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Zenith IF Transformer help, K706 Chassis

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dobulee

Electrical Engineer
Hi Gang - Not sure exactly where to post general Radio Tuner questions...

But I'm working on a Zenith K706 Chassis that has a bad IF CAN in the FM section

The zenith PN is 95-1864

It crosses to the following
Miller 1463
Merit FM-256
Stancor RTC-9046
Workman T654

Does anyone know of sources for these? They come up occassionally on ebay, but the only one currently available (Miller 1463) has a $50 asking price, which exceeds the value of this entire radio :D

Any help appreciated.
 
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Probably bad humor but if the one on ebay sells you ought to part out your unit and sell the other good ones. :)
 
Or find another chassis, to part out? Use the part you need and sell the rest for a profit.
 
Those "cans" are nothing more than a shielded enclosure for a slug-tuned coil. Have you pulled the coil out of the can? I'm not sure what would go wrong with the coil - it's not like high power could burn it up in the IF section. It might be a simple repair. Just a thought before spending $50.

Good luck
 
In this case, the ferrite tuning slug is cracked - either overtorqued or dropped at some point -- , rendering the tunable range of the transformer to be way off. There are other things that can go wrong...the coils occasionally corrode through or fail mid winding, and there are occasional silver migration issues with the internal mica caps.

If there aren't any old radio guys out there with stashes of things they are willing to sell, then buying a similar radio chassis is likely the least expensive route.
 
awesome! Thanks watthour. Often getting small quantities from micrometals or Mag Inc can be a challenge, but Ill give it a try.

To ask a totally rookie question, do the cores readily unscrew the from the top of the can? the bottom? Both from one side? each from its own side?

any experience appreciated!!
 
I've seen variations, but the cans and slugs I've had to get "surgical" with have allowed the cores to simply back out the top, although sometimes there is a waxy sealant in there which makes it tougher, and that can be eased a little by heating the core.
 
I have seen IF transformers where there is a thin rubber filament stuck between the core and the coil form so that wherever the core stops, it stays. Theoretically, if it was assembled that way, it can be disassembled and reassembled the same way. But if you get mediaeval with ferrite cores, they will crumble.
 
That's probably preferable to this method:

MilwaukeeSawZall.jpg
 
Quick Update -

So, will a little bit of heat, these slugs actually just readily spun out of the cans. I looked into the slugs from Micrometals...that'd be just the thing, except they're difficult to purchase in small quantities.

Anyhow, I found a throwaway donor transformer (a 455kHz AM CAN) that had a core with the same threadpitch -- little bit of heat and that one came out too. Screwed it in, and I'm back up and running.

I took the opportunity to use my newly acquired mid 70s wavetek sweep gen and post marker generator to align the FM. As often happens, the alignment I got using the 'correct' procedure wasn't as good as exercising the 'just tweak it by ear' routine :D I have 2 sweep generators, and I can never get either one to give me a nice clean 'pip' at the marker frequencies like you see elsewhere...the pips are there, but ugly, broadband, and undefined. Oh well. Really need an old master to run me through that sometime.
The sweep generator IS nice for centering and tuning the ratio detector though...the visual 'S' curve on a scope does make that easier. The AM IF alignment is easier to do, and it was actually pretty far out -- hopefully that doesn't mean these IF transformers have mica wafer problems...time should tell.

I live in a busy area for FM in the east coast megalopolis, and the selectivity of FM on this set is about a 7 our of 10 and the FM reception with a short wire antenna is about an 7 also. Its not bad really. It'll pull in a weaker station on a 2nd adjacent channel to a local 50kWer, but won't resolve 1st adjacent channels that some better tuners willl -- but, hey, to be expected for a modestly priced little set.

Either the band is so crowded that the AFC just doesn't 'hone in' very well, or I've got a bad AFC rectifier diode under the shielded FM section. However, getting that shielded section open really doesn't look like a ton of fun, so for now I'll probably just let it go.

The AM on this set is about a 9 out of 10 for a table top. Overall an enjoyable little radio that should clean up nice. Overall there was really less to change in this chassis than I expected - Zenith build the thing with almost entirely disc and mica caps, so there's really very little to change. The few paper caps in there are into low voltage filtering applications (AGC, audio coupling), but I changed them out for good measure (gotta feel like you did something!).

Thanks for the comments and the levity :) Glad I didn't have to break out the old DeWalt, haha.
 
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