Hello everyone! Just picked up a really nice zenith 25CC50 (model T998DE11), see photo. Just had to change a few tubes, and the set works great. Since a new CRT was installed in 1985, and the TV was put way shortly after that. This set does have an annoying problem however. The vertical sync seems to "flicker" at the lower half of the screen. Sometimes it acts up, sometimes it does not. I replaced the vertical output tube, the vertical oscillator, sync sep, and even cleaned all of the vertical controls. It still acts up every once in a while, can be kind of irritating. Any thoughts?

Welcome to the dark side!

Your zenith set is quite the looker. If its been sitting then most likely the problem your experiencing is due to failing capacitors. They have dried up while in storage and have started leaking current through them. This can cause the erratic behavior in the vertical section. I will usually re-cap an older set like this one if I plan to use it regularly to make it as trouble free and reliable as possible. This is referred to as re-capping and you'll see it alot.


There are many TV nuts here around you. Feel free to poke around and ask questions

Again, WELCOME!

Oh ok, thanks for the advice, much appreciated. Re-capping this particular set looks like it might take a while, lol.

The zenith chassis have plenty of room under them. If you want to save money, use individual caps for the power supply and don't worry about trying to find replacement filter cans. If you haven't worked on electronics before, don't fear. Edit your profile to show your location and someone may be nearby to help you out.

The caps to replace are the electrolytic and wax caps. Most sets of this recent age don't have wax caps in them anymore, but you can never be sure. Re-capping the set may only take you an hour or two if you already have the parts on hand. If you don't have them yet, then I recommend either justradios.com or tubesandmore.com for replacement capacitors. I believe most of this chassis just unplugs from everything else. 4 bolts to remove the chassis from the cabinet.

Just make sure to DISCHARGE THE CRT before trying to remove the chassis, or you'll get a BIG shock when trying to disconnect the CRT from the high voltage cage. Attach a clip lead to the chassis, and the other end of the lead to a long, well insulated flat-blade screwdriver. slip the screwdriver under the rubber anode cover and move it inward until you feel it stop . It should be touching the anode now. let it sit there for a minute or more to fully discharge the high voltage. Now you should be able to take the chassis out without fear of shock.

After you pull the chassis, I would also recommend checking to see if there are still residual voltages on the power supply filter caps before trying to unsolder them. if there is anything more than 500mv, discharge them using a 100 ohm 1/2 watt resistor to the chassis. Otherwise you could get a big spark across your soldering iron when trying to unsolder them. Also be very cautious of polarity. Some of the filter cans may have a negative ground or positive. installing power supply capacitors backwards may cause them to overheat and explode or smoke the chassis.

Overall, the set is well worth the trouble. Zenith TV sets by far are the most reliable and produce the best picture of any vintage color set. Get it working properly and it will serve you well.

I also have seen that problem, and I found that it's bad ground connections where the terminal strips are soldered to the chassis tabs; usually at both ends and sometimes in the middle. Do all of them all over the chassis, and you will save many future headaches!

Charles

Thanks again for the advice, this set has superb color, brightness, and contrast, with superfine resolution! So, the only real problem is that annoying compression or flicker on the lower half of the screen. I am definitely keeping this one. I have found earlier, and later Zeniths, but never quite one from around the 1970-1973 timeframe.

Nevermind, trying to get pics to upload

Very nice looking TV.
I think there is a zener diode on this set off the 24 volt supply to one of the vertical transistors. I remember having a flickering screen and found it to be a leaky zener on a 19CC19 chassis. I'll check my schematic on this chassis and let you know for sure.

Oh, I see...Yes, let me know what you find out about the zener! Like I said, the set is in superb condition. When I got it home I noticed only two of the tubes (not including the CRT) had ever been changed, and it had just a light coating of dust on the chassis!

I pulled out the service manual for this chassis. Its very similar to a 20CC50 chassis which I have a few of. I was thinking this was a more solid state type chassis like a 4 tube hybrid at first. Did you check any of the transistors on this set? They can be pulled from the chassis as they are plugged into sockets. Check the 121-587 vertical blanker. The sencore super cricket is an excellent tester to get off ebay if you don't already have the ability to test solid state devices. Video amp transistors can cause vertical jump too. Some of the transistors in zenith sets can cause problems in other circuits you would least expect unless you had a schematic. Check those first as its an easy test.

I dont have a transistor tester, but I will try to swap them, since there is an NTE dealer near my huose. They carry some of the older obsolete transistors as well. Will let you know......

Might try a little cleaner on the transistor sockets as well.

Bill R

I'll try some cleaner first, see if that fixes it........GOOD ADVICE, THX...