Zenith 25DC56 Restoration

[BeamT]

Greetings,

I have been looking for some time for an early 70s Zenith to restore and finally found a 25DC56 on FB Marketplace. The model sticker on the back is hopelessly faded. I believe this could be the Medford D4754W or the Bennett D4754W9. In the Zenith dealer pictures I can find, they appear identical, one being an early 1973 model, the other later in the year. I've not been able to locate any model information inside the set beyond the chassis stamp. The tripler, focus divider, and all of the safety caps are original.

Sams lists an 2 Photofacts 1312, and late production 1375.
Is there a way to determine which build it is?
Is Zenith CM-121 correct for either build of this chassis?

I'm hoping to get some service information before I start....

More pictures to follow...

Thanks in advance!

[Electronic M]

I just got a metal table model of this that's dead. It's a ways down my priority list though. I think the chassis may have been covered in multiple service manuals by Zenith.

Any manual for the chassis should at least be close even if there are production differences. Look for an original paper Sam's on the used market it'll be cheaper and better than the lousy xeroxes and scans Sam's sells on their website.

[zeno]

The style of set was only in the D- line brochure. as a D4754W.
The -9 means a later production with usually minor changes. They
are usually in the HV.
CM-121 sounds right BUT they may have supplements
like CM121 -S1 etc. Only the Zenith index will tell you.
Same idea with Sams The later # MAY cover the -9 sets.
Sams also have PCB's that are not on the web & you need.
Best bet is find an original Sams 1375 paper copy. A vast amount of these
sets can be fixed with any SM. Even the CC2 chassis is almost the same
just wrapped different !

BTW you HAVE to change ALL the 22-5001 with the right ones if they
are white. Orange ones a good. Change them dont test them.

73 Zeno
LFOD !

[JohnCT]

Originally Posted by zeno

BTW you HAVE to change ALL the 22-5001 with the right ones if they
are white. Orange ones a good. Change them dont test them.
:
LFOD !

Do not overlook this advice, and don't even run it without changing those caps first. I was there when these first started failing, and boy did they drive us nuts.

Otherwise the early horizontal SS chassis was a very solid and reliable design. Most of the problems was the weak design of the module to chassis contacts. Back in the day I used to Dremel all the module contacts of these and the later vertical chassis with a wire brush and give them a thin coat of dielectric grease to avoid getting callbacks for random brightness/tint/contrast, etc. changes.

John

[BeamT]

Thanks for the replies! I haven't powered up the set yet and wasn't going to until I had some more information and the safety caps. All 5 are original. The Zenith 22-5001 capacitors appear to be scarce now. From what I have been able to learn the Orange Drop version was a .0018mf @ 1.6kv (715P Series)

Mouser appears to have a CDE 715P Series Orange Drop 1800pf @ 1.8kvdc 5% tolerance. Mouser 598-715P1825800LC (I'm not sure this is actually 1.8kv)

Mouser also stocks a Panasonic PolyPropylene "RF Microwave Film Capacitor" 1800pf @ 1.6kv 3% tolarance. Mouser 667-ECW-HA3C182HB.

Would either of these be an acceptable substitute?

This set also has the original 212-139 Tripler and stick focus divider. Does anyone have a Zenith Cross Reference to a newer Zenith Kit that combined the two? Maybe a ECG/SK/NTE Part number? Was there such a part that combined the two?

[JohnCT]

Originally Posted by BeamT

Mouser appears to have a CDE 715P Series Orange Drop 1800pf @ 1.8kvdc 5% tolerance. Mouser 598-715P1825800LC (I'm not sure this is actually 1.8kv)

Mouser also stocks a Panasonic PolyPropylene "RF Microwave Film Capacitor" 1800pf @ 1.6kv 3% tolarance. Mouser 667-ECW-HA3C182HB.

That Orange Drop is 800V, the Panasonic is 1600V. I would think the Panasonic would work but I don't know exactly how much voltage is across that cap in circuit.

John

[zeno]

From 1978 parts book

212-139 subs to 800-791 tripler kit $32.07
It dont say whats in it. Also be sure be sure there is no dash number
after -139
As for the divider get the 63- number & I will look it up. Pretty
sure the divider was different than the one for hybrids.
Also watch the CRT socket. If too much HV they arc & get toasty at the focus pin.

