Author Topic: Toroid dual 240V primary  (Read 2859 times)

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Offline BurningTantalum

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Toroid dual 240V primary
« on: January 20, 2019, 09:33:44 am »
I have acquired a Yamaha P4500 450W per channel stereo amplifier that was blowing mains fuses. I expected to be unsoldering output devices but it appears to be a short circuit primary winding. There are two, assumed identical, primary windings, one of which is connected to the 240V live when the front panel switch is on, and the other from the same switch contact via a N.O. relay contact. The winding lead-out wires are both red and the PCB does not discriminate between them in the overlay- both labelled as 'Red'.
I assume that the reason for this design is to reduce the huge inrush current that this toroid would have if a single winding was connected to 240V.
I am interested if anyone has come across this design before, as I haven't.
More importantly: The 'good' winding seems to be around 6 ohms to the black N lead. The 'bad' winding seems to be 0.01 ohms to the best of my measuring equipment. Has anyone experienced this type of failure? Any tips on removing the outer shrunk plastic and delving inside? I cannot see any discolouration through the clear wrap.
Attached is the relevant part of the Yamaha service schematic, but the relay is shown in the Neutral wire, with a thermistor across the contacts. There is no provision for this on my PCB and the contacts are connected in series between F102 and Cn116.
The offending red wire is the one with the sleeve and was the one connected direct to the switch via F101.
BT
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #1 on: January 20, 2019, 12:38:13 pm »
Could it be intended for 110v or 220v windings and only 1 fuse must be installed, not both fuse, since each fuse is rated 15A each.

and regarding the inrush, lesser windings [winding divide 2] lesser inductive reactance and more inrush, so its unlikely the reason.

What did the manual say about the fuse?

 

Offline BurningTantalum

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Re: Toroid dual 240V primary
« Reply #2 on: January 20, 2019, 01:12:59 pm »
The diagram definitely shows both fuses fitted, and I have found pictures on the net showing the same. The 'HB' and 'JU' variants appear to be for 120V and 240V supply, but on this PCB the overlay info for the fuses is redacted apart from the legend of '5A fuse' by F101 and F102.
The winding is definitely shorted regardless of how it should be connected.
I have only the schematic and no other service info unfortunately.
The toroid is buggered so I may as well start to peel off the wrap and have a look. If it cannot be repaired then the very nice amp will be scrap, or as they say in these parts "a boat anchor".
What I haven't done is power up the toroid using just the 'N' and the good 'L' wires. I assume that I can then determine the main winding voltage which is very helpfully just described as 'B+' and 'B-' on the schematic but must be around 80-0-80VAC.
I suppose I could find another toroid of the correct rating, sort out a soft start, and add another much smaller transformer for the 27-0-27VAC output.
Regards, BT
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #3 on: January 20, 2019, 01:36:01 pm »
Oh yah, I think someone applied 240v into the center tapped 110v winding and fused it already.

As it is the center winding, I suppose the toroid Xfrm need to be rewound.

I search the manual, and yah.... it certainly lack description of the fuses. They did mention speaker fuse, however.



 

Offline SilverSolder

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Re: Toroid dual 240V primary
« Reply #4 on: January 20, 2019, 01:41:18 pm »
Your understanding of how it works is right -  while the relay contacts are Open, the mains current is forced to run through the thermistor labeled TH101 on the diagram.  This prevents a large inrush current.  Once the relay closes, it is taken out of circuit.

If a winding is shorted, you won't be able to power up the transformer on any of its windings without the short drawing an enormous current. 

Opening it up is probably the only way forward.  At worst, you can rewind the whole thing with new enameled wire.  Fiddly work, but not rocket science! (There are youtube vids on this)

« Last Edit: January 20, 2019, 01:53:14 pm by SilverSolder »
 

Online ArthurDent

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Re: Toroid dual 240V primary
« Reply #5 on: January 20, 2019, 03:21:38 pm »
It looks like SilverSolder is correct. I have redrawn the circuit to make it clearer and there appear to be 2 identical 250 VAC windings with a common neutral, not two 120 volt windings, maybe so they could use smaller diameter, easier to use, wire to wind the primary. The reason for the relay is that TH101 gets hot and low resistance after the amp switches on and the relay that is powered by the secondary voltage energizes to bypass TH101 and allows it to cool.

Normally I'd want to disconnect the secondary windings and try to power just the good primary winding with the fuse removed from the other side but where there is such a difference in resistance readings, it does look like a short.
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #6 on: January 20, 2019, 03:28:32 pm »
Not that you might be wrong. But that's open up the question of why it had blown.

