Zenith 25EC58 Restoration Thread

[BeamT]

Do you have the shield for the Subcarrier module?

[TinCanAlley]

Originally Posted by BeamT

Do you have the shield for the Subcarrier module?

This is the alternate module (9-69) for this chassis and it doesn't require the shield. If you look closely at the board, there is a shield around the sensitive section only. I think the cover is the reason the pots on the original were messed up. They were poking through the holes for make adjustments and messed the up. They were pretty mangled.

[TinCanAlley]

Do know if this set will run with the video IF module removed? If so, I can connect my generator's signal via it's injector connection directly to the C1 test point into the video amp section. I am unsure if connecting my generator to this point while the IF is working would be a bad thing as it seems there's roughly 6V on it. This would let me see if the jail bars are being injected from the IF stage.

[BeamT]

I believe you can run this set without the IF module in place. The 125v B+ supply interlock does not run through the IF module. You wouldn't need to feed a signal to the IF output, you should still have a raster. You might need to turn the brightness up a bit to see it.

If you are going to replace C202 in the horiz blanker, it needs to a temperature compensated N750 Capacitor.

[TinCanAlley]

Originally Posted by BeamT

I believe you can run this set without the IF module in place. The 125v B+ supply interlock does not run through the IF module. You wouldn't need to feed a signal to the IF output, you should still have a raster. You might need to turn the brightness up a bit to see it.

If you are going to replace C202 in the horiz blanker, it needs to a temperature compensated N750 Capacitor.

If by the IF output you mean connection 5 on the IF module (Video-Sync-Chroma), that's where TP C1 is located, so I could clip the lead of the generator onto that point on the top of the chassis.

I have the two capacitors in my cart now and they are the part numbers from the SM and the C202 replacement is N750.

[TinCanAlley]

Here is the section of the generator instructions related to injecting the video signal. Maybe it will help identify exactly where it should be connected.

[BeamT]

I'm assuming you have the set back up and running, and the jail bars are visible.

Try this:

1) Gently disconnect the IF Input cable from the tuner to the chassis. At the chassis it is very similar to an RCA connector. Power up, jail bars still visible? Power down.

2) Remove the IF Module. Power up, jail bars still visible? Power down. Leave the IF Module out.

3) Drive the video stages with one of your generators. Most generators will block a small amount of DC but to be safe, connect the 50 Ohm video output through a .1 uf or so capacitor to block the DC. Connect to the C1 test point you have identified. Shield to a nearby chassis ground. Power up, adjust video level from the generator, try different patterns, what do you see?

Safety-- This is a transformer based set, so you should be reasonably safe using grounded test equipment, unless something is seriously wrong.

I appreciate my Sencore PR57 everytime I use it

[TinCanAlley]

I managed to get the other 3 leads from the cut capacitors out. It seems a bigger tip and holding in on the side of the solder bucket helped, but I think extra pulling also helped. I'm attaching a pic of one of the leads I got out. I'm not sure what that stuff is, but it is hard and was on the three leads I couldn't get out, but not on the one I did get out. So maybe that's why only one lead came out.

Also note the crud on the lead. Is that old flux? I'm assuming the discolored lead is only from the removal and it wasn't that way in the solder.

So when I work to install the new capacitor leads, should I put a drop of flux in the bucket first? I figure I'll let it heat up for about 20 to 30 seconds before trying to insert the lead.

Oh, and with the amount of time it takes to head up the bucket to melt the solder, is the heat transfer to other components like resistors bad? They get pretty warm/hot during the process. Should I clip a heatsink to the leads of the other components sharing the bucket?

[TinCanAlley]

Question about transistors. I've noticed that several transistors noted in the SM, each with different Zenith part numbers, have PNs for other manufactures, but those part numbers are all identical. Does this mean when you buy from another manufacturer you're getting replacements that aren't the exact specs, but close enough to work? For instance, transistors 695, 888, 931 and 447 replacements are all ECG123A or NTE123AP. The same happens will all other manufacturer's replacements (RCA RT102).

