Zenith 25EC58 Restoration Thread - Page 8 - Videokarma.org TV - Video

TinCanAlley · #**106** 04-24-2026, 01:02 PM

Success on the tuner! All caps replaced, reassembled with gears properly aligned and cleaned up. After installing it back in the set, all tests went perfectly. It no longer hesitates, fails to move to next channel and the AFC is working again. I know I got the gears aligned correctly as the channel lit matches the corresponding tuning knob.

So now my next task is to remove the IF module and connect a composite signal directly to C1 using the cable I made with a 75ohm resistor and 47uf capacitor. I have a sacrificial DVD player that volunteered to make the connection.

TinCanAlley · #**107** 04-27-2026, 01:53 PM

While I had the scope out to check the signal on C1, I figured I'd check other waveforms in that area. I checked the base of the ABL (Automatic Brightness Limiter) and I got the waveform that in the past made me think it was the reason for my jailbars, but was told by others here it was okay. Comparing what I get to the low definition waveform in the SM, I still think mine is not right. I have the peaks like in the example, but between them is a long flat line in the example where mine has ripples from tall to short along that line. If that's the way it should look, wouldn't the example show it? I'm attaching an image.

TinCanAlley · #**108** 04-27-2026, 05:34 PM

More on the ringing seen on the brightness limiter circuit. I have looked over recent pics of the chassis underside after studying the schematic. There is a 7w resistor that comes off the tripler that connects to the flyback connection where a black wire goes under the chassis. Where it connects is a 7.5V Zener diode noted as a power regulator. From there a wire travels along front side of the chassis and connects to the brightness limiter pot. I can see in a recent pic that I never changed that diode. In the attached pic you can see it and the leads seem discolored from heat. Again, my limited knowledge in circuits had got me again. Still, I believe this setup is designed for power to flow and be limited to 7.5V by the diode, right? If that diode is failing, or out of spec, can that cause the ringing? I have 2 new diodes from Zenith, but they don't look like the current one. These are glass and smaller. The color bands on them give me 194 and that matches the SM part number of 103-194.

So I think it's time to change that diode. No point testing it as to do so requires it coming out, a bench power supply and DMM. Might as well just put the new one in and save some time. While I'm at it I'll replace the setup switch as well.

TinCanAlley · #**109** 04-29-2026, 01:03 PM

Can someone help me interpret this adjustment from the SM?

I think the reason the new IF module didn't work right was due to the detector level not being set properly. While all other parts the IF can be aligned outside of the set at the factory, the detector level might be set dependent. The SM has the procedure and I'm sure the first part of it is just to setup for the main alignment in the section below this one, so isn't necessary for this procedure.

I'm attaching the procedure. What it doesn't note is whether the set needs to be on or off. I'm assuming off and I need to connect my power supply, set to noted voltage and make adjustment while measuring with DMM. Is this correct? If the set is on, won't my injected voltage be thrown off by the voltage provided to the IF?

TinCanAlley · #**110** 04-29-2026, 06:32 PM

So I have a question about brightness control and where it should start affecting the brightness. I never paid much attention to it, but today I decided to follow "proper" adjustment of contrast and brightness. Brightness is supposed to be first and you bring it up to bring out detail in dark areas, but not so much that they turn grey. Then you adjust contrast to bring up the overall brightness of the image. When I turned both controls down, I started to up the brightness, but noticed there was no affect until about 2/3 of the rotation range or so before it started to show. I thought it might be because I had the contrast down too low, so I set that to about center. Still, the brightness didn't start kicking in until around the 2/3 rotation.

Is this correct? I have no knowledge of being able to set the brightness range on this set and the SM has not info, either. All I can set is the brightness limiter. If the control should have more of a range, then maybe I have an issue somewhere in that circuit.

old_tv_nut · #**111** 04-29-2026, 09:23 PM

The brightness control may have a wide range just to accommodate circuit tolerances - as long as you can reach the correct setting without being near one end or the other of travel, it should be OK.