One fuse blow, the other one suffer?

Oh BTW, if you are Right, then the Neutral should come out with 2 leads instead of 1. Conveniently hidden inside?  ;D
 

Online ArthurDent

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Re: Toroid dual 240V primary
« Reply #7 on: January 20, 2019, 03:46:56 pm »
If there were two separate 120 volt windings that could be wired series or parallel, I'd expect four wires but the photo clearly shows three wires. Here is another schematic I found that shows the same three wires for the primary. It wasn't what I would have expected but the photos and two schematics seem to verify there are only three leads on the primary. 
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #8 on: January 20, 2019, 03:53:01 pm »
Please do us all a favor not to confuse with your un-highlighted or unclouded addition of your own drawn transformer there.
The original schematic is not like that.

I think you must be confused between schematic representation and "Practical Transformer Winding" in the real world.

2 windings = 4 leads. Is a Fact.

Period.
 

Online ArthurDent

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Re: Toroid dual 240V primary
« Reply #9 on: January 20, 2019, 06:18:40 pm »
Armadillo – Sorry you’re having trouble following the schematics and other posts by the OP in this thread. Here is yet another photo of the inside of a Yahama amplifier showing the THREE primary leads.

Please do us all a favor not to confuse your misconceptions about the imaginary missing 4th wire with the actual photo from the OP, the photo below, and the second Yahama schematic that clearly shows only 3 wires.

I think you must be confused between what you believe should be and the real world.
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #10 on: January 20, 2019, 06:32:59 pm »
My Goodness, ArthurDent, it was in response to your own posting at Reply #5 that its 2 identical windings so that thinner size wire can be used. This is also drawn in your schematic of yours showing 2 windings in Parallel.

All I am telling you what you have tried to allude to others, is 2 Parallel windings = 4 leads, which means YOU ARE WRONG!   :-DD

Do I need you to dig for photograph when the OP first posting already have the 3 leads showing so clearly.

I was saying, all the while, is a series winding center tapped 120 / 240 windings consisting of 3 leads.

I must have over-rated you.   :-DD

Or is it that I don't appreciate your drawing?   :-DD

to be 2 identical 250 VAC windings

Not that you might be wrong. But that's open up the question of why it had blown.

One fuse blow, the other one suffer?

Oh BTW, if you are Right, then the Neutral should come out with 2 leads instead of 1. Conveniently hidden inside?  ;D
 

Offline floobydust

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Re: Toroid dual 240V primary
« Reply #11 on: January 20, 2019, 07:30:41 pm »
You can inject say 12VAC to the secondary, and see what comes out the primary.
A shorted primary would draw lots of (secondary) current, compared to the usual magnetization current.
 

Offline Chris56000

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Re: Toroid dual 240V primary
« Reply #12 on: January 20, 2019, 10:06:08 pm »
Hi!

Full service documentation here:–

https://elektrotanya.com/yamaha_p1600_p3200_p4500_sm.pdf/download.html

(Don't be put off by the Japanese – the manual is written in English as well!)

My recommendation is to disconnect all the winding connections from the transformer to the DC pcb  (full wiring diagrams are in the bookwork I linked to above!), then see if you can identify the three primary leads for definite – what I think Yamaha have done is simply bring the outer end of primary 1 and the inner end of primary 2 in one single piece of sleeving together, and this will be the "115V" tap – if this is the case the '0' and '230' ends of the overall primary pairs will be single leadout wires!

However if you have a transformer that is Jap/American Market ONLY, then the taps may be '100' and '115', in which case the '6 ohms' mentioned in the OP's post will be the '0' and '100' taps and the 'fraction of an ohm' will be between the '100' and '115' taps.

Anyway, there should be a part no. on your transformer which should match one of the version nos. given in the full parts list – print out the Manual I've linked, study it carefully along with your suspect transformer, and come back if you're still stuck, and I'll advise on replacing it!

Chris Williams

PS!

