Zenith Roundie Changes, Improvements, & Issues

[bhegges]

I am digging into two Zenith TVs I picked up. In searching old topics and studying the schematics I am curious what others have experienced, what was good, what should I look out for?

The first chassis is a 25MC33 which I believe was their 4th-ish generation color roundie chassis from about 1964 and the second is a 24MC32 which followed. I have read that vertical output transformer failure may be common and some of the plastic coil bobbins can disintegrate. The 24MC32 chassis does not have a horizontal linearity coil and also have different part numbers for the horizontal and vertical output transformers.

Zenith had just a few years selling color roundies before rectangular sets came out. Were the above noted changes along with other changes for cost savings or did each color roundie chassis provide some incremental improvements?

[Electronic M]

The 2 letters in the chassis number were model year M and the color TV chassis designator C. The first digits are tube count and the last were feature level. I've had both a 25MC33 plain Jane console and a 25MC43 remote set and the main difference was addition of the socket and wiring for the remote reciever chassis and the motors on the tuner and tint pot.

Zenith had 5 chassis years of color roundys the J, K, L, M and N year chassis.

The 24MC chassis you have has 1 less tube it could be a cost saving measure or a case of engineering trying a SS part and needing 1 less tube. It should be the same year as the other set you have.

The linearity coil forms can be brittle and or melted treat them with care...Zenith's don't eat flybacks like RCAs so I typically don't tweak output current unless it's fairly out to lunch. Even into the rectangular era Zenith's have been noted to have vertical transformer failures.... I've been lucky enough to not experience it yet (knock on wood) having at least 9 different tube color Zenith's come through my hands.

[trinescope]

The Zenith vertical output transformers have a habit of developing shorts between the primary and one of the convergence windings, which essentially shorts the vertical stage B+ to ground through all the low resistance parts in the convergence circuits. For whatever reason the insulation on the wires as well as the paper tape materials break down. I had this failure on a CTC-5 as well, due to the way the windings were done it allowed all four secondaries to short to B+, which smoked all three DC convergence adjustment pots. I have already rewound that transformer, and is working, just need to find a working substitute for the three pots (100 ohm with a tap at 75%).

[Jeffhs]

Originally Posted by trinescope

The Zenith vertical output transformers have a habit of developing shorts between the primary and one of the convergence windings, which essentially shorts the vertical stage B+ to ground through all the low resistance parts in the convergence circuits. For whatever reason the insulation on the wires as well as the paper tape materials break down. I had this failure on a CTC-5 as well, due to the way the windings were done it allowed all four secondaries to short to B+, which smoked all three DC convergence adjustment pots. I have already rewound that transformer, and is working, just need to find a working substitute for the three pots (100 ohm with a tap at 75%).

I am amazed Zenith color TVs even had this problem (shorts in the vertical output transformer) in the first place. I would have thought Zenith's quality control department would have been aware of this and corrected the problem long before it reached this point. Zenith, after all, was one of America's best radio and TV makes (if not the best, hands down) since the company's founding in 1918. Why they would allow defective components such as vertical output transformers knowingly get into their televisions (unless it was a one-time fluke) is beyond me. I always thought their QC (quality control) division would have caught this problem before it became anywhere near this serious. If Zenith were not already as well-known a company as it was, I would think a problem like this would have put the heck of a dent in its reputation. Zenith's decades-old slogan, after all, was "the quality goes in before the name goes on"; a problem like this would have all but ruined the integrity of that slogan (and put the company out of business for good) in no time flat.

[zeno]

Zenith VOT's are known to go BUT ....
I worked from 1970 on at one of the biggest Boston area Zenith dealers.
I was also jig man so I would have repaired most VOT jobs on big sets.
In all those years I may have replaced 4 ? of them all the way up to hybrids.
I put more in 9" & 12" Panasonic B&W's, so many we stocked them.

This seems to be an age related problem not overall mileage.
As far as QC goes I dont remember who built them. If someone can look
at one & get the EIA number that will tell who. Dont forget also TV's
were NOT supposed to last 50 yrs. If VOTs are popping 5 yrs down the line
QC probably isnt to blame.

BTW I have the price / substitute guide form @1977 & can check for subs etc.

73 Zeno
LFOD !

[trinescope]

So far every vintage style transformer I have unwound has been constructed of the same materials; enameled magnet wire, some sort of thin, wax coated paper between winding layers, and kraft paper covering the outside of the winding assembly. It's the wax paper layers that get burnt through, creating a carbon path and a winding short when the enamel on the wire fails after age and there is enough voltage stress to cause an arc. Voltage pulses in the vertical output stage can reach up to 2KV or so. Modern single build magnet wire insulation is good for 700V, and Kapton tape for 7KV/mil; these are what I have used so far in my transformer rewind projects. I'm sure Zenith had a few failures at most when the sets were current, these failures are occuring decades after most of these sets were retired by their original owners.

