Quote:
Originally Posted by drh4683
I started to pull it apart this evening. As you can see in the photos, the TV was backed up against a forced air supply duct. This obviously blew a ton of dust into the TV. Grey dust luckily, it was not a smokers set. I can handle that. I was able to tape the tube layout diagram back together. The loose screws were the top screws that mount the TMA to the bezel. Why those were removed was beyond me. Its a simple key slot mount. Thats why I think the home owner pulled it apart, and rubbed his arm against the side of the cabinet while doing so tearing up the layout diagram.
So far, most of the tubes are shot. Since I never had a 25CC50, I was suprized to see a not so common zenith tube. They used a 6JG5 for the 3.58 mhz oscillator which Ive never seen. Thats a one of a kind tube that you never see in other zenith color sets. Zenith almost always used a 6GH8 for that. I can name every tube used in all the color zenith chassis in my sleep except for this one. The 6LB6 is so dead the filament might as well be open. This is very alarming when I see a dud horizontal output. It means it was running way to hot, and this was a replacement sylvania so something is definitly wrong. The belfuse for the cathode is missing too as well as the 6JK5 regulator. Something strange happened to this TV, yet to be determined.
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Doug,
I have a 25CC50 in a sort-of danish modern style cabinet that's my main living-room set. It was my parents' 1st color set they bought in late 1971, and has been in near daily use except for a year or two around '95-'96, before they gave it to me. I did all the service on it, until I graduated college and moved away in '83, and then they ran it until it wouldn't go no more, and replaced it with a Sony in '95.
When I got the set, the CRT was shot and most of the tubes tested bad. I had another 25CC50 in a metal cabinet with a good rebuilt CRT, but with several other issues, so I sacrificed that one to get my parents' console working.
Anyway, the 6JG5 isn't the 3.58 oscillator. It does use a 6GH8 for that. The 6JG5 is either the burst amp or the chroma bandpass amp. The other tube is the 6E(H/J)7. The rest of the chroma processing is done in the IC on the plug-in module.
On that chassis, the heaters for the damper and HV regulator are wired in series (probably to keep the HV from coming up if the regulator tube is removed). There should be a label on the HV cage door with two specific regulator/damper pairings because of different heater currents. If your set has a 6DN3 damper, you'll need a 6JH5 regulator (2.4 amp heaters).
I've had two problems with the horiz. sweep in the metal-cased set I had. One of them was a 33 or 39k 1 or 2 watt resistor going to one of the plates on the 6U10 horiz. osc/disch. tube going intermittent, and causing loss of drive to the horiz. output. This would toast your 6LB6. I replaced it on my console as a preventative measure. Also, the VDR in the horiz. output circuit went open, causing insufficient width but not overheating anything.
(Sorry I don't have exact values on these parts, I'm going from memory, and I haven't been inside my set in almost two years.)
Another problem with this chassis is the 1.2 or 1.5 Meg resistors around the HV adjust pot drifting out of tolerance. They are 5% tolerance, and when they drift out, you won't have enough range on the HV adjust pot to get 25kV. If you run out of range on the focus control, you'll have to replace the focus divider. If the original one is still in there, it's probably bad by now. It crosses to an ECG HiDiv-4, discontinued years ago.
Also, I've had trouble with early '80s NOS compactrons cracking where the base is attached to the bulb. I've had it happen on the HO, damper, and regulator tubes over the past few years, both GE and Sylvania-made tubes.
I figured that's from sloppy maufacturing, as by that time those tubes would only be used for replacement purposes.
Good luck with your set, if it has a good CRT it'll have that typical excellent Zenith picture.
Mike