View Full Version : My new Zenith 25MC36!!

03-09-2017, 06:02 PM
Just went and picked this up today in the really "good" part of town for $20. I'm convinced that it has no less than 6 million hours on it, the cathodes are absolutely burned black. I brought the CR70, and at first, it showed a dead HK short. I reseated the socket and was able to get a better connection. The CRT is very tired, but it does have emmission. More to come later. On the road home.

Please pardon my stupidity on the title. I obviously meant 25MC36, not 56.

Also, a Huge Thanks to Madrad45, who found this set for me.

03-09-2017, 09:41 PM
Been checking toobs and cleaning this thing for the past three hours. One things for certain - it needs a buttload of work. The CRT socket is toasty, the blue lateral and purity ring assembly is toasty, it needs no less than 14 tubes replaced. Amazingly, the picture tube responded very well to the Auto Restore function on my CR70. I haven't even made it to the tuner yet either, with has a frozen motor.

Electronic M
03-10-2017, 06:54 AM
Zeniths have a hard time coming to to terms with not being immortal....Sets that don't die easy tend to accumulate hours....My 71' hybrid probably comes close to what that set has seen.

03-10-2017, 07:39 AM
I was up past midnight replacing the CRT socket and freeing up the tuner motor. Just gotta find 14 tubes now.

03-10-2017, 09:52 AM
That's a fine chassis! They can make a good picture. Watch out for the power supply electrolytics and vertical integrators. The integrators are notorius for linearity problems. If it looks like a keystoned yoke, check your electrolytics, especially if it has been brought back to life for a while. The electrolytic feeding the vertical output transformer dies and will look just like a bad yoke.
My NC chassis CRT at it's neck base is black too!
Space Command too! A lightweight TV to carry! (RIGHT!)

03-10-2017, 10:28 AM
Yeah, I'm not really looking forward to replacing those capacitors. It did produce a dim, confetti filled raster when I bought it, but I have not turned it on since.

03-10-2017, 11:26 AM
For $20, I would have bought and made room for that one too! Congratulations Dave. Rare is the Zenith of this vintage that does NOT produce a raster. Enjoy!!!

My only TV-stereo combo is a non-remote 25MC36 in the same style-finish as yours. That one I have not gotten around to power up yet, so this is fun to watch. :yes: Make sure to stabilize that horizontal efficiency coil so it doesn't pop-apart on you, at the rear left corner under the 6BK4

That late 1965 chassis was followed by two more roundie chassis. It is actually a like the 24MC32 chassis with a pincushion circuit added for the "new rectangular tube", also omitted on the last similar round-screen chassis 24NC31, etc. The 25NC38 was the second series of rectangular tube TVs. Another workhorse predating the 20X1C36/38, first to use the 6HS5 pulse regulator in place of the 6BK4C-6EL4A shunt reg.

Use an old toothbrush to clean the top-sides of the terminal strips and make sure they are still soldered at the chassis grounds. High hours combined with some vibration challenges even the most robust dip-soldered service-saver chassis.

EDIT_ looking at your chassis, the efficiency coil is under the chassis and next to the damper tube.

03-10-2017, 11:51 AM
I was up past midnight replacing the CRT socket and freeing up the tuner motor. Just gotta find 14 tubes now.

Checking my stock - if I have any you need, they're free for postage, as I have Zero Zenith TVs...9 radios though..

03-10-2017, 03:57 PM
Checking my stock - if I have any you need, they're free for postage, as I have Zero Zenith TVs...9 radios though..

I found all the tubes I need with the exception of a 6JS6 and two 6JH8's. I'll have to see if anybody around here has before I ship any in.

03-10-2017, 06:32 PM
Is there any particular reason why I can't find a SINGLE 6JH8 in my entire big box o' tubes? Been searching for a 6JH8 for hours, and it's hopeless. Found about 357 6GH8's though.

03-10-2017, 07:01 PM
Some days you get the bear; others it gets you! I've had that feeling with valves too.

03-10-2017, 09:35 PM
Is there any particular reason why I can't find a SINGLE 6JH8 in my entire big box o' tubes? Been searching for a 6JH8 for hours, and it's hopeless. Found about 357 6GH8's though.

I think I might have those two tubes you're looking for in my tube stash somewhere, I'll take a look when I get a chance, seeing as I'm probably never going to find an old Zenith Tube powered Color TV anytime soon near me, I might as well give the tubes to someone who can use them.

