Re-winding a Toroidal Transformer

[paul_g_787]

I have an audio power amplifier here which uses a toroidal transformer. It is a Crown XLS-402D

This amplifier is suffering from a very common fault with this model (according to Crown themselves when I phoned them) where the transformer is shorted.
I discovered this by connecting the transformer to the mains with the intent of measuring the output voltages, only to have it trip the RCD (of course because the Live to Neutral is shorted).

I am wondering if any of you here have some experience re-winding Toroidal transformers and could give me some pointers?

Why is it faulty?

Unfortunately, crown in their wisdom, decided to not put any insulating material on the inside of the lid of this model amplifier, resulting in,  when something heavy is placed on the lid, the lid touches the top of the transformer mounting bolt. 

Now as far as I understand it, this effectively makes the mounting bolt another secondary winding, which being only a half turn will have a very very low voltage but a huge amount of current and therefore short the primary coil?? That is my educated guess.

I can clearly see that the lid of the amplifier is dented in right on top of the transformer, there is a little bump where the bolt head used to be (before the bolt was lost by the previous person to attempt repairing this amp). There is also a charred mark in this dent which I gather is where there was some arcing right before the death of the primary coil.

So I am pretty sure this is the fault. Does anyone agree with me on this?

What next?

I asked crown about a replacement transformer and they do not sell them any more.  Boo!

I would like to point out that this amplifier was given to me free by my local music shop that gave up on it. Some people might just say to bin it! But that is no fun!

I would very much like to learn how to re-wind the transformer as a little project, I have nothing to lose so might as well try to rescue the amplifier from going for scrap.

Now first of all there is a label on the transformer which shows the output voltages.

There is a single primary winding which is for 240V.
Then there is a single secondary winding which has a centre tap marked 0V, two more 18.5V taps then two 67V taps.

Kind of like this:

                                     Secondary Winding
67V                   18.5V                0V                   18.5V                 67V
  |                        |                        |                        |                        |
  |--------------------|--------------------|--------------------|--------------------|
==================================================
  |----------------------------------------------------------------------------------|
                                    Primary Winding (240V)

But the label does not stat the power(W or rather VA) of the windings at all!

This is where I am a little confused as usually when I have looked at a transformer there will be a Watts or VA rating on the label.

I believe I have tracked down the Manufacturer: http://en.fsnre.com/

But that is all I can find for now, I can't find a datasheet.

[andy3055]

Will this help?

https://www.parts-express.com/pedocs/specs/xls-d-series-ac-power-draw-thermal-dissipation-info-specifications-45385.pdf

Edit:

Is there anyway for you to mount a standard transformer? You could probably work out considering the max input being 935w and leaving a safety margin. Just a thought.

[Doug Brewster]

There's a guy (kevzep) over on audiokarma who I think has professional experience on Crown equipment. He may be able to help..

[WattsThat]

Rewinding is nothing more than sourcing the correct replacement wire, polyester tape and lots of patience.

Unwind the overall insulation tape, noting how many layers, save it as it should be reusable. Start unwinding the secondary and count carefully, write it down and take pictures of how the tap(s) come off. Unwind the secondary-primary polyester insulation tape (should have no adhesive) and see what you have left of the the primary. Unwind/count/document.

Measure the diameter of the wire used, measure lengths and source replacement wire. Make a wooden bobbin, wind on the primary wire and start rewinding. Thread the bobbin. Tighten the windings. Again and again. Insulate. Wind secondary. Insulate. Done. Google hand winding a toroidal transformer. It’s not difficult, just labor intensive. A 1kw core will be a handful.

Personally, I’d shop eBay for a replacement. They do show up now and then.

Just be sure to insulate the bolt.

[TheMG]

Unless you have a toroid winding machine... it's going to be a very tedious process to do yourself, since you have to pass the entire length of the wire through the core with each turn!

As for determining the transformer rating, did you ask Crown if they could provide the specs for it?

Anyways, they do list the current consumption with 1/3 power pink noise at 4 ohm stereo to be 4.9A @ 230V, so 1100VA would be a good starting point.

This transformer should work: https://www.antekinc.com/an-10464-1000va-64v-transformer/

It's 1000VA and 64V @ 230V input, so at 240V would be right around 67V. As for the VA rating, I wouldn't worry too much about being 100VA smaller. For starters, that was just an estimate based on manufacturer's reported specs for the entire amplifier's power consumption under pretty much a worst case scenario. Transformers in audio amplifiers are pretty much never rated to deliver the full output of the amplifier continuously. Unless you plan on pushing the amplifier HARD for prolonged periods of type with music content having a very high average power, it's not going to be a problem.

The transformer doesn't have the 18.5V taps but you can easily use a second smaller transformer for that. It's only for control and input circuitry, so opamps, LEDs, fans, that kind of stuff. No big power requirement from the 18.5V, on the schematic the 18.5V goes through 1N4004 diodes, so probably less than 1A. Something like this should suffice: https://www.antekinc.com/an-0218-25va-18v-transformer/

I've purchased transformers from that company on a few occasions. The price is hard to beat and they seem to be of decent quality, I've never had any issues with them.