[DavGoodlin]

This problem of vertical output transformers has been reported on L and M series chassis, but not the 24MC32 and 24NC31/Z,
the last roundies in the narrower 1966 and 1967 chassis. While this seems to be an age problem, a difference in circuit designs may give clues.

Checking and comparing the schematics, there are differences between say a 27KC20 using a 6EM7 and a 25LC20 using a 6HE5.
Primarily the 6HE5 has a higher b+ voltage on the plate and the sweep pulse is much higher amplitude.
I have two 1964 models using 25LC20 chassis and I'm not ready to restore them in due to this fact.

Taking a page from utility power transformer testing procedures for turns ratio, isulation test

Borrowing a piece of medium voltage equipment called a "Megger insulation tester," I was able to actually measure a VOT's
primary to secondary winding leakage in the Gig-ohms range, using approximately 1000 volts DC. Using this as a baseline
reading, I should try the following methods to increase this value:

1. Pass just enough DC current through the lower-resistance secondary winding to heat up and drive moisture out of winding
using heat without a potential difference applied.
2. Dip transformer in hot insulating varnish, so voids where moisture may have attacked exposed copper windings are filled.
3. Retesting leakage after using any combination of the above methods.

[trinescope]

Originally Posted by DavGoodlin

This problem of vertical output transformers has been reported on L and M series chassis, but not the 24MC32 and 24NC31/Z, the last roundies in the narrower 1966 and 1967 chassis. While this seems to be an age problem, a difference in circuit designs may give clues.

Checking and comparing the schematics, there are differences between say a 27KC20 using a 6EM7 and a 25LC20 using a 6HE5. Primarily the 6HE5 has a higher b+ voltage on the plate and the sweep pulse is much higher amplitude. I have 2 1964 models using 25LC20 sets and I'm not ready to plug them in due to this fact.

Borrowing a piece of medium voltage equipment called a "Megger insulation tester," I was able to actually measure a VOT's primary to secondary winding leakage in the Gig-ohms range, using approximately 1000 volts DC. Using this as a baseline reading, I should try the following methods to increase this value:

1. Pass just enough DC current through the lower-resistance secondary winding to heat up and drive moisture out of winding using heat without a potential difference applied.
2. Dip the entire transformer in insulating heated varnish, in hopes that voids where moisture may have attacked exposed copper windings are filled.
3. Retesting leakage after using any combination of the above methods.

It happens on the rectangular sets as well.

[bhegges]

Looking closely over the schematics see below for key changes from the 25MC33 vs my late model 24MC32 which according to the SAMS date came after the 24NC31/Z. To my earlier question it does not appear that the 24MC32 was built as a cheaper tier chassis, but if price was the driver it resulted in equal or better performance. It will be hard compare with 50+ year old electronics but someday I will get them running well side by side.

• Combined the audio detector and audio output into one tube, a 6j10
• Removed the horizontal efficiency circuit - curious if there is value in adding this, while no has complained about burning up horizontal output transformers in these sets this would maybe provide some adjustments for further safety???
• Added blanking gate diodes
• Changed the color killer circuit to include removing the 6kt8 triode and replacing it with a diode, this left the 2nd chroma bandpass amp as just a single element tube, a 6jc6
• Changed vertical output transformer and horizontal output transformer, note these are the same as was used in the 24NC31/Z chassis
• Changed the power transform resulting in lower source voltages, 330v vs 410v, there are lots of resistor value changes so cant immediately say components saw lower voltages

On the topic of rewinding vertical output transformer and/or dipping then in varnish is there anyone that currently offers this service?

[Electronic M]

Originally Posted by DavGoodlin

This problem of vertical output transformers has been reported on L and M series chassis, but not the 24MC32 and 24NC31/Z,
the last roundies in the narrower 1966 and 1967 chassis. While this seems to be an age problem, a difference in circuit designs may give clues.

Checking and comparing the schematics, there are differences between say a 27KC20 using a 6EM7 and a 25LC20 using a 6HE5.
Primarily the 6HE5 has a higher b+ voltage on the plate and the sweep pulse is much higher amplitude.
I have two 1964 models using 25LC20 chassis and I'm not ready to restore them in due to this fact.

