Zenith 25EC58 Restoration Thread

[TinCanAlley]

Question about 23.5V in the set. How important is this? The reason I ask is I tested the regulator and it seemed fine. However, what I noted was the installed regulator is 121-808 and the sheet attached inside the set has it as being 121-966. In order to find the differences between them, I had to compare the part numbers from the other manufactures listed in the SM. That lead me to compare ECG152 (121-966) to the ECG184 (121-808). I found datasheets on them from AllTransistors and they are definitely different, but I don't know what those difference would translate to or if it's even an issue. Since this voltage is in just about every circuit of the TV, it must be important.

Can someone with the necessary knowledge take a look?

Thanks!

[TinCanAlley]

Since this TV has a transformer, it should be possible to bypass the IF and inject composite video and audio. If I understand this correctly, I should be able to remove the IF module, connect the composite to C1 and ground, then connect the audio to connection 4 of the sound module and ground. To reverse it, I just remove connections and reinstall IF module.

The input voltage of the sound module is .25V, down from the SIF out of .47V going through L215. I believe the fixed audio out of a DVD player can be as high as 2V, so will something need to be added?

I'm attaching the relative portion of the schematic and circled the points of connection.

[TinCanAlley]

I removed the IF module and connected the video out of my generator to C1 which is noted as being the picture detector output. The generator's manual says to connect its video output after the detector, so this is the correct point. When I did, I had to set the output on the generator to max in order to see anything. What I got was dim and the horizontal and vertical was not stable.

Looking over the schematic, the detector out is 6.99V. From reading up, the composite signal is a max of 1V. The detector's out also feeds the video processor module and that is 6.24V. The video processor module sends out H and V synch as well. So does the composite signal I'm feeding it need to go through the video processor as well to? I see 3 destinations for the detector output.... The video processor module, the chroma module and the section with the video amps, vertical blanker, blanker amp, etc.

So am I correct that the composite I feed needs to hit all those points and can't just simply wander into the video outputs. The signal needs to drive the vertical and horizontal modules, etc?

If this is the case, how do I duplicate the voltage on the detector output for all the other modules? The odd thing is, the manual for my generator says nothing about removing the IF module or anything other than connecting the lead to a point after the detector. So maybe there is something in the circuit of the generator to allow it to inject its signal over the detector's?

Such a crazy day.

[BeamT]

Troubleshooting 2 issues, jail bars, unable to produce one or more service lines in setup.

When you disconnected the IF input from the tuner, jail bars yes/no?
When you removed the IF module, jail bars yes/no?
When you inject the video source from your generator at C1, IF Module out, jail bars yes/no?

The 6.99v shown at C1 in SAMS is a DC value, to the right 1.4v P2P is the waveform involved and is roughly what your generator is supplying. Not every set has a IF Module that can be removed.

The 24v DC supply is very important, 121-808 looks like a good sub for 121-966. Higher values and a different case, but looks good. You noted in prior threads the voltage at the test point was good. Did you replace this transistor before?

What is the "ground loop" you are referring to?

The strategy behind measuring the DC values on the CRT is to rule out a circuit issue and unfortunately implicate the CRT. You appear to have good gray scale with what you have, good focus, convergence is really good.

I've never worked on an Avanti, which I believe your set is, do these NOT have a Service Saver plate on the bottom?

Might be interesting to test your setup switch with your ohmeter before you go to the trouble of replacing it.

[TinCanAlley]

Originally Posted by BeamT

Troubleshooting 2 issues, jail bars, unable to produce one or more service lines in setup.

When you disconnected the IF input from the tuner, jail bars yes/no?
When you removed the IF module, jail bars yes/no?
When you inject the video source from your generator at C1, IF Module out, jail bars yes/no?

The 6.99v shown at C1 in SAMS is a DC value, to the right 1.4v P2P is the waveform involved and is roughly what your generator is supplying. Not every set has a IF Module that can be removed.

The 24v DC supply is very important, 121-808 looks like a good sub for 121-966. Higher values and a different case, but looks good. You noted in prior threads the voltage at the test point was good. Did you replace this transistor before?

What is the "ground loop" you are referring to?

