Quote:
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.
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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.
Quote:
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.
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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.