Author Topic: Toroid dual 240V primary (Read 5732 times)

BurningTantalum · « **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

Armadillo · « **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?

BurningTantalum · « **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

Armadillo · « **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.

SilverSolder · « **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)

ArthurDent · « **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.

Armadillo · « **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?

ArthurDent · « **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.

Armadillo · « **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.

ArthurDent · « **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.

Armadillo · « **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!

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.

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

Quote from: ArthurDent on January 20, 2019, 03:21:38 pm

to be 2 identical 250 VAC windings

Quote from: Armadillo 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?

floobydust · « **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.

Chris56000 · « **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!

BurningTantalum · « **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

BurningTantalum · « **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

ArthurDent · « **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.

Armadillo · « **Reply #16 on:** January 21, 2019, 10:15:54 pm »

Oh my goodness, I must have been wrong.

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.

Cheers;

SilverSolder · « **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!

BurningTantalum · « **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

Zero999 · « **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.

glarsson · « **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.

BurningTantalum · « **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

IanB · « **Reply #22 on:** January 22, 2019, 03:28:06 pm »

Quote from: BurningTantalum on January 22, 2019, 03:12:59 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?

strawberry · « **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

Bud · « **Reply #24 on:** January 22, 2019, 06:59:22 pm »

Quote from: glarsson 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.

Quite an ingenious way to handle the task

glarsson · « **Reply #25 on:** January 22, 2019, 07:07:08 pm »

Quote from: Bud on January 22, 2019, 06:59:22 pm

Quite an ingenious way to handle the task

A friend acquired a very expensive amplifier (cheaply) with the transformer completely toasted. Someone had twisted all secondary wires together and applied power.

BurningTantalum · « **Reply #26 on:** January 23, 2019, 01:49:19 pm »

The secondary winding is/was about 45 metres of dual 2.0mm copper.
Strawberry- Unfortunately the varnish is burned or blackened all the way through the windings. Had it just been on the surface I think your suggestion would have worked. There is a disturbance in the layers at this point, as if there was a kink in the wire when the transformer was wound, which may have been the case as it was only about 50mm from the lead-out, so maybe it has been a hot spot for all its life and failed one day when it was being thrashed in high ambient temp.
Bud- Nothing will surprise me anymore about electricity and 'the great unwashed'. The number of times I have seen a failure where the owner has started swapping wires around to 'get it working again...' This is the same process as the chainsaw/strimmer/mower that stops and won't start so the owner screws both carb mixture screws around wildly.
BT

ArthurDent · « **Reply #27 on:** January 24, 2019, 05:48:36 am »

If you counted all the turns on all three windings so you know the amount and size of wire to rewind and if you don't have friends to run laps, you could use a method that was used in the mills in the northeast U.S. in the 1800s. To weave cloth they had 2 sets of threads that alternated up and down and a shuttle with a bobbin of thread that would be passed from side to side to make the cloth. If you can make a crude version of the shuttle that will be small enough to pass through the window of the toroid when it has all the wire on it, you might be able to wind it by yourself. Here is a photo of one of these shuttles to give you an idea. You would probably have to snug each turn tight or if you wind it too loosely you could run out of room to pass the shuttle through.

If there was some way you could test the rest of the amplifier by temporarily using external transformers before you spend a lot of time and effort to rewind the toroid, only to find that isn't the only major problem, and it really isn't worth fixing, that could save you a lot of grief.

BurningTantalum · « **Reply #28 on:** January 24, 2019, 12:58:17 pm »

Arthur- This is what I will end up doing when I have exhausted all other avenues, and it has returned to the top of my inbox.
Actually I lived for a large part of my life in a World Heritage area where these looms were invented. The designs were a guarded secret until an enterprising New Englander got a guided tour and memorised the designs until he got back to Pawtucket, or somewhere.

IanB- I chopped off the primary today and counted 460 turns (actually 920, but they are in parallel and can be ignored for voltage purposes).
The schematic showing the connections labelled 'AC27' cannot be correct as it would give 38VDC, and the reservoir capacitors for this rectified output are 35V working.
37 turns/460 turns x 240 volts =19 volts, which is 27VDC by coincidence, and more likely to have 35V capacitors, so maybe the Yamaha diagram has been mis-written.
This doesn't help with the main output as 280/460 x 240 = 146V which is 206VDC. The res capacitors are listed as 160 working voltage.
Or have I missed something? I ran a Dremel once round the toroid and each short piece of copper would be 1 turn. I divided the number by 2 as there are 2 windings, in parallel and in phase (unlike the secondaries.)
Regards, BT

ArthurDent · « **Reply #29 on:** January 24, 2019, 03:22:54 pm »

First I'd like to apologize for my ancestors copying your weaving methods. They probably thought of it as 'innovation'.