[james_s]

I wound a large toroidal power transformer by hand once, it came out beautifully and works perfectly but yes it was very tedious. If you enjoy tasks like knitting you might enjoy winding a transformer, but unless you have a lot of free time and really want to do it yourself, you're going to be better off getting a company that has a winding machine to make one for you. It's even possible that there's an off the shelf transformer that will work.

[mag_therm]

There is ( at least) one USA manufacturer that offers various sized standard toroidal kits that are wound primary only.
But I checked and all primaries are 120 V. The prices are quite reasonable, I would buy one for hobby use.
The idea is that they provide info for the user to wind the custom secondaries which have less turns.

Maybe there is a similar provider in EU or UK ?

[Crumble]

Two things may be of help:

• Did the RCD open or did you have a circuit breaker pop? Maybe you have RCBO's? A short should not trip an RCD, and if a circuit breaker opened it is possible that there was a short while turning it on, or no secundary load while doing so, which is known to cause large current spikes, especially with large transformers. If there is a leakage to earth you should be able to find it quite easily: it will clear if you use the transformer outside of the (presumably grounded) enclosure. Do keep in mind it may have exposed live wire somewhere on its surface, so put it on a piece of plastic and don't hug it;
• The weight of a transformer is usually an indication of the power. Just put it on a set of scales and compare the weight with some known ones, for example using transformer datasheets on distributor sites. Using the TME parametric search I see that 500VA transformers weigh between 3,5 and 3,9kg.

Maybe you already figured these things, but this is the way I would have started solving this kind of problems.

When you find this transformer is broken you may need to measure the gauge of the wires to find how the current was distributed from them. I guess this is what Douglas Self calls a "Class G" amplifier: an amplifier that uses a double set of output transistors to merge efficiency with high peak power. It may even be running the output transistors in class A. The 67V rails are likely to have a lower current capability, and if you want this to be repaired you may need to know what the wire gauges were.

I agree with the other guys that I would not wind the transformer by hand, it gets quite tedious quite quickly.

[james_s]

There is ( at least) one USA manufacturer that offers various sized standard toroidal kits that are wound primary only.
But I checked and all primaries are 120 V. The prices are quite reasonable, I would buy one for hobby use.
The idea is that they provide info for the user to wind the custom secondaries which have less turns.

Maybe there is a similar provider in EU or UK ?

I think the one I dealt with was Bridgeport Magnetics, I bought some cores from them, the toroidal one I wound and a few cut C cores and bobbins. I remember they also offered cores with the primary wound and they will do custom winding to whatever specs you want although I didn't check the price. Due to the weight of transformers it's probably best to try to find a supplier in the country you are in.

[dietert1]

Before spending a lot of effort on the transformer, i would try to run the amplifier from a lab power supply. Apparently it is a class H design and you need about +/- 25 V. You can connect the supply to both pairs of rails and try the amplfier. For levels up to about 50 W it should work that way. You can select a replacement transformer by mechanical size. The Antek transformer mentioned above does have a 18 V secondary and  another 12 V one that will give you another 18 V tap with some additional windings you can add by hand.

Regards, Dieter

[Inari]

This is not a class H.

There is only ONE hi-current/voltage dual wired secondary (2x67V).
Other, lower 18,5-0-18,5 is not hi-current windings.


It can be replaced easily with two separate transformers.
For example: https://www.newark.com/hammond/1182q60/transformer-toroid-120v-750va/dp/54X7542?st=2%20x%2060v%20toroidal  for Power amps

and this: https://www.newark.com/multicomp/mcta050-18/toroidal-transformer/dp/38K4883 for lower power circuitry.

Only thing which good to follow is that you need to connect both transformer primary wires to main switch - to switch both transformers same time.

Maybe this bigger one which I linked is little too big (you need to measure old transformer diameter and height to choose best for you. Bigger is better but they may not fit if it is very big. And you need also room for ta smaller one if you choose this 2 transformer approach.


Seems like I have also one of this transformer, with burned primary
By dimensions this is somewhere 450-550VA power capacity. If you dismantle it - what is primary winding wire diameter? This may help to choose new transformer.

[dietert1]

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

[Inari]

Yes, this is right.

https://uk.farnell.com/multicomp/vtx-146-625-155/transformer-625va-2x-55v/dp/1675103  can be used here, it also fit. And https://uk.farnell.com/vigortronix/vtx-146-050-218/50va-toroidal-transformer-2x18v/dp/2817653 for control circuit.

[dietert1]

Here is a video of the transformer repair. I think somewhere he mentions that it took him "several months".



Regards, Dieter

[paul_g_787]

OK that is a very helpful video. That is the next model up to mine. It has a sightly bigger transformer but it is essentially the same amp!

I have contacted a local toroid transformer manufacturer to see if they would be interested in re-winding my transformer. If they are not, or it is too expensive then I will have a go at this myself like this guy did.

