Zenith 25EC58 Restoration Thread

[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.