In the diagrams provided by the OP, Yahama have made confusion worse counfounded by apparently showing the transformer connected for both 115V and 230V simutaneously, which is not only nonsense but will destroy the transformer and the amplifier's semiconductors!
« Last Edit: January 20, 2019, 10:21:30 pm by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline BurningTantalum

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Re: Toroid dual 240V primary
« Reply #13 on: January 21, 2019, 01:50:28 am »
Crikey, I had no intention of starting an on-line fisticuffs!
Let's just rewind a  moment:
As I stated, the schematic that I posted is NOT correct to the PCB layout that I have.
The amplifier has not been modified or opened prior to my acquisition.
There are two windings that are energised by 240V- one at switch on and the other after a delay. There is no provision for a 110V input except by a different transformer on manufacture. The back panel clearly states '240V'.
I understand the thermistor in the schematic but this relies on both windings being energised, albeit gradually, at switch on but my version does not have this.
Now:
Arthur- your schematic is very close to what I have, although my relay contacts are actually between the fuse and the transformer rather than the PCB edge and the fuse, but the same thing in effect.
Thanks for the link to the service manual- I had not managed to find this. I have not downloaded it yet as two browsers have given an error, but I will try later.
Chris- I have the transformer removed from the amp, and I am most certain that the winding is shorted/damaged. When I return from work I will power it up on the good winding and measure the secondaries, then attack it with a scalpel etc.
As I see it- one winding will supply the stated secondary outputs but at only half the rated VA, so should be OK to determine the secondaries to specify a replacement if I need to.

I think everyone has become a bit fixated and confused that there is an option for 110VAC supply, which I am sure is not the case. On opening the amp there was a fuse fitted in both F101 and F102 positions. The F101 fuse had failed catastrophically with the link sputtered on the glass.
If, as has been suggested here- for some supply condition- F101 was not fitted, then the amp would not work as the relay would be open and the transformer isolated so unable to power the relay circuit to energise it.

Just to note- I think that the '250' legend by some of the connections to the PCB refers to 1/4" AMP tag type terminals rather than a voltage.
I will report back later. Thanks for the suggestions and interest.
Regards, BT
 

Offline BurningTantalum

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Re: Toroid dual 240V primary
« Reply #14 on: January 21, 2019, 09:38:01 am »
OK, I powered up the 'good' primary which promptly took a large amount of current and changed its resistance to around from 6 ohms to around 1 ohm.
I had a dig into the transformer and managed to get at the joints of the windings and the lead-outs. As you can see the black wire was a problem because it was under the outer secondary winding. There I found our old friend- a thermal fuse, intact, and the ends of the two windings commoned. I separated them and found that there is a short between both windings somewhere.
I may as well unwind the whole caboodle as I have nothing to lose. I throw myself at the feet of Chris56000 and anyone else for info/tips about doing this and rewinding it afterwards.
I have rewound alternators before (Japanese motorcycles) and a vacuum cleaner motor (commercial carpet cleaner) and voice coils so I am not cack-handed.
BT
 

Online ArthurDent

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Re: Toroid dual 240V primary
« Reply #15 on: January 21, 2019, 02:11:07 pm »
Although I hadn’t seen a three-lead 240 volt only transformer before, the photos and the schematic that BurningTantalum showed indicated that there definitely were two identical primary windings with one common start wire and two separate end wires. Checking the parts list for the amp in the service manual verified this theory since it listed three separate transformers for 120, 230, and 240, depending on the different supply voltages for different countries. This evidence didn’t fit the view of one poster who then chose to launch an ad hominem attack rather than discuss the facts.

It is common (and probably regulation now) to have a thermal fuse in the transformer to protect from overtemperature and the photos of the tear-down of the transformer confirms there is a thermal fuse in the transformer in question. It also appears that from the OP’s posts this amp has a soft start feature that initially connects one of the two primaries to the line to lessen inrush current which can be rather high in toroidal transformers and the thermal protection fuse may be in series with only one primary winding, perhaps the one that doesn’t see the inrush current on start-up. Because there are two separate windings used, each has a regular fuse in series with it and for 240 volt operation they appear to be 6.3A fuses. If either or both primaries have a problem, each has a fuse in series with it for protection.  Another reason for the two-pole paralleled switch and two fuses is to double up because of the high current drawn on the primary side. It seems that a lot of engineering went into this amplifier and is probably reflected in the selling price.

The clues that this wasn’t a universal voltage transformer was it had only three leads instead of four, the schematic and the manual do not mention setting the input voltage connections, and there are three versions of the amplifier for the three common supply voltages used is different countries. Also the thread title: "Re: Toroid dual 240V primary" was a definite clue.

On rewinding the transformer, if you’re lucky you may find that taking off the first winding or two you’ll find that there is one spot where the insulation was compromised and you can repair that without unwinding the entire transformer. Probably the secondaries are toward the outside and once they are removed you can see where the primary problem is.  If you’re lucky you may be able to reinsulate that spot and rewind the other windings with the proper type of insulation. I can see a new transformer being quite expensive so it’s worth a try. Good luck.
 

