Airline 7" tv & am/fm radio phono : Help with radio if transformers

[John Folsom]

I am restoring my Airline 94GSE-3018A 7" TV & AM/FM radio-phono. The TV restoration has gone well, but after recapping the radio chassis, I powered it up and ....NOTHING! Just a slight hum and a bit of crackle, on AM and FM positions. Injecting a 10.7mhz FM IF signal and/or a 455khz AM IF signal resulted in no output, except when driving the secondary of the detector transformer. This lead me to believe the 1st & 2nd IF transformers were possibly bad.

After removing the shields from the IF transformers, it became obvious they were the types with the build in mica capacitors in the base. I have read threads bout the "silver mica disease" and metal migration, and I find these transformers suffer from this disease.

As you can see in the photo, disassembling these transformers and removing the flulty capacitors in the base and replacing them with discrete silver mica caps is a very delicate process, and I predict my chances of accomplishing this feat are slim.

Being a TV guy, I don't have much radio related data. Did Merit, Miller or Thordarson make replacements for these dual FM/AM IF transformers?

The Airline part numbers for the transformers are 20E451 and 20E451-2, and the radio chassis is 20E493A.

Any help on replacement part numbers or advice on how to precede with repairs on the transformers I have would be GREATLY appreciated.


Thanks!

[bob91343]

Two-frequency IF cans were very popular. Nearly every AM-FM set had either those or separate transformers for each band. Separate ones took more chassis space and installation labor so that was used more in higher end units.

So if you can scavenge some old transformers you might be okay. Assuming, of course, that your diagnosis is correct.

[Reece]

If the set indeed has silver mica disase, after the bad internal caps are cleaned out you can install approximately 120pf silver micas under the chassis on the transformer terminals for the AM portion. The value isn't too critical as the slug is adjustable. Don't know what values the FM transformers would need but someone may post with the answer. In any case the process is not so touchy as the cap is under the chassis and not in the can.

[John Folsom]

Reece, thanks. The touchy part is getting the transformer apart to remove the built in caps. Once that is done (if I can pull it off), I can measure the inductance range on the AM and FM windings and calculate the required PF value. But if someone can identify a replacement part from Merit, Miller, meisner or others, I would prefer to go hunting for a replacement.

[Chad Hauris]

I do a lot of these IF can repair jobs and 22 PF for FM, 100 PF silver mica caps for AM seems to work fine on every one I did.
Pretty much every tube type radio I work on now except the older type from the 30's and 40's with the trimmer caps in the IF cans has this mica cap problem.

I have some videos on my youtube channel of repairing the cans.

These with the phenolic base will hold up easier than the type with the plastic base...either type I carefully hold the base in a pair of vise grips and carefully drill out the rivet holding the top on, remove the mica, trim up the clamping pieces which held the mica so they don't short, then just put the new caps under the chassis and sometimes in the larger type like this they can fit up inside the can. Some of these earlier type like on Zeniths have a nut holding the top phenolic piece on and are easier to disassemble.

Sometimes I have had the wires break off from the coil but I just very carefully scraped off the enamel insulation with a file and re-attached to a piece of bare wire soldered to the terminal. Sometimes I have had to carefully superglue the coil form back on. Also watch out for sticky ferrite cores. Re-align the set with the new caps and you're off to the races again and it should hold up a lot better than the flaky mica wafers.

I would do a repair job on these rather than getting a new old stock unit. I think these things end up corroding/oxidizing due to the air and the NOS unit would be subject to the same forces of deterioration.

Sometimes they don't show the problem until they have been used a few minutes. I just did a job on an RCA AM/FM chassis, replaced all paper/electrolytic caps, powered it up and it worked great, after about 1/2 hour audio gets noisy and then disappears and it is the bad mica cap problem showing up.

[old_coot88]

Originally Posted by Chad Hauris

...Sometimes I have had the wires break off from the coil but I just very carefully scraped off the enamel insulation with a file and re-attached to a piece of bare wire soldered to the terminal.

Check out the 'aspirin trick' for stripping fine enameled wire, beginning at post# 26 ...

http://videokarma.org/showthread.php?t=257295&page=2

Bet it'd be great on Litz wire too.

[wa2ise]

Originally Posted by John Folsom

Reece, thanks. The touchy part is getting the transformer apart to remove the built in caps.

If the two wafers are held together with a nut and screw, you should be able to remove the nut and screw and be able to access the mica cap wafer. I've been able to do this, and I just used a scribe tool to scratch away the migration paths between silvered cap areas. Keep track of which way the mica wafer was in there (which side was top, and orientation. So you can get it back in again. If you are successful, you may not even have to touch the alignment.

Sometimes they don't show the problem until they have been used a few minutes. I just did a job on an RCA AM/FM chassis, replaced all paper/electrolytic caps, powered it up and it worked great, after about 1/2 hour audio gets noisy and then disappears and it is the bad mica cap problem showing up.

I had an Emerson AA5 that drove me nuts back in high school with this symptom. Did a recap, no good. Ended up tossing it, before I learned about silver mica disease,

[drdave3]

Chad,

What is the name of that video? I am having trouble locating it.

David

[Chad Hauris]

Here is a video where I show the whole job:

http://www.youtube.com/watch?v=uu4QQPrxIQM

I just did another one of these jobs today and it came out perfect with no damage...the key with the plastic type is in how you work the drill...you need to very slightly angle it and drill very slowly so the bottom part of the rivet just sort of falls apart and then you can pull the top part off.

I personally have not tried re-using the mica after cutting but personally I prefer to just remove all traces of it and use external caps, that way you never have to open that can again in case some other trouble might come up with the caps. Sometimes in addition to the silver migration I think intermittent contacts between the contact fingers and the mica sheet may come in to play.

When I was younger I too tore apart many radios in frustration trying to get them to work due to the IF can cap problem and getting no sound after fixing all the other problems I knew about (bad electrolytics, paper caps, etc). By the time I got to college I figured out what the problem was but ended up scavenging working IF cans from junk radios, not being brave enough to open up the cans and tackle the problem directly.