Re-winding a Toroidal Transformer

[paul_g_787]

Then there were the two 0-18.5V windings to un-wind. 36 turns each.

For some reason this image "fails the security checks" and won't upload. Whatever that means??

This is as far as I got on Thursday. Took about 2 hours 45 mins so not too sad.

[paul_g_787]

Made a little progress sunday. The next step was to remove the second layer of insulating tape then inspect the primary coil.

It took about 35 mins to remove the insulating tape. There was actually two layers of tape on this section.

[paul_g_787]

Then I found, just like the guy on youtube, I have a burned out section, but not as bad as his.

I have tried to upload the image, but again, I get "Your attachment has failed security checks and cannot be uploaded. Please consult the forum administrator." Where do I contact the administrator?
The image is only 900kb. What security checks???

My plan is to do what the guy on youtube did. Which is to cut the wire here. Lose a little of this winding and re-join it.

He used these little white insulating sleeves the same as on the wires on the transformer. What is this stuff called and does anyone know if I can get some? have a look at 20:47 in the video.

[andy3055]

Fiberglass sleeving. It is used in most motor and transformer winding due to its super resistance to high temperature in addition to the insulating properties.

May be you can use Kapton tape with the adhesive backing so that you can cut a piece and wrap it around the joint. The transformer should not get that hot. May be even two layers of heat shrink tubing will work.

[james_s]

It's that stuff that's made of silicone rubber impregnated fiberglass sleeve, it's often called "spaghetti" but I don't know if there's a more official name for it. I have some fiberglass adhesive tape that I've used in transformers, and the stretchy clear tape is polyester. I bought some from tapecase.com when I wound my big toroid years ago, no idea if they're still around but they had all different kinds of tape.

[andy3055]

It's that stuff that's made of silicone rubber impregnated fiberglass sleeve, it's often called "spaghetti" but I don't know if there's a more official name for it. I have some fiberglass adhesive tape that I've used in transformers, and the stretchy clear tape is polyester. I bought some from tapecase.com when I wound my big toroid years ago, no idea if they're still around but they had all different kinds of tape.

They are still around!

[Circlotron]

Yes, it is a class AB, which is terrible design at +/- 95 V supply and will never work reliably, except maybe for home use - at the typical 1 W output level. When playing music at a 2x 30 or 40 W output level with 200 W peaks it will dissipate about 400 W of heat in the output stage plus another 100 W or more in the transformer. Circuit diagram seems to be here:

https://www.crownaudio.com/zh/product_documents/135277-1_2_xls2u_main-pdf

Regards, Dieter

In that schematic, Q4A has only a capacitor connected to it's base and nothing else. That's a bit odd...

[dietert1]

Probably D4A and R16A are meant to generate the base voltage of Q4A and somehow the connection got lost in the diagram. The amplifier won't work at all without, so you may safely assume the wire exists on the board. A typical copy error that happens when cloning existing products.

Also interesting in the schematic: It's rare in that it generates the bias without that nasty potentiometer that everybody else is using. Adjustment is by selecting R14A. And under load will be as unstable as +HT2.

Regards, Dieter

[Circlotron]

Also interesting in the schematic: It's rare in that it generates the bias without that nasty potentiometer that everybody else is using. Adjustment is by selecting R14A. And under load will be as unstable as +HT2.

Q7A and Q5A are a effectively a current source and sink respectively so any change in HT voltage shouldn't affect the bias that much.

[paul_g_787]

Yes, it is a class AB, which is terrible design at +/- 95 V supply and will never work reliably, except maybe for home use - at the typical 1 W output level. When playing music at a 2x 30 or 40 W output level with 200 W peaks it will dissipate about 400 W of heat in the output stage plus another 100 W or more in the transformer. Circuit diagram seems to be here:

https://www.crownaudio.com/zh/product_documents/135277-1_2_xls2u_main-pdf

Regards, Dieter

In that schematic, Q4A has only a capacitor connected to it's base and nothing else. That's a bit odd...

That is not the correct schematic. That is for the regular XLS series. This is the D series. My model is the XLS 402-D

Here is the right one: https://elektrotanya.com/crown_xls_d.zip/download.html

[dietert1]

When you look at the two schematic diagrams, they look very similar. Then you notice that someone with little experience made the changes for the D model. In the newer model the long tail pair runs at a gain of about 2x (220R versus 470R), which i would consider a severe mistake. In the original version gain was 10x (100R versus 1K). Then you notice he introduced gain limiting also for the second pair (R121, R122). Probably that amplifier has significant THD at low levels (above 0.1 %). Don't know what R126 and R127 shall be good for. The diodes around C102 are an improvement though.

