Transformer refinishing questions

[k7mdo]

I am in the process of building a small plate modulator for another project and have a couple of questions.

The modulation transformer I have is the one from an Eico 730 and the power transformer and filter choke from the same unit.  Unfortunately none of the transformers were treated well in their youth.  The black paint is flaking off of the modulation unit and all of the items have different paint shades of black. 

I am a bit afraid to take the can covers off of the modulation transformer for refinishing having never done this before.  It looks like just four through bolts and the can havles would come off... any dangers in doing this to the integrity of the transformer... tips?  Also, the modulation transformer core has what appears to be a mylar tape spanning the distance between the case halves.  This stuff is sort of flexible and has been painted over but because of its "looseness" it looks bad.  Is this tape common or available?  I have not seen a transformer with such a wrap before so am hesitant to remove it or try to replace it...  ideas?  If the tape is somehow "integral" I will simply leave it alone but if I could replace it the transformer would sure look better....

thanks, Tom

[KA2DZT]

I've repainted many xfmrs.  You should be able to remove the end bells without any problems.  Not sure what the wrap you're talking about it but don't worry about it.  I'm assuming that the xfmrs all have lead wires and not fixed terminals.  You may be able to repaint the xfmrs without taking them apart, but flaking paint is more work so you may have to.

Fred

[WBear2GCR]

Post a picture or two?

Never heard of a "mylar wrap" that was visible outside the transformer.

My method for really bad end bells and xfmrs is more or less as follows (if you want to know):

Remove xfmr from chassis.
Write down where stuff goes first.
Take a digital pic for good luck.
Remove bolts/screws through xfmr.
Gently remove end bells from wires.

Wire brush the end bells.
I use a wire brush mounted on the arbor of a grinder actually...

depending on the result of that one or more of the following, but not all at once:
- oven cleaner etch
- acid etch
- vinegar + salt bath overnight
- milder alkaline degreaser (Mean Green/Thunderbolt - brand examples)
- neutralize/rinse (inbetween steps and at the end)

If pitted, prime with "spot filler/primer".
Bake, low temp
Then sand down, reprime.
Fine grit sand.

Paint with appropriate shade and luster paint. (this matters if you want to emulate the original
or make it look in concert with the rest of the unit)
Bake.

For the lams, I do the wire brush routine followed by a thin coat of lacquer.
If a lower gloss is required, I put the lacquer on with a cotton ball. No bake

This usually gives a very good result, equal to new in many cases.

Try to not get the wires wrapped around the spindle when doing the rust removal on the lams...

Re-assemble, sometimes with fresh hardware, sometimes with buffed hardware, sometimes
with painted hardware. Sometimes fresh fiber washers.

                         _-_-

[k7mdo]

Thanks for the input!

The modulation transformer came off of a scrapped Eico 730 and uses PP EL-34's on one side then has taps for numerous output impedances up to about 10,000 ohms on the other.  I am building it to modulate a homebrew transmitter.  

I think the "tape" around the laminations outside worried me at first but I begin to think it was put on by someone to keep their paint form getting into the laminations of the core.  I don't plan to slop the paint on so thick as to have much enter so it just can't be such a big deal.  I just wondered if anyone else had seen such a thing wrapped around the laminations for any reason.

I am going to forge ahead and remove the "bells", clean them up and paint them and remove the tape around the laminations and spray them for looks.

Thanks again for the thoughts.

 Tom

[N6YW]

In addition to what was just mentioned, using the edge of a soft pine board, like a 1x2, and a hammer to gently tap around the edges of the end bells. Using a hair dryer before, to warm up the surface helps also. Just be careful to not bend the wires too much and allow them to remain "relaxed". Once the bells are off, proceed with the normal refinishing routine. For the lamination's, I use high temp epoxy paint. For the end bells, I usually use black wrinkle or a hammer tone finish. It's a really nice touch to perfect your project.
73 de Billy N6YW

[WD5JKO]

Re-assemble, sometimes with fresh hardware, sometimes with buffed hardware, sometimes
with painted hardware. Sometimes fresh fiber washers.

   Don't forget the fiber washers. These are not cosmetic. Omitting them will increase the transformer core loss, and act somewhat like a shorted turn on one of the windings.

  Also, if the wire lead insulation is brittle, or cracked, then at a minimum, sleeve the wires with heat-shrink, or better yet use Teflon tubing. I've had to snip the wires off inside the Bell, and splice on new wires, and heat-shrink the connection. This gets iffy when the Bells crunch the wires at the solder joint. Sometimes good to add a layer of fish paper between the inside of the Bell and the wires.

Jim
Wd5JKO

[k7mdo]

Well, I spent an hour removing the tape around the laminations.  It seemed to be adhered (roughly) with something like bees wax.  It was uneven and irregularly applied.  I wonder if it was used to "damp" the laminations?  They look fine and I was able to remove the material as it was slightly soluable in MEK.  Still a puzzle to m as to its purpose.  The tape over the top of the goo was like clear packaging tape.  It was old stuff and partly dried to a stiff consistency.

The bells came off without a hitch first and the wires are all pretty flexible with ok insulation.  They are long enough for me to work with.

Once painted, it will live an easy life in my shack.  It is rated in the literature as able to produce 50 watts.  It also has a feedback winding.

Seems like I am low on Mod transformers and long on Power transformers.... I haunt a local electrical surplus place and buy everything that looks OK... the wife is becoming afraid the house will collapse with all the iron on the second floor!

I "acquired" and old 860 I am itching to try but will finish the power supply and modulator first....  thanks again for the encouragement.  Tom

[WBear2GCR]

A little after the horse has left the barn, but if you had put up a picture of that "tape" it
might have been clear what was up with that...  no digital camera?