The strategy behind measuring the DC values on the CRT is to rule out a circuit issue and unfortunately implicate the CRT. You appear to have good gray scale with what you have, good focus, convergence is really good.

I've never worked on an Avanti, which I believe your set is, do these NOT have a Service Saver plate on the bottom?

Might be interesting to test your setup switch with your ohmeter before you go to the trouble of replacing it.

When I disconnect the tuner to the IF, I get a bright white screen, and see no jailbars, but I don't see them with any of the bright test patterns, so I can't say for certain if they are still there.

When I remove the IF module, the screen goes completely black like the set is off, so no way to tell if jailbars are there.

Connecting to C1 with IF module removed doesn't bring up the pattern from the generator and if I increase the signal on the generator I get a scrambled image like there's no horizontal or vertical sync. Adjusting the vertical and horizontal hold makes it more stable, but it's not a useful image.

Since the manual for the generator doesn't specifically state the connection for injecting their signal needs the one from the IF be disconnected, I'm tempted to just leave the IF module in, disconnect the tuner and connect the generator to C1. The worst that can happen should happen to the generator, not the TV. I can't imagine they'd make such a feature and not implement safeguards in their system. Also, having the IF module removed might remove the 6V needed for something else in the video chain?

The regulator installed is what was installed when I got the set. Since it didn't match the paper in the set, I got the correct one. Never got around to installing it, but did so this time round as I had it. And yes, the voltages from the installed were correct and still so with the new regulator. I know I didn't need to change it, but it bothered me it wasn't what was called for.

I'm assuming it was a ground loop. After reinstalling the chassis and making all the internal connections (and double checking), I plugged it back in. It immediately made a deep buzzing from the speaker and possibly other sources. I immediately unplugged it and check all connections again. I plugged it back in and it still did it. I left it plugged in and tapped on all connections in the area of the tuner, IF and sound. I found that just my hand touching certain areas was enough to make it stop. That's what made me think it was a ground issue. I reverse the plug and it stopped and hasn't come back since.

What is a "service saver plate"? Never heard of it. Only thing on the bottom of the set is a removable metal cover to access the underside of the chassis.

I just got the new switch in the mail today. It is identical except for the fact it is much tighter than current one. The one in the set not only slides up and down, but in and out as if there were a spring between the slider and brass contacts. The new one has no play at all. Since all the resistors and capacitors on the setup side of the switch have been replaced, I don't know where else to look and changing the switch is an easy task that will help rule out another piece of the chain. This would also allow me to disassemble the old switch to inspect.

Tomorrow I plan on getting all the voltages from the CRT. I have it dialed in pretty well and thought I'd display the color SMPTE pattern with the pluge stuff on the bottom. That should keep voltages stable so if I test again I know the video information sent to the CRT is the same and any changes in voltage aren't from displaying different signal.

[BeamT]

Zenith marketed the access plate on the bottom as the Service-Saver Chassis because it allowed service from the bottom of the set without having to remove the chassis. Components replaced, or measurements taken with the set running, tipped on its side.

With the IF Module removed, can you turn up the brightness and get a raster?

Yes, you can connect your generator 50 ohm video output to C1 with the IF module in the set. Rock or position the tuner slighlty to mid-channel, or you can remove the IF in cable.

Google "Servicing a 1974 Zenith 19DC12 4 tube hybrid color tv" you might find this YouTube interesting. No connection to the author.

[TinCanAlley]

Originally Posted by BeamT

Zenith marketed the access plate on the bottom as the Service-Saver Chassis because it allowed service from the bottom of the set without having to remove the chassis. Components replaced, or measurements taken with the set running, tipped on its side.

With the IF Module removed, can you turn up the brightness and get a raster?

Yes, you can connect your generator 50 ohm video output to C1 with the IF module in the set. Rock or position the tuner slighlty to mid-channel, or you can remove the IF in cable.

Google "Servicing a 1974 Zenith 19DC12 4 tube hybrid color tv" you might find this YouTube interesting. No connection to the author.

After all the times I've been inside the set I never noticed that there are 4 threaded inserts in each corner. I've never see an Avante with anything other than the plastic base, so I wonder what they are for. I was thinking of making some legs and screw them in so I can take off the base and open the bottom and not have to put the set on its side.