Please note that these amplifiers have a SOA-protection for their output stage (Q108, Q109 and their circuitry). They are meant to limit output current depending on output voltage to guarantee safe operation. That means the amplifier will drive ohmic loads well up to it's clipping limit (with LED indicator). But the amplifier will also limit with resonant loads like huge bass drivers or compound speakers with built-in passive crossover networks. There won't be any indication of that type of limiting, except bad sound. By the way: this circuit of SOA-Protection ignores the fact that SOA depends on temperature. So you can kill the amplifier with a single half-amplitude 10 msec pulse after running it hot for a while.

In my opinion class AB with +/- 95 V supply will not run reliably. Above in this thread there was the proposal to buy a new transformer with somewhat lower output voltage, lets say 2x 55 V. That sounds more realistic and i think 2x 40 Vrms are enough. Then the amplifier will run at about +/- 55 Vdc and output peak power of 300 W into a 4 Ohm load. With PA speakers 2x 300 W is extremely loud and for home use you will rarely listen at more than 10 W or so.

Regards, Dieter

[GerritMax]

He used these little white insulating sleeves the same as on the wires on the transformer. What is this stuff called and does anyone know if I can get some? have a look at 20:47 in the video.

https://www.ebay.co.uk/i/184374770519?chn=ps&var=692075820665&norover=1&mkevt=1&mkrid=710-134428-41853-0&mkcid=2&itemid=692075820665_184374770519&targetid=1000147648740&device=c&mktype=pla&googleloc=1007222&campaignid=10199419372&mkgroupid=101401897425&rlsatarget=pla-1000147648740&abcId=1145983&merchantid=119207815&gclid=CjwKCAiAiML-BRAAEiwAuWVgglPHC8XySKgyAzvvWk0NeZnYJR2HYGLaz01_IYW3K9AjS2euQWcHXxoCqYUQAvD_BwE

[paul_g_787]

Hi Everyone. So I came back to this project yesterday and finally was brave enough to attempt to repair the primary coil!

I found one wire was completely burned through just like the chap on YouTube and the two immediately next to it were partially burned through also and all three were welded together!

The primary coil is actually two windings, one on top of the other joined in series. I assume that in 120V countries they would be joined in parallel. These two windings were joined by the weld about halfway, causing the short.

So I snipped them and joined them on the outside of the transformer just like he did. I only lost 3x ½ turns from it which is the best I could do. I don't think this would affect it much.

See attached picture of the join.

I added some plastic strips for insulation and the Fibreglass insulation tubing on the bends. I could have got a smaller diameter insulation but this will do for now.

I plan to slip more fibreglass over the solder joints then another layer of plastic before winding the polyester tape back on.

[paul_g_787]

Next step was to test it! I ran the transformer in series with a 100W tungsten lamp.
Live -> Lamp -> transformer -> Neutral.

And what do you know! It works, no lamp lighting up and I measured the correct mains voltage across the transformer and approx 250mA dropping to 100mA of current after the coil charged up. Left it running for 5 minutes and it is not even warm.

So I would say the primary is now good!


(I know the picture shows a little bit of a dangerous set up, but it was all I could do for now. Perhaps the next project is to build a box with a lamp and a 13A switched socket).

[Crumble]

Ow, well done!

I would not have attempted doing a repair like this, must have been a chore.

[paul_g_787]

I have insulated the live and neutral terminals of the transformer.

There are two windings on the primary in series for 240V. (I assume these would be wound in parallel in 120V land) so there is a third original join here.

[paul_g_787]

I have also insulated the repaired joins I have made.

[paul_g_787]

So now this is insulated nicely I covered the joins in another layer of plastic and taped them down.

[paul_g_787]

And last week I got round to wrapping the insulation tape back onto the primary.

I have tested the coil again in series with a lamp as before and it all appears good!!

Now just to re-wind the secondary coil......

[amyk]

Look up "split hoop toroid winder" for some ideas if you want to make the rewinding process a little easier...

[paul_g_787]

Look up "split hoop toroid winder" for some ideas if you want to make the rewinding process a little easier...

Yes I have seen those on youtube. Very useful but not worth the cost for one single transformer.

Might pick one up if I have to do more of these in the future.