03-11-2017, 11:26 PM
Got the Zenith up and running today. It needs A HOLE LOTTA HELP though. I'll have to go in and check out the sweep circuits, as you can see in the pix, they need help. I'll be getting two NOS 6JH8's tomorrow, I'm sure that will help the color. May have to adjust the 3.58 MHz oscillator. Making progress though. The third pix is after a very crude convergence job and vertical height/linearity adjustment.

Electronic M
03-12-2017, 10:23 AM
You may want to recap he convergence board and replace the convergence rectifiers if the convergence won't improve....Though that raster distortion seems indicative of additional problems that may be affecting both systems.

03-12-2017, 11:44 AM
Just discovered a very annoying problem that has me stumped. I noticed that when I tried to adjust fine tuning from the front, nothing would happen. I replaced the little rubber belt, thinking it wasn't turning the little gear inside the tuner. However, the little gear inside the tuner isn't even turning the tuning slugs at all. It, just free wheeling, and not adjusting the fine tuning. WTF?

Electronic M
03-12-2017, 12:12 PM
The single gear that tunes the slug gears is usually (in most tuners) on a mech that only meshes it with the slug gears when turned or pushed and turned. See if you can find such a mech on the tuner....It may be gummed up.

03-12-2017, 01:52 PM
I thought about that, but I could find no such mechanism. The whole thing is on a pulley system, and I don't really see how pushing it in could work. I did manage to fine tune channel 3 - by taking the whole tuner out and adjusting the slug with a screwdriver. I worked on the set for about 3 hours today, and check out the image that I finally got. The purity is still bad though.

Admins, would it be possible to change the title of this thread 25MC36 instead of 25MC56? That misteak is driving me crazy.

Electronic M
03-12-2017, 02:40 PM
Nice work.

Took care of that title change for ya.

Do you have a separate serviceman's degausing coil? If so now is a good time to use it. If not You can make one out of a dead BPC CRT set easily.

03-12-2017, 02:53 PM
FT should be push & turn otherwise it will engage with normal
channel selecting. Dont remember them all though, SGVG
tuners most used the clothespin style FT mech or the wish bone.

Dont remember on non-SC sets but SC sets have a rocker switch
to kill the motor when it hit a channel. They would gum up
& sometimes break. Also had another disc with twist tabs for
channel skip. Also on some a blank channel was used as a TV
off position.

73 Zeno:smoke:

03-12-2017, 03:22 PM
I'll take another look at the fine tuning assembly when I get back home. Thanks for the title change, Electronic M. And yes, I do have a degaussing coil, silly me didn't even think to use it. Unfortunately, I won't be able to go get those 6JH8's today, maybe one day this week after work I'll be able to go.

03-12-2017, 03:36 PM
April and July 1966 PF Reporters have several "Symcure" cards for your set, starting on page 8 or 9:'s/PF-Reporter-1966-04.pdf's/PF-Reporter-1966-07.pdf

We always annotated them on the Sams - saved us a callback or two, I'm sure.

03-12-2017, 06:59 PM
I finally stopped being an idiot and PUSHED IN the fine tuning knob, and voila. Perfectly clear raster with excellent colors and good convergence. I'd say this TV's done for now. It's a perfectly watchable picture. I used the degaussing coil while the NTSC generator was set on red raster, and the purity instantly became perfect before me eyes.

Jon A.
03-13-2017, 04:11 AM
Couldn't hurt to give the pincushion controls a little tweak.

03-13-2017, 10:07 PM
For such a high hour set, I'm honestly very thankful that it works this good. When I saw it in person for the first time, I was tempted to just walk. But I didn't, and I'm glad. The true quality of ZENITH has really proved itself to me on this set. I wasn't expecting to get it working this good, if ever. The quality of ZENITH has exceeded my expectations. About the pincushion, I did change the 6C4, but I forgot to adjust the control. I'll do that when I change the 6JH8's, which I'm getting tomorrow.

03-14-2017, 10:35 AM
check any new tubes going in for short before installing.

03-14-2017, 10:57 AM
You are lucky! The 25MC36 was a wonderful set. Worked on many over the years, and they are a well designed workhorse!

03-14-2017, 03:19 PM
check any new tubes going in for short before installing.

Oh yes, I always check every single tube in the set before I even turn it on.

03-14-2017, 05:55 PM
Alright - now which one of y'all heathens came over here and broke my TV? I got home today, turned the set on, and there is NO COLOR! I replaced the 6JH8's with NOS ones, and I got the B-Y demodulator to work, but the R-Y is still dead. What gives?

03-14-2017, 07:18 PM
I don't have any idea how I was getting deflection on the vectorscope with a black and white image. But then, I probably didn't do something right.