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 Post subject: Hints for IF transformer repair - (silver mica disease)
PostPosted: Feb Mon 18, 2008 8:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12406
Location: Haledon, NJ, usa
I had occasion once again today to repair/rebuild an Emerson IF transformer with "dreaded silver mica disease".

I thought it might help a bit here to go over a few details to help those of you who may face this one day as well.

To gain easy access to the guts without any serious destruction... I decided this time to carefully drill out the bottom crimped circular clamping rivet.

I used a nice sharp new 3/8" and a 1/2" drill bit in a little hand drill. Turning this slowly I was sure to not bugger things up too badly and the rivet just fell out nicely.

With the outer can removed now as well ...I was then able to get my long-nosed pliers in there and pull out the old mica sheet and then clip off the lead tabs a bit so they don't touch each other.

I decided to add the new caps under the chassis to make sure of easy access for tweaking.

First of all let me mention that even after repair there is always a little slug tuning/trimming to be done, of course, and it has aways been MY problem that I can never find an appropriate "diddle stick" or tweaking tool.

There are two externally threaded, hollow, tunable ferrite slugs in there. The slugs move up and down over the coils which are under and/or inside the hollow slugs.
...These slugs increase or decrease the permeability and hence the inductance of the coils.

There is one on top for the primary winding and the other has access below for the 2ndary.

The slot in the slug is long ...ok.
.. but it is a bit too wide for any flat blade tool I have ...because the access hole in the top and bottom is so small that any flat blade tool that can fit through the access hole usually is barely wide enough to grab in the slot to turn the slug.
Very frustrating.

I actually fashioned a home-made tweaker this time from a chop-stick using a flat file to form the blade. It made it through the hole but barely grabbed that slug... so I am glad there was little tweaking to do.

I mention this to encourage you to get everything as close as possible so there will be little need for too much tweaking.


In the past I've simply used a 100pf external cap across both the primary winding and the 2ndary too.

However using those "guestimate" values has caused much need for tweaking and the consequential tool slippage and even more frustration.

So this time I decided to try to be more accurate.

I made an investment last year in a handy dandy accurate little inductor/capacitor meter (L/C Meter IIB $120.00) which came in very handy today.

The primary winding read 1.4MH in the center of the slug range. With a range of +/-400uH (1MH to 1.8MH total)

The Secondary winding read 800uH in the center of the slug range with a range of +/- 300uH or so. ( 500uH to 1.1MH total)

I took a quick web hop over to a nice, on-line L/C resonance calculator site:
http://www3.telus.net/chemelec/Calculat ... ulator.htm

...and while there I found that the optimal capacitor calculated for 455-khz was 88pf on the primary winding and 155pf on the secondary.

So I used a 90pf and a 150pf.

Before reinstalling the IF into the circuit I FIRST positioned the slugs in the center of their range so as to avoid too much tweaking.
I also used some white lithium grease on the slug threads to help them move easily... and boy was that a big help too.

Well everything came out just "peachy"!!

I powered up and the radio started playing nicely right away.

But to be sure I was properly aligned I fed in a modulated 455-khz signal from my signal generator as exactly as my frequency counter can help me get it.

The bottom slug needed almost no tweaking at all!!! Less than a single turn.

The top slug however did need about a three revolution upward tweak to peak.

All in all... it was a bit time consuming but at least I know the thing will tweak properly now and I can probably re-tweak again if ever needed now without worrying about rounding the slug's slot.

I hope this helps those who are uncertain in this area of repair.
Good luck.

_________________
" To be a man, Be a non-conformist, Nothing is as sacred as integrity of your own mind." Emerson


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 Post subject:
PostPosted: Feb Tue 19, 2008 1:46 pm 
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Joined: Nov Wed 28, 2007 5:24 am
Posts: 121
Location: Fredericksburg, VA
Very good write up Peter. This example is for 455 kc, and I assume the 10.7 mc FMs would be similar in practice.

But my question is how to determine the cap value for the discriminator coils. What is the frequency of resonance?

I have been doing a few Zeniths recently by just cleaning the mica and reassembling, but am looking for a more permanent solution.

Frank DuVal


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 Post subject:
PostPosted: Feb Tue 19, 2008 6:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3526
Location: cereal city usa, mi
discriminator are often all different. heres some zenith values which seem to fall right in the middle of the road.

http://www.sparkbench.com/zenithifxfmrs.gif

david


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 Post subject:
PostPosted: Feb Wed 20, 2008 3:43 am 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 810
Quote:
it has aways been MY problem that I can never find an appropriate "diddle stick" or tweaking tool.


I know what you mean, believe me. I have been experimenting with this and so far have used a toothpick, cut in half and filed to a screwdriver shape on one end. This works if you have relatively easy access, such as antenna coils. The slugs have to be pretty loose or the toothpick will break. Better than breaking the slug though. I also made a "diddler" out of a toothbrush handle, which is pretty tough plastic. This worked good on some stubborn trimmer caps that needed substantial force to turn. For IF transformer slugs, I used a piece of styrene plastic bar stock, filed to size. These are available at hobby shops. This fit nicely in the deep transformer holes, but was prone to breakage because it is very soft plastic. I had to refile the tip several times before completing the alignment. I am still searching for the ideal material to form alignment screwdrivers out of.


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