I like the idea of the bobbins he made to wind the wire onto. Will definitely make some of those!

[Audiorepair]

There is ( at least) one USA manufacturer that offers various sized standard toroidal kits that are wound primary only.
But I checked and all primaries are 120 V. The prices are quite reasonable, I would buy one for hobby use.
The idea is that they provide info for the user to wind the custom secondaries which have less turns.

Maybe there is a similar provider in EU or UK ?

I quite liked this idea.  Is this actually a thing in Euroland?

[GerritMax]

Here is a video of the transformer repair. I think somewhere he mentions that it took him "several months".



Regards, Dieter

Cheers for posting that video, i've got a Technics SU-V8 amp with a dead transformer.
Looks like there might be hope of getting it working again.

[paul_g_787]

A little update. Crown contacted me back regarding a replacement transformer. They have said they do not provide any parts for any of their amplifiers at all and I would need to buy a new amplifier. However they don't stock parts even if you bought a brand new amplifier!! I am quite shocked! I don't think I would like to buy anything new from them if they don't even have spares for new products!

So I asked is there a datasheet for the transformer and they told me it is a complete secret. Lol  Even though the amp schematics are on their website??? Weird.

Still haven't heard back from the transformer manufacturer in china and I don't think I will ever hear back from them.

So anyway I had a few quotes from some transformer re-winding companies and the cheapest is £160!!! and that is not even replacing the wire with new, that is for cutting out a section and joining it just like in the video.

Seems extraordinarily expensive to me. I am sure it would take only minutes to remove and replace the two windings with new wire, of course if one had a winding machine.

So I think I will just try what the chap in the previous youTube video did. It can't be too difficult, just time consuming.

Wish me luck!!

I found this cool video of how fast it can be done:



This guy made his own winding machine! Quite cool.

[langwadt]

Yes, this is right.

https://uk.farnell.com/multicomp/vtx-146-625-155/transformer-625va-2x-55v/dp/1675103  can be used here, it also fit. And https://uk.farnell.com/vigortronix/vtx-146-050-218/50va-toroidal-transformer-2x18v/dp/2817653 for control circuit.

or just add windings for the low current control voltages on the big transformer. Start with a few turns and check the voltage per turn,
it probably won't take that many turns to get 18V

[james_s]

A little update. Crown contacted me back regarding a replacement transformer. They have said they do not provide any parts for any of their amplifiers at all and I would need to buy a new amplifier. However they don't stock parts even if you bought a brand new amplifier!! I am quite shocked! I don't think I would like to buy anything new from them if they don't even have spares for new products!

Wow, I will make sure I never buy anything from Crown, for some reason I had it in my head that they made good products. You should make sure to tell them that due to their lack of support for their own products you will also not consider purchasing any of their products in the future.

[langwadt]

A little update. Crown contacted me back regarding a replacement transformer. They have said they do not provide any parts for any of their amplifiers at all and I would need to buy a new amplifier. However they don't stock parts even if you bought a brand new amplifier!! I am quite shocked! I don't think I would like to buy anything new from them if they don't even have spares for new products!

https://www.crownaudio.com/en/support/parts ?

[Refrigerator]

So anyway I had a few quotes from some transformer re-winding companies and the cheapest is £160!!! and that is not even replacing the wire with new, that is for cutting out a section and joining it just like in the video.

Seems extraordinarily expensive to me. I am sure it would take only minutes to remove and replace the two windings with new wire, of course if one had a winding machine.

So I think I will just try what the chap in the previous youTube video did. It can't be too difficult, just time consuming.

Rewinding transformers is a pain in the arse. Rewinding toroidal transformers is an even bigger pain in the arse because you can't just wind on a bobbin and assemble the laminations but you have to loop the wire through the core over and over again.
Especially if your primary is shorted, because that means you have to undo all the windings.

Here's an older video that i used as a reference maybe 4-ish years ago when i was into DIY-ing custom transformers.

[paul_g_787]

A little update. Crown contacted me back regarding a replacement transformer. They have said they do not provide any parts for any of their amplifiers at all and I would need to buy a new amplifier. However they don't stock parts even if you bought a brand new amplifier!! I am quite shocked! I don't think I would like to buy anything new from them if they don't even have spares for new products!

https://www.crownaudio.com/en/support/parts ?

Yes that is who I got in contact with.

[paul_g_787]

So I made a start of this Thursday. First I tested that the transformer was indeed faulty by connecting the primary in series with a 100W tungsten lamp. With nothing connected to the secondary.
The lamp lit up.

Best to double check before starting this huge task! 

So then I made four little bobbin things like the ones in the video but I made mine from some scrap wood.

[paul_g_787]

Then I completely took off the wrappings making sure to save the plastic tape for later. I used a tin of beans to wrap it around for safe keeping.

Then I un-wound each of the coils one at a time onto the wooden bobbins. The two second-halves of the winding came off first (the lengths from the 18.5V to 67V taps). 94 turns each.