Zenith Manual CM-108

[Penthode]

I have been looking for the original Zenith service manual for the 24MC32Z chassis. I believe the CM-108 carries this chassis.

Does anyone have a spare manual or a photocopy of this?

Thanks in advance.

Terry

[Steve K]

Terry:

I have the CM-106 manual which has the 24MC32 (no Z) chassis. If you think that is the same I can photocopy it for you.

Steve

[Penthode]

Thanks Steve.

I recall last repairing a 24MC32 last in 1972! I do not know how or why I can recall this but I believe that there may be a minor difference in the luminance amplifier circuit between the 24MC32 and the 24MC32Z: I think one had DC coupling and the other had AC coupling with DC restoration.

I haven't dealt with Zenith color sets since 1980. In the last 40 years I have learned a lot and forgotten a lot but for some darned reason the above has stuck in my brain.

I may have recalled this incorrectly. So for some fun, if you could post the luminance amplifier section and we compare with the 24MC32Z (if someone has it or I trace the circuit), we will see if my 40 years of memory is intact.

Thanks,

Terry

[Kevin Kuehn]

Terry,

Here's part of the 24MC32Z schematic. I don't have the complete CM-108 service info, although I do have CM-107 which includes the 24MC32. This schematic I just posted is part of a condensed 1961-1975 manual I recently acquired. I doubt you'll be able to see much detail on the small forum picture format. If you PM me an email I can send a better scan.

[Kevin Kuehn]

Maybe this is more focused on the part you're interested in?

[Steve K]

Terry:

Sorry I couldn't get back to you sooner but it has been a long day at work. Based on the schematic that Kevin provided it looks like there are a few differences to the circuit. It doesn't look like the CM-106 manual will be of much help to you.

Steve

[Penthode]

Thanks Steve and Kevin.

Yes many years ago I recalled a difference. The 24MC32Z appears to have a DC coupled luminance amplifier. I am still curious however if my recollection is correct with regards to the AC coupling of the 24MC32?

I am in the process of refinishing the cabinet of the set I recently found and shall get around to the chassis shortly.

[Kevin Kuehn]

Here's the 24MC32.

[Penthode]

Thanks Kevin. My brain never fails to amaze me.

Actually I recall it was 39 years ago (1973) a 24MC32Z arrived at the TV repair shop I worked at on weekends when I was in my final year of high school. It was traded in for a brand new Zenith. The set had no raster and the customer was told the CRT was bad.

The circuit diagram referred to diagnose the problem was the 24MC32. If you look at the 24MC32 diagram, you will find a d.c. blocking capacitor. The HV measured okay and from the diagram it was concluded the CRT was indeed bad. I was offered the TV if I would remove it along with some other old sets from the shop. This was my first color TV.

My Dad helped me fetch it home and I obtained the 24MC32Z diagram. I saw the DC coupled cathode follower and concluded that maybe the cathode follower was bad. I changed the tube and was gratified to find a bright raster on the screen. I eventually gave the set to my grandmother who lived up the road and the set gave many years of faithful service.

I have not looked at the diagrams in 39 years or so. The memory of the this specific difference in the production runs must have permanently set in my brain.

Terry

[Kevin Kuehn]

Yep it's funny how those little things can stick in a guys brain. Then again there's stuff I did last week I can't remember...
It must be like that selective hearing that my wife claims I have.

Is that set you found last month a 24MC32Z or 24MC32?

[Penthode]

Originally Posted by Kevin Kuehn

Is that set you found last month a 24MC32Z or 24MC32?

Kevin,

Thanks for the illustrations.

The one I found was the 24MC32Z. It is the same as the one I had many years ago.

It is interesting the that the 24MC32 blocks the DC video component to the CRT and there is no DC restoration. Perhaps this was because this was Zenith's low end chassis and they trid to get away with it? It must have been problematic which led to the revision.

Anyhow, I am glad they restored the DC component on the "Z" chassis.

Terry