Offline Armadillo

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Re: Toroid dual 240V primary
« Reply #16 on: January 21, 2019, 10:15:54 pm »
Oh my goodness, I must have been wrong.  :-DD  :-// Just what am I doing, now that I am thinking.!

ArthurDent could be right with the Parallel Connection as shown in his drawings. [Re-attached here]

Give it a try, again.  ;D

Cheers;  :clap:
 

Offline SilverSolder

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Re: Toroid dual 240V primary
« Reply #17 on: January 21, 2019, 11:14:09 pm »
If you've rewound a Japanese motorcycle alternator, you won't find a toroidal transformer too much of a challenge!   :-+

 

Offline BurningTantalum

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Re: Toroid dual 240V primary
« Reply #18 on: January 22, 2019, 01:15:31 pm »
Well, SilverSolder, give me a GS750 alternator any day !
I should have chopped off the secondaries with a Dremel, but I foolishly was thinking that I could save the copper wire and rewind the beast. It took me all afternoon to unwind it, then found the damage to the primary. I think that it will be a right royal pain to rewind it, but I will mull it over.
Mrs BT said that in 36 years she has never known me say, "I don't think I can do that." Well now she has. I was impressed that she incorrectly identified it as a motor. I remembered that she is the daughter of a man who owned an electrical rewind business all his life (father and company long deceased.)
I found some YouTube vids of toroid winding machines- things of beauty indeed. For a few moments I was off thinking- "If I make one of those..." but then reality bit.
Also pictured is why fans in equipment should always have filters. I have no idea how this came to be full of Aussie red dust. It must have been used as a PA amp at a country horse race.
Thanks everyone,  BT
 

Offline Zero999

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Re: Toroid dual 240V primary
« Reply #19 on: January 22, 2019, 01:48:26 pm »
Why not simply buy a new transformer?

Even if it isn't exactly the same, it's possible a standard toroidal transformer with two 120V windings in series will do, with a some modifications to the starting circuitry.

It's generally not economical to rewind small transformers like this. Now you've separated the copper from the iron, you may be able to recoup some of its value in scrap metal.
 

Offline glarsson

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Re: Toroid dual 240V primary
« Reply #20 on: January 22, 2019, 01:50:19 pm »
We completely rewound a toroid like this on the early eighties. Three or four people running back and forth in a long corridor... We didn't need a sexy machine.
 

Offline BurningTantalum

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Re: Toroid dual 240V primary
« Reply #21 on: January 22, 2019, 03:12:59 pm »
Zero: I probably will obtain a suitable toroid and there is room for a separate small 27-0-27 transformer, but the amp cost me nothing and toroids are actually quite expensive. I can't sell the thing as I am in the middle of nowhere and shipping to a population centre is not on. I wanted to learn what had happened, and usually like to have a crack at things just to extend my skills.
An exact replacement would be a huge cost, although the manufacturer still exists- Chain-Mate Industrial Co., Ltd, Taiwan.
I wonder what caused this failure? The loose wire in the picture is direct from one of the red lead-outs- hence the measured dead short. It can't have been a long term overload as the thermal fuse was OK, and the fuse F101 had not ruptured due to a slowly increasing fault current, but a catastrophic failure.

Glarsson: I have the long corridor... but with the sort of friends I have, I would need a sheep-dog.

I am still not sure what the main secondary voltage was. The schematic shows the small secondary as being 27-0-27VAC and was 37 turns bifilar. The main winding was 280 turns bifilar. The main res capacitors are 160V.
BT
 

Offline IanB

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Re: Toroid dual 240V primary
« Reply #22 on: January 22, 2019, 03:28:06 pm »
I am still not sure what the main secondary voltage was. The schematic shows the small secondary as being 27-0-27VAC and was 37 turns bifilar. The main winding was 280 turns bifilar.

Can't you just multiply? 280/37 x 27-0-27 = 204-0-204?
I'm not an EE--what am I doing here?
 

Offline strawberry

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Re: Toroid dual 240V primary
« Reply #23 on: January 22, 2019, 06:30:14 pm »
clean damaged area with IPA and solder burned wires,
then use warnish on to damaged area
isolate soldered wire using mylar tape
rewound secondaries
done that many times
 

Offline Bud

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Re: Toroid dual 240V primary
« Reply #24 on: January 22, 2019, 06:59:22 pm »
We completely rewound a toroid like this on the early eighties. Three or four people running back and forth in a long corridor... We didn't need a sexy machine.
:-DD
Quite an ingenious way to handle the task
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