Okay, I'm going to give the generator to C1 with tuner disconnected as I am unable to set the tuner between channels as it's motor driven.

I'll also try upping the brightness with the IF removed.

[TinCanAlley]

Connecting to C1 with tuner, generator doesn't bring up anything useful. I can tell the generator is doing something as the screen changes a bit as I set the output level, but it never brings up anything even close to a pattern. With the IF removed, no amount of brightness setting brings up any raster, etc. I'm going to go over my pics of the underside and trace exactly where the IF out goes if not directly to C1.

I tried testing the voltage on the video out of the generator, but it only went up as I moved the output level and then dropped to 0 after moving it. I think I'll connect it to my scope and see if it actually putting out a composite signal.

If I'm not mistaken, I should be able to connect the center of a video out from my old DVD player to C1 and the outer to ground and get an image? I'm not worried about and if it works, I can let it go into screensaver mode as that really shows the jailbars.

[TinCanAlley]

I watched the video. Some interesting content. I've tested my CRT with 2 separate testers and each one shows it good. While I can't get a red or green line, I can get a faint blue line. However, if I do greyscale outside of the setup with a greyscale ramp, I can get very strong colors from each gun and good brightness. The thing I haven't been able to figure out is the gain taps. I understand they work through a series of resistors to help remove color from the white parts of the picture. If I am understanding the schematic correctly, if you pull the tap off, it disconnects the video output to the crt and that removes the 142V. So why, when I pull the red tap in setup do I get a bright red line and lose it when I put it back? Like I said, if I understand the schematic, why red line with 0V and no line with 142V

Rejuvenating is risky from what I understand. I don't want to run the risk of damaging the CRT.

There was one thing I didn't fully understand from the video. When he was reading voltage from each of the resistors, what exactly was he seeing that pointed to the CRT's blue gun? One side of all of them seemed to have the same 26xV and the other side of each had lower, with blue not matching what red and green showed. Not sure how that translates to a bad blue gun.

[TinCanAlley]

I'm posting a couple of screen shots. I have to admit this is the best the set has looked so far. These pics don't do it justice as I am getting better shots from the camera adjustments advised by others, but they are still a far cry from how good it looks in person. Got quite a few compliments on the image quality. The youngsters didn't know a CRT image could look this good. They thought all you'd see is large scan lines and blurry images.

Tested the CRT again today and it tests as good and the image produced (color, brightness, contrast) seems to agree. So the inability to bring up lines in setup doesn't seem to be the fault of the CRT. Oh, and I got the greyscale set so well, I didn't have to touch the gain taps at all this time. Usually I had to move one and deal with the whites not as white as I'd like, but now they are pretty much dead on. Not too warm and not too cold.

Now if I can just get rid of the darn jailbars, I'd have a damn near perfect set.

[TinCanAlley]

What are the odds the jailbars are from the flyback? Reason I ask is I can get an NOS replacement for ten bucks. Even if it isn't the cure, are flyback usually something that hardly ever needs replacement, so a spare isn't necessary?

[TinCanAlley]

New question. I don't want to pull it again, but I find that the SM doesn't have anything on my tuner setup other than the remote board. I have no clue if the non turret schematics are for mine or not. I had it out months ago to fix the power on/off issue. I replaced all the trigger caps and other electrolytics on the board and it fixed the issue. I was just going over some pics I took and noticed that there is another board in the tuner/remote assembly and it has three electrolytics on it. I don't know if it's a board connected to the tuners or the board used for the channel tuners on the front side.

While on the subject of tuner, does an OEM IF module come pre-adjusted, or is that something that is done once installed and connected to the tuners?

Also attaching a pic of a piggybacked resistor that came in a batch of Zenith parts I got. It came in a plastic bag with no real indication on what it meant for.

[old_tv_nut]

The top resistor looks like 820 ohms +/-10%.

https://circuitdigest.com/calculator...ode-calculator
decodes the 5-band resistor as 1.02 ohms +/-4%, which makes no sense to me.

Where did this batch of parts come from? Kind of seems like someone's bench sweepings unless someone can tell us what the 5-band device really is.