Now one other thing. Several sets back in the day & a few on this forum
came out perfect BUT they had jail bars. Never found why. I suspect
its from subbing the tripler to an ECG etc. Try to keep it OEM & save
the old parts.

73 Zeno
LFOD

[BeamT]

Thanks John, Zeno!

I have the capacitors ordered from Mouser. There is a scattering of 22-5001 on the auction site but only in bundles of other parts that will do me no good with this set.

The Tripler in the set is a 212-139, no dash after the 139. So the 800-791 Kit should be correct if I need to go that far.

The Focus Divider in the set is a 63-9012-02. Sams calls for a 63-9896. Is there a revised kit for it?

Thanks again...

[zeno]

63-9012-02 subs to the -1 $10.05
There is also a kit for hybrids.That has a long list of subs in the pkg.
You would have to see that to see if it subs. Its an 800-616 I would guess
not because hybrids run 25 KV and yours 31,500 KV IIRC. Bigger resistor.
Finally there is the ECG HvDiv-4. Also a long list of subs including yours.
I would trust that.

I recommend the following after capping it, before full power up.
Check the hoz out transistor case to GND. Should be very high ohms.
Next take the 2 screws out & run the set at @60 VAC a few hrs.
There is an oil filled cap ( ovalish one). In the day not many went
but after 45 yrs they are often popping after a short time.
Then put screws back & test.

73 Zeno

[BeamT]

I wasn't able to find enough Zenith 22-5001 Safety Caps at the moment so I replaced them with Panasonic ECW-H16182RJV from Mouser. Polypropylene, 5%, non-inductive, high frequency circuits, flame retardant. Its getting harder to find capacitors with longer leads. All 3 capacitors in my set were originals as is the 212-139 Tripler and the 63-9012-02 Focus Divider. All three of the original safety caps measure 200-300pf high and no leakage. All 3 of the new Panasonic caps measure right at 1800pf (.0018uf)

A couple of signs the set has been worked on, the horiz hold stop has been clipped off, and one of the plastic pull strips was missing from under the 9-57 Horiz Module, laying off to the side of the chassis. One of the power supply diodes is different than the other. Missing one of the chassis to cabinet screws. I'm pretty sure this is a 25DC56 Early Production.

I followed Zeno's advice:

1- Replaced the 3 Safety Caps
2- Measured from the HOT to ground, no low resistance, HOT, damper, and both boost diodes test good. Low voltage diodes test good. 24v Regulator tests good. B+ rails look good.
3- Removed the HOT screws and brought the set up to about a 60v line. Sound came up right away. I let it run for a couple of hours. Screws back in.
4- Brought the set up to a 117v line. HV came right up to 25kv. Only a dull raster, no snow. Turned out to be a very dirty Set-up switch. No amount of flipping back and forth could get it to go into either set-up or norm. Finally had to clean it with DeoxIT.

I realize I could have just plugged this TV in but I would like to take my time.

I appreciate all of your advice!

[JohnCT]

Originally Posted by BeamT

I realize I could have just plugged this TV in but I would like to take my time.

Maybe! I can't recall exactly, but I'm sure a lot of those caps would check OK but still give the chassis fits (like int shutdown).

John

[BeamT]

This is what I have so far:

I touched up the static convergence, grayscale is not too bad, and it seems to track ok, dark to light. Its needs a good patient set-up.

The picture came up initially, pulled up from the bottom. I can adjust the Vert Size to fill the screen. The Vert Linearity is off, stretched at the top. When I adjust the Vert Lin, as I approach a better overall linearity, the Vert Hold becomes unstable. I pulled the Vert module, cleaned the pots, transistor sockets, the module connectors, re-tensioned the connectors, and put it back in. The Vert Hold is better but I'm still stretched at the top. Its not drifting at all.

I'm having a hard time getting the contrast down without having the brightness up too high. The contrast control acts more like a brightness control. I'm using a Sony DVD player and I wonder if I'm driving this too hard?

The HV comes up right away to 25kv and then over 30min it will very slowly gain another 500v and then stay stable. I have a really good focus point with the focus control about mid-range, but I have to wait for it to settle down to adjust it. Nice crackle on power-up, totally quiet after that. Horiz hold is rock solid.

So, plenty to work on...