Zenith 25EC58 Ghosting Problem - Videokarma.org TV - Video

ghjkl67 · #1 06-05-2012, 11:39 PM

I started two threads like this relating to the same issue before at both Videokarma and antiqueradios, but haven't had much success. I figured I will give it one more try before I give up.

This thread is for a Zenith Chromacolor II with a 25EC58 chassis and space command.

You can see in the pictures the trouble I am having. Ignore the blurriness and just look at the bright white outline around some of the images. In one of the pictures, you can actually see the horizon start to go through a person's body in the foreground. The bright outline is most distinct when there is a large contrast in the same frame, like a dark object on a light background. Also notice that there is a black outline on the left side of the image as well. I thought this issue could just be because of the VCR or tape, but it does the same thing with a cable box also. Tuning and AFC doesn't help.

A repairman looked at it and noticed that the focus control on the back of the set seemed to adjust the brightness. He told me to replace the HV tripler or the focus dial. I tried both and neither of them worked. The focus still controls brightness. I am not sure if the issue with the focus control is even causing the initial ghosting problem.

Most people at antiqueradios.com told me that it might be caused by a bad electrolytic capacitor somewhere in the set. Since I have never actually troubleshooted a circuit board at a component level and since my meter is very inaccurate when set to ohms, I was hoping someone here at videokarma.org could give me a more definite answer. I don't know where to even begin looking for a bad cap and am not sure if that is even the problem.

Feel free to sell me any parts for this television that you think might help. At this point, I am willing to try anything. Also check out my Videokarma classified thread for a 9-57-VCR module that I am looking for.

http://www.videokarma.org/showthread.php?t=254352

Zenith26kc20 · #2 06-06-2012, 09:50 AM

It's been a long time since I worked on one of these but I remember a small electrolytic on the AGC line going to the tuner that would open and cause this. An ohmeter won't do to much for those electrolytics. You would need a ESR meter to really check them.

Zenith26kc20 · #3 06-06-2012, 09:56 AM

Also, be on the safe side and change those white capacitors in the high voltage stage. They regulate the high voltage and when enough of them fail the neck gets cracked by too much high voltage. A warning of this coming is the set develops a width problem and the turn on "crackle" gets very pronounced. Try to get the original Zenith replacements. I can't figure why the focus affects brightness, unless the control was hooked up wrong in the past or possibly the CRT socket has some corrosion on the focus pin to the spark gaps in the socket. Be careful moving the tuner assembly around. If I remember right that set uses an incandescent based optocoupler to turn on the triac in the remote circuit and age makes it fragile and the set will go dead when it fails.

ghjkl67 · #4 06-06-2012, 10:23 AM

I know where the tuner is, but I am not sure where to find the AGC line. Is it on the AGC module? If I am correct, I believe it is also called the delay line. It is a large coil under the chassis. If that is what you are referring to, I checked its continuity and it tested good. It isn't an electrolytic cap, so you are probably referring to something totally different. Have you seen this symptom before? Do you think the focus control is related to this ghosting issue?

Since I probably won't be able to pinpoint an exact capacitor, is there any larger component where the capacitor is on that I can replace? I would rather replace an entire piece, but I don't know what the AGC line looks like.

Also, thanks for the advice on replacing the white caps. The television crackles a lot, even after it is started and warmed up. Quite often, there is a light snap, and a black horizontal line flashes on the screen. I don't see any width problem though.

Zenith26kc20 · #5 06-06-2012, 03:08 PM

the tuner is a small silver box on the front assembly where all the tuning wheels are located. It is a bit of a bear to remove the whole assembly but the tuner may be visible without removing the entire assembly. its about 3 inches by 5 inches and has a couple of coax cables connected to it. Look at the wires going to it for a pin with a small (usually silver) electrolytic about 1/2 to 3/4 inches long to ground on the other end. I'll try to find my EC schematic tonight and look for the cap and see if I can help narrow it's location a bit better.

ghjkl67 · #6 06-07-2012, 09:58 PM

I think this is the tuner, but please correct me if I am wrong. If the capacitor is on this piece, I will probably just have to replace it, rather than finding another tuner. Tuners are so hard to come by. I couldn't quite understand when you described where to find the electrolytic. Is it on the tuner itself, or somewhere else near the tuner?

ghjkl67 · #7 06-10-2012, 07:55 PM

I noticed that the tuner uses an RCA plug that goes to the chassis. Would anybody recommend plugging a composite video signal directly to this terminal in the chassis? I did not try it yet because I was afraid that it would damage something or short something out. If I can bypass the tuner entirely, that would tell me if the tuner is causing the picture problem.

Electronic M · #8 06-10-2012, 08:31 PM

I highly doubt that the tuners RCA jack carries composite video. That RCA jack is most likely the IF line from the tuner to the IF strip. If you connect composite video to that line you will likely get no reception or video at best, or damage things at worst.

ghjkl67 · #9 06-11-2012, 11:39 AM

Thanks for the tip. The last thing I want to do is create another problem with the TV. Any chance of bypassing the tuner?

Electronic M · #10 06-11-2012, 02:46 PM

If you have the schematic(don't try this with out one) look for a diode called the video detector. If the signal voltage on the output of the detector is about 1V your can inject composite video directly to that point by doing the following...Take a volt meeter and check to make sure that there is not any voltage between the outer (ground) lead of the video cable if it is more than a volt or 2 give up, but if it is less than that connect the outer lead of the video cable to chassis ground (I recommend using a video source for this that you don't mind damaging until you have it working) then measure the voltage between the center pin of the connector and the output of the detector if it is still under 2 volts try connecting it and see if you get video(you may want to insert a small capacitor between them instead of using a direct connection). If not or if black and white are inverted then it either can't be done or you need to use one of the video amp schematics designed for this that if floating around on the net and build a video amp.

None of this is for the non technical and even with all of my experience I tend to shy away from this approach for fear of causing damage and because it is usually easier for me to get acceptable video by fixing bugs in the tuner and IF in of most of my sets. I've only done direct video injection for testing and with equip that has designed to inject a test signal into a TV and not be easily dammaged.

ghjkl67 · #11 06-11-2012, 02:53 PM

This method is way out of my reach and I don't have a schematic . Thanks for the suggestion anyway. I am going to try looking for the bad cap that the other member in this thread described.