I have only done research and modified transformers to measure outputs of primary windings. I know what size gauge to use and how many turns to yield the desired output on the secondary. I’m not concerned about figuring out the math and variables involved, if that’s what your asking? I will gladly accept your input on the matter.
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I will gladly accept your input on the matter.

You're welcome, I have an approach that I've used before, though I've never used MOTs, most of mine came from redundant telecoms and computer equipment before SMPS became commonplace. So linear supplies with 24v 100A DC output, repurposed/rewound for 75v 25A CNC stepper drives

 

That's what I thought was happening. Here are some things to try:

1) Use a Dremel with small circular cutting blade.
2) Dremel also makes a tool with an oscillating stroke that has metal or wood cutting blades ( I would not buy it just for this purpose though.)
3) If you use a hacksaw type blade, go diagonally across the secondary, not perpendicular where you risk nicking the primary.
4) Use a chisel and hammer (be gentle). Don't try the entire depth in one step. Go 1/8" to 1/4" (3 mm to 6 mm) at a time. Peel the layers. It gets pretty easy.
5) Covering the primary with plastic tape will offer some protection. A shield of thicker plastic offers more. I found no need to do that though.

 

You're welcome, I have an approach that I've used before, though I've never used MOTs, most of mine came from redundant telecoms and computer equipment before SMPS became commonplace. So linear supplies with 24v 100A DC output, repurposed/rewound for 75v 25A CNC stepper drives

Were they the ei type?

 

That's what I thought was happening. Here are some things to try:

1) Use a Dremel with small circular cutting blade.
2) Dremel also makes a tool with an oscillating stroke that has metal or wood cutting blades ( I would not buy it just for this purpose though.)
3) If you use a hacksaw type blade, go diagonally across the secondary, not perpendicular where you risk nicking the primary.
4) Use a chisel and hammer (be gentle). Don't try the entire depth in one step. Go 1/8" to 1/4" (3 mm to 6 mm) at a time. Peel the layers. It gets pretty easy.
5) Covering the primary with plastic tape will offer some protection. A shield of thicker plastic offers more. I found no need to do that though.

Ya I get carried away when I struggle with things. I started out careful and ended with chiseling both side off and drilling it out aggressively.

 

Were they the ei type?

Mostly. some toroids and some very uncommon C-T cores.

Some EI cores can be separated and the bobbins removed individually. You need to inspect them to see how they've been assembled. Sometimes they are just held together with a clamp others are spot welded along the EI junction. It may be possible to drill, grind or chisel the welds apart to get the bobbins out. That obviously makes rewinding easier.

 

Mostly. some toroids and some very uncommon C-T cores.

Some EI cores can be separated and the bobbins removed individually. You need to inspect them to see how they've been assembled. Sometimes they are just held together with a clamp others are spot welded along the EI junction. It may be possible to drill, grind or chisel the welds apart to get the bobbins out. That obviously makes rewinding easier.

I picked up another unit today and the secondary winding was very brown and black and had hard black “stuff” oozing from the inside of the winding only on the secondary. The microwave was being thrown away because it wouldn’t heat up. I’m assuming it is because of the transformer and the secondary winding has some kind of issue. Would this mean that the whole thing is bad or just the secondary that got very hot?

 

Almost certainly a shorted turn or more in the secondary winding...

 

If the core is not welded, I would dissemble the core itself and then you can work on the bobbin and windings easier.
That is the way I've done it in the past. Old TV transformers were used for my early power supplies.
Getting the first lamination out can be a bit tricky, but after that, it should be ok. Just be careful not to bend or scratch them badly. Once you have the first couple out, sliding a scraper between the laminations will separate them for easy removal.
Practice on the damaged ones. You could even carefully unwind part of the damaged primary and try to solder the cuts. Then rewind them with some insulation in place around the wire if you feed up to that.

 

Removing the secondary is simple. All it requires is a little patience. No 16# sledge hammer is needed. No disassembly. No extraordinary protection for the primary.

Here's one I did about 15 years ago:



I pulled one secondary lead up to show the primary. The lamination welds were not removed. The magnetic shunt is in place. It still worked despite my laziness. The OCV as I recall was about 3 V to 5 V.

 

I'd add one thing to the descriptions of how to remove the secondary, clamp the transformer down. Either in a bench vise or with a C-clamp to the bench. Fighting it moving around when doing the removal just adds to the frustration.

 

I'd like to see VA (output) versus VA (input) with and without those more drastic changes. Remember what started this thread. Keeping it simple does have its advantages.

 

Almost certainly a shorted turn or more in the secondary winding...

How does that happen? The heat and vibration?

 

If the core is not welded, I would dissemble the core itself and then you can work on the bobbin and windings easier.
That is the way I've done it in the past. Old TV transformers were used for my early power supplies.
Getting the first lamination out can be a bit tricky, but after that, it should be ok. Just be careful not to bend or scratch them badly. Once you have the first couple out, sliding a scraper between the laminations will separate them for easy removal.
Practice on the damaged ones. You could even carefully unwind part of the damaged primary and try to solder the cuts. Then rewind them with some insulation in place around the wire if you feed up to that.

All the cores are welded and that’s why I haven’t gone that route. I’m not a great welder. I can do it, but it’s not my strong suit.
I was wondering if I could use some solder to fix the knicks in the wire or, kind of splice in new insulated wire of the same gauge. Lots of experiments to try.

 

I'd add one thing to the descriptions of how to remove the secondary, clamp the transformer down. Either in a bench vise or with a C-clamp to the bench. Fighting it moving around when doing the removal just adds to the frustration.

Yes, I need to be doing that.

 

I'd like to see VA (output) versus VA (input) with and without those more drastic changes. Remember what started this thread. Keeping it simple does have its advantages.

I will start testing the damaged primaries.

 

Ok gents and ladies, I have removed the secondary successfully without primary damage, unfortunately the leads attached to the ei frame was broken due to the short and heat. This is where my lack of understanding will be clear, where can I solder my new secondary winding to the transformer? The pictures posted show a river with some solder where the old leads were located. Should I just solder to that existing rivet?

 

That 'rivet' looks like a solder tag and in that case, yes, you can connect a wire to it.
It is not essential for your new secondary to connect to the transformer core.

 

That 'rivet' looks like a solder tag and in that case, yes, you can connect a wire to it.
It is not essential for your new secondary to connect to the transformer core.

Man electricity is cool! Why would they use it in the original transformer then? Seems like more work? Longevity, I suppose.

 

I'd just leave it as flying leads, no need to terminate on the core, or finish in a suitably sized connector block.