[vol.2]

Originally Posted by TinCanAlley

Question about transistors.

So the thing about transistors is that they have a lot of characteristics, but which ones make a difference entirely depends on your application.

When a manufacturer is deciding which transistor to use in which spot, they have an entirely different set of considerations than a general repair-person does, or in fact you or I have many years later.

Usually, they are working with a bunch of boxes of parts they buy in bulk, and that is all down to contracts and maybe just what's around at the moment. They try to make it as cheap as possible while still doing the job, but that might actually mean using a way over-spec'd part if that's what they have a whole lot of in the back room right now.

The repair manuals are looking at the whole field of parts available and giving you choices because different repair shops have their own stock and their own prices based on the volume of parts they can afford to buy at the same time.

Sometimes (a lot of times) the exact transistor you chose doesn't make a heck of a lot of difference. As long as it sort of fits the voltage and current requirements as the junctions, it's going to do the job.

Other times, the gain characteristics and/or the transition time of the transistor is going to make a big difference, and you have to pay attention to it in the circuit. In these cases, even a single transistor replacement can require reverse engineering the whole portion of the circuit that it is found in.

If you are seeing a single transistor type shown as a replacement for many different transistors in BOM, then it's likely that is a common replacement part used for a variety of transistors from different manufacturers. And so is more of a general replacement than an exact fit. They are usually pretty much okay, but I have seen situations in which Sam's replacements didn't work in a circuit. Not often, but I've seen it.

[TinCanAlley]

Well it seems the replacement caps won't be here until tomorrow. No biggie as I was cleaning the G2 pots with D5 and F5 and decided to give the setup switch it's 5th or so cleaning when I decided to look at it closely with my video microscope. I couldn't see anything but the pads at the end for the setup mode and the contacts on the slider. They were pretty much black and it appeared the slider part wasn't making full contact even though the spring in it was providing pressure. Not sure this at fault, but it got me searching for a replacement and I managed to find an NOS switch and bought it. Should be in by end of week. So I'm going to replace the switch while the chassis is still out.

The G2s now rotate much smoother than before, so it can't hurt to have clean pots.

[TinCanAlley]

I reinstalled the chassis after inspecting all components. I connected all internal cables and verified proper locations based on pics taken before removal. I plugged it in and it immediately started a deep buzz from the speaker. I unplugged it and double checked everything and it is all correct. Plugged it back in and it did the same thing. I have it unplugged now and haven't tried turning it on until I find out why it is making this sound while off. Any ideas?

[TinCanAlley]

Never mind, ground loop. First time it's ever happen. Rotated plug and it's gone.

I haven't gotten around to doing any real adjustments as the room is too bright. I'll do them this evening. The new setup switch is late, so I left the old one in for now. Still won't bring up any lines from the G2 controls. Hopefully the new one will fix that issue. As for jailbars, here's hoping the new caps will fix it. If not, I'm going to look at removing the IF module and injecting my generator directly into the circuit on C1. Was told these bars can also come form bad RF, IF, etc.

We're getting there!

[TinCanAlley]

I've done all the adjustments. I got the convergence pretty much dead on, except for a little blue horizontal on the very right edge. Seems the 2 new pots and a very extensive D5 and F5 cleaning of the other pots helped greatly. It is the best I've been able to get it and I'm more than happy with it and will not touch it again.

I'm starting to think the jailbars are coming from some other source (IF?). They are still there with all new electrolytics using SM part numbers, new diodes and new ceramics in the horizontal circuits. I disconnected the tuner to IF and I got a bright white raster, so no way of telling if the jailbars were there or not. My next step is to remove the IF module and connect my generator directly to C1 and inject the signal. I'm not sure if I can bring up anything that will show if the jailbars are there or not. What about a direct composite feed? I'm sure it's not as simple as connecting the composite out signal to C1 and ground, right?

As for the peak pix control... Is that off fully counter clockwise, or does that over soften the image? I really can't tell, but it seems soft CCW and noisy fully CW. So maybe the center is where it should be?

Attaching a couple of pics. They're from a laserdisc.

[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.