                      _-_-

[KA2DZT]

Re-assemble, sometimes with fresh hardware, sometimes with buffed hardware, sometimes
with painted hardware. Sometimes fresh fiber washers.

   Don't forget the fiber washers. These are not cosmetic. Omitting them will increase the transformer core loss, and act somewhat like a shorted turn on one of the windings.

  Also, if the wire lead insulation is brittle, or cracked, then at a minimum, sleeve the wires with heat-shrink, or better yet use Teflon tubing. I've had to snip the wires off inside the Bell, and splice on new wires, and heat-shrink the connection. This gets iffy when the Bells crunch the wires at the solder joint. Sometimes good to add a layer of fish paper between the inside of the Bell and the wires.

Jim
Wd5JKO

Jim,

Interesting, your comment about the fiber washers.  I never considered that they somehow prevented increase core loss.  I've worked on many xfmrs (including painting) and not all of them had fiber washers.  I guess they may mimic the shorted copper bar around a hysteresis motor.  I'm not completely sure that is the case.  I often wondered why fiber washers were on xfmrs.  I thought maybe to allow for some expansion of the core with heat build-up,  not really sold on that either.  Then I thought maybe to prevent tightening of the screws from damaging the paint.  Not too sure of that one.  Maybe some further research may answer this question.  Some xfmr also have fiber sleeves covering the screws.

Fred

[WD5JKO]

Interesting, your comment about the fiber washers.  I never considered that they somehow prevented increase core loss.  I've worked on many xfmrs (including painting) and not all of them had fiber washers.  I guess they may mimic the shorted copper bar around a hysteresis motor.  I'm not completely sure that is the case.  I often wondered why fiber washers were on xfmrs.  I thought maybe to allow for some expansion of the core with heat build-up,  not really sold on that either.  Then I thought maybe to prevent tightening of the screws from damaging the paint.  Not too sure of that one.  Maybe some further research may answer this question.  Some xfmr also have fiber sleeves covering the screws.

Fred

 Fred, I found a good reference in Google books, "Fundamentals of Magnetics". See image below.

I have seen transformers were the laminations were literally welded together to save cost. These without exception ran extremely hot, and were very noisy.

Jim
Wd5JKO

[KA2DZT]

Interesting, your comment about the fiber washers.  I never considered that they somehow prevented increase core loss.  I've worked on many xfmrs (including painting) and not all of them had fiber washers.  I guess they may mimic the shorted copper bar around a hysteresis motor.  I'm not completely sure that is the case.  I often wondered why fiber washers were on xfmrs.  I thought maybe to allow for some expansion of the core with heat build-up,  not really sold on that either.  Then I thought maybe to prevent tightening of the screws from damaging the paint.  Not too sure of that one.  Maybe some further research may answer this question.  Some xfmr also have fiber sleeves covering the screws.

Fred

 Fred, I found a good reference in Google books, "Fundamentals of Magnetics". See image below.

I have seen transformers were the laminations were literally welded together to save cost. These without exception ran extremely hot, and were very noisy.

Jim
Wd5JKO

Thanks Jim,

I've been working with xfmrs over 50 yrs and you can learn something new everyday.  Welded laminations, one reason why I never consider using MO xfmrs for anything but scrap.  I lost count years ago, but I probably have 1000 xfmrs and chokes, counting the small ones.

About 25 years ago I scrapped about 100-200 xfmrs and chokes.  I think there may have been some good ones in that mistake.  I made a machine to remove the wrappings and unwind the copper once the laminations were removed. I was taking the steel to the scrap yard 1000 Lbs each time.  Don't remember how much copper.

I'll have to pay closer attention to that fiber hardware in the future.

Fred

[k7mdo]

Sorry on the pictures, I didn't think they would show much as the tape was all painted (poorly) black....

I don't think I hurt anything though and my guess is that the beeswax on the lamination edges was just some attempt at keeping moisture out of the laminations?  I just have not seen it before and the crudeness of its application indicated it was not some factory application.

The transformer had no sleeves on the screws nor fiber washers, however, the power transformer that came off the the same discarded unit did have fiber washers under the screw heads only... 

I can probably make some of the sleeves and washers when I get ready to put it back together.

Happy holidays and take care, Tom

[KA2DZT]

Tom,

If the xfmr had no sleeves, then no need to add any as there may not be enough room through the laminations for the screws plus sleeves.  As for that tape you mentioned, just clean all the wax or whatever it is off the laminations.  Use mineral spirits (if you have any) and a wire brush to clean everything up before painting.

For just plain old black, I use the cheap paint (black is .96 other colors now 1.52) that Walmart sells.  If you want to go better, use Krylon black satin or gloss.  As with all these spray paints follow the directions.  Pay attention to the time between coats.  Walmart paint, put all coats on within 4 hours or less.  If you miss the 4 hour mark DO NOT repaint for at least a week.  Krylon claims to put coats on within 1 or 2 hours.  I use 1/2 hour between coats.  If you miss the 1 hour mark Krylon claims you have wait 24 or 48 hours to re-coat. Don't believe them,  with Krylon it is best to wait at least 5 days or more.

These two paints dry fast but it is best to let them dry for at least a day before handling.  Krylon can take 3-4 days to fully cure.

Tape off anything you don't want painted, like the lead wires.  These spray paints create a lot of spray dust.  So, don't do any painting in your living room or anywhere in the house.

Fred

[WBear2GCR]

you'd be surprised at what someone can figure out from a few pictures...

                        _-_-

[N6YW]

And what ever you do, DO NOT USE YOUR WIFE"S OVEN TO BAKE THE PAINT!!!!!!