Taking a page from utility power transformer testing procedures for turns ratio, isulation test

Borrowing a piece of medium voltage equipment called a "Megger insulation tester," I was able to actually measure a VOT's
primary to secondary winding leakage in the Gig-ohms range, using approximately 1000 volts DC. Using this as a baseline
reading, I should try the following methods to increase this value:

1. Pass just enough DC current through the lower-resistance secondary winding to heat up and drive moisture out of winding
using heat without a potential difference applied.
2. Dip transformer in hot insulating varnish, so voids where moisture may have attacked exposed copper windings are filled.
3. Retesting leakage after using any combination of the above methods.

Back at the ETF I was talking to someone who IIRC had a 24NC31 with a bad vertical output transformer...I told him I had a parts chassis and he only wanted an aftermarket transformer because he didn't trust the stock ones...Seems like overkill to me, but then again the audio output transformer was open on my keeper set and I had to scrounge one off the parts set.

Originally Posted by bhegges

Looking closely over the schematics see below for key changes from the 25MC33 vs my late model 24MC32 which according to the SAMS date came after the 24NC31/Z. To my earlier question it does not appear that the 24MC32 was built as a cheaper tier chassis, but if price was the driver it resulted in equal or better performance. It will be hard compare with 50+ year old electronics but someday I will get them running well side by side.

Sam's date is fairly meaningless. They were sometimes a year late in covering some stuff... Sam's was a company that bought electronics and reverse engineered it... Because of that their coverage is far from perfect.

The only way I'd buy an M year being newer than an N year is if the model number has a newer year letter (both the model and chassis contained year letters). Zenith would keep building a chassis with the same chassis number for multiple years if it was successful and a product they didn't feel could benefit from more engineering effort....The 14N22 monochrome chassis that ran into the 70s is a good example of that.

The narrower chassis was a cost saving design. When Zenith went rectangular only they had a wide chassis for premium sets and a narrow one for cheaper sets...The circuit changes you mention all sound like cost cutting methods to me. Zenith wasn't going to go to printed circuit boards to cheapen their TVs in the 60s.

[zeno]

QUOTE The only way I'd buy an M year being newer than an N year is if the model number has a newer year letter (both the model and chassis contained year letters). Zenith would keep building a chassis with the same chassis number for multiple years if it was successful and a product they didn't feel could benefit from more engineering effort....The 14N22 monochrome chassis that ran into the 70s is a good example of that. END QUOTE

Sams coverage was typically 9 mo to 1 yr later. Younger sets were under warranty anyways & if you were factory service you would have an OEM
subscription. Zenith model & chassis ##'s did follow a pattern 95% of the
time. A few examples are
The 14N22 had IIRC a seven yr run, same chassis.
The 25EC58 "flat chassis" was dropped mid year & replaced with the
25EC45 CC2 chassis BUT was used in a few top end combos for F line.
Model ##'s also had a strange mix especially at the high end.
Why was a E4025W 19" CC2 not called a E1925W to reflect the
screen size ?? Later they would, ( 17" = 37, 23" = 45, 25" = 47 IIRC ).

[trinescope]

Originally Posted by bhegges

On the topic of rewinding vertical output transformer and/or dipping then in varnish is there anyone that currently offers this service?

I could do rewinding, I have done three different parts with success; the 15" Westinghouse vertical convergence transformer, a CTC-5 vertical output, and the CTC -10 and up vertical output. As far as trying to re-varnish an existing transformer, it probably wouldn't make a difference. The varnish really only seals the ends of the winding anyway. If you ever try unwinding one of these transformers, the winding turns come off easily except at the ends where the varnish is.

On the subject of replacements, they are all constructed the same way and with those same vintage transformer materials, so going OE vs aftermarket doesn't make a difference as far as reliability.

[Electronic M]

Originally Posted by trinescope

I could do rewinding, I have done three different parts with success; the 15" Westinghouse vertical convergence transformer, a CTC-5 vertical output, and the CTC -10 and up vertical output. As far as trying to re-varnish an existing transformer, it probably wouldn't make a difference. The varnish really only seals the ends of the winding anyway. If you ever try unwinding one of these transformers, the winding turns come off easily except at the ends where the varnish is.

On the subject of replacements, they are all constructed the same way and with those same vintage transformer materials, so going OE vs aftermarket doesn't make a difference as far as reliability.

If by 15" Westinghouse you mean the H840CK15 model, then I'm actually looking for a new convergence transformer for one. The one in my set is still good, but I hear they tend to short and take out unobtainable potentiometers so I'd like to buy one for preventative maintenance.

[Electronic M]

Remote and HiFi tone controls were optional equipment too. I suspect that is what R and H stand for, but haven't tried to decode the model suffixes to know.