TinCanAlley · #**112** 04-30-2026, 02:58 PM

Quote:

Originally Posted by old_tv_nut

The brightness control may have a wide range just to accommodate circuit tolerances - as long as you can reach the correct setting without being near one end or the other of travel, it should be OK.

Over brightness is achieved just before end of travel.

TinCanAlley · #**113** 04-30-2026, 04:52 PM

Some success, and a setback.....

I tested all CRT voltages and they are pretty much what the SM has. I then ditched the pattern instructions for contrast and brightness and started with brightness til blacks started turning grey and back off til black again. Then adjusted contrast to bring up overall brightness and went back an forth between them to fine tune. From there I use a blue colored filter to do color and tint. Used another test pattern to adjust peak picture till all ringing was gone, but not over softened. From there I brought up the geometry patter and adjusted vertical linearity. After all was settled, see attached pic 1 and 2 for results.

Now look at pic 3. While viewing, I completely lost horizontal sync. No adjustment of the control would bring it back. I then removed the board and reseated which gave me back sync, but the image is still unstable as it is wavering every few seconds like it's about to lose sync again. The SM says it's either horizontal phase detect, oscillator or AFC. Each one of those is a socketed transistor and they were recently installed to put correct PNs on the board. Maybe one is failing? More likely they need to be tested and then soldered in place.

Pic 4 is a bonus.

old_tv_nut · #**114** 04-30-2026, 05:21 PM

When you have a symptom coming and going, it is not a failed transistor.

Tap, press and wiggle things to look for an intermittent connection.

TinCanAlley · #**115** 04-30-2026, 05:40 PM

Quote:

Originally Posted by old_tv_nut

When you have a symptom coming and going, it is not a failed transistor.

Tap, press and wiggle things to look for an intermittent connection.

Could it be the transistors in those little metal sockets? I've had the set running for more than an hour now and it stopped wiggling like it was about to lose sync. I wish I could remember better, but when I first got the set it had horizontal sync issue, so I put in a new board. It didn't work and I wound up swapping the oscillator transistor from the old board and it worked. Put the other one back and it stopped working. So the current board has the original transistor. Maybe I should look for a new transistor. I'll see if I can find the one that was on the new board and test it. I have a box of old parts, so I think I still have it. Been 13 years and the brain isn't serving up memories like it use to.

consoleguy67 · #**116** 05-01-2026, 06:43 AM

The picture looks like a typical Zenith; sharp, with great color!

TinCanAlley · #**117** 05-01-2026, 10:41 AM

Quote:

Originally Posted by consoleguy67

The picture looks like a typical Zenith; sharp, with great color!

Now if I can only get the horizontal sych working again.

TinCanAlley · #**118** 05-01-2026, 11:49 AM

I think I found the horizontal sync issue, maybe. The 755 horizontal driver transistor tests good as far as not being shorted, etc. However, the hFE is zero. According to the specs of the ECG157, it's replacement, it should have a minimum hFE of 30.

Ordered an OEM replacement.

TinCanAlley · #**119** 05-01-2026, 12:06 PM

I just reread the IF detector output adjustment again. I waited for someone to chime in, but I'm just going ahead and do it. I realize now the set needs to be on while doing this as the voltage I'll be inserting is 7.2V and I'm adjusting for an output of 8.5V. Not an electronics wizard, but I don't think that is possible without the set's power supply having a hand in it.

The insertion point is on the line going between the AGC connections of the IF and Video Processing Modules. SM says normal voltage of the AGC is 5.79 to 7.51V. The voltage being measured while adjusting is on C1 (detector output).

Not sure, but to be on the safe side, I might put a capacitor on my positive lead of the power supply to keep any DC from heading up my way.

Wish me luck.

old_tv_nut · #**120** 05-01-2026, 01:20 PM

You do not need or want a capacitor in the external power supply lead. That would defeat the insertion of 7.2 volts. The 7.2 volts is to force a known AGC voltage in the system when making the adjustment of the output.