my way of thinking about it is correct, but I feel that if none of the changes have even affected the bars, it must be somewhere else.

Yeah, it's extremely tricky.

I still think that it's probably a case that the set always had those bars, but there was something acting as a low pass filter which provided some kind of anti-aliasing just as a side-effect of the component tolerances at the time.

Or it's a shielding problem or a grounding problem.

[TinCanAlley]

Originally Posted by vol.2

Yeah, it's extremely tricky.

I still think that it's probably a case that the set always had those bars, but there was something acting as a low pass filter which provided some kind of anti-aliasing just as a side-effect of the component tolerances at the time.

Or it's a shielding problem or a grounding problem.

I think I'm finally going to find out if it's an IF issue. I watched a video on someone taking a portable set from '76 and connecting composite directly. He had to short a resistor in the IF section, but I won't need to do that as the entire IF module comes out on mine. Also, the point where he inserted the composite he use a 75ohm resistor and a 47uf capacitor. I am going to modify an old 75ohm RCA by cutting off the head and attaching the resistor and capacitor and terminating it with a barrel connector big enough to fit over the C1 test point to insert directly into the video amp like the IF does. If it works, I'll not only have the video image, but I'll be able to see if the bars go are gone or not. Even if they are still there, I'm going to create a way to leave the IF in and switch from RF to Composite as needed.

[TinCanAlley]

Okay, so on the remote motor setup, I think I know what's what. The motor's setup is spring loaded and puts pressure on the tan gear thing. The other end side of that gear connects to a gear in the channel selector unit and that gear has metal contacts that also act as sprints pushing from the other side towards the motor. The tan gear thing doesn't actually mate up perfectly with the black one. The teeth only match up around 1/3 of the circumference. It is designed to slip over the black one when the motor turns, but stay in place all other times. With pressure from the spring on the motor side and the channel selector side, it's enough to let the tan one turn and then turn the channel selector gear. Still would like to get my hands on a replacement for possible future need.

Once I've gotten the replacement caps delivered, I'll install and reassemble. Hopefully the cleaning and reassembling will cure the channel changing issue.