Yes, the turns count confused me as well. I assumed the main secondary should be 75-100VAC for each winding given the 160 volt caps and the '27' winding could be 17-19 volts given the 35 volt caps.

If the '27' secondary is about 17 volts and has 37 turns that is about 2 turns per volt so the primary for 240 volts should be 480 turns, which is close to what you counted. That would make the 280 turns you counted on the main secondary 140 volts which makes sense using math but seems very high from the capacitor rating. With low audio output the load on the power supply should also be low allowing the main supply voltage rise to near peak value or around 200 volts. If the main secondary voltage was 70 volts then the number of turns on that winding should be exactly half of the 280 you counted or 140. Is it possible you counted the entire length of the windings and not just one half?

Another way to double check this is to measure the primary wire size in circular mils and the same for the main secondary which would give you a rough ratio of the current rating for the windings. If the primary was AWG 24 with a cir mil area of about 400 and the main secondary was AWG 18 with a cir mil of about 1600 then the ratio of 4 would mean the secondary might have 4X the current but would have to have 1/4 the voltage so each winding would have about the same VA rating. I've ignored the '27' winding which is low current and won't influence the calculations that much. Other than that, I'm at a loss to explain this

BurningTantalum · « **Reply #30 on:** January 25, 2019, 09:36:10 am »

Hi Arthur, and thanks for persisting with this!
I counted 860 of the single turns pictured below. I had already pulled off 30 turns of double wire before I got fed up and chopped the rest off with the Dremel, so I halved the 860 to 430 to give the actual turns, and added the 30 = 460. That seems valid to me. It is a mystery. A new 500 or 650 VA transformer is $200 plus shipping so I will leave this for a rainy day, and sweet talk my mechanical friend about the wonders of toroidal coil winders, and mention Joseph Arkwright and the water powered cotton loom.
Incidentally, the primary is 0.9mm diameter over the varnish; the secondary is 2.1mm over the varnish. This suggests 1/2 the voltage (very roughly) =120VAC which is more in the ballpark. My guess is that it should be 100 or 110VAC.
Thanks again for the help.
Regards, BT

tautech · « **Reply #31 on:** January 25, 2019, 10:20:09 am »

I'll drop this here in case it's of assistance:
https://ludens.cl/Electron/trafos/trafos.html

Good luck.

ArthurDent · « **Reply #32 on:** January 25, 2019, 03:16:05 pm »

BurningTantalum - "Incidentally, the primary is 0.9mm diameter over the varnish; the secondary is 2.1mm over the varnish. This suggests 1/2 the voltage (very roughly) =120VAC which is more in the ballpark. My guess is that it should be 100 or 110VAC."

The diameters of the wire seem reasonable and close to stock wire sizes but I said "measure the primary wire size in circular mils', not the diameter. Using cir mils would give a ratio of about 5:1 which is probably a somewhat high ratio. All these calculations are wild approximations but a voltage in the 50-80 volts range would make more sense than 100-120 volts.

SilverSolder · « **Reply #33 on:** January 25, 2019, 05:28:23 pm »

You know the total redneck fix for this amp... is to wire the remaining toroid so it powers both left and right!

This is assuming you are OK with 200W per channel rather than 450W, of course!

ArthurDent · « **Reply #34 on:** January 25, 2019, 06:43:10 pm »

SilverSolder, I believe you'll find the open space is for the input board and the filter cap board, not for a second transformer. There is only one transformer.

BurningTantalum · « **Reply #35 on:** January 27, 2019, 03:54:08 am »

Thanks for that Tautech, I will peruse that extensive info when I have a moment.

I have not come across 'circular mils' (When I went to school in 1961 we learned the cgs metric system, despite being in the UK, as it was a primary school that was intended to feed into an elite grammar school.) but it appears to be an area measurement so I will have a think about that when I have a moment.
BT

SilverSolder · « **Reply #36 on:** January 27, 2019, 04:53:38 am »

Quote from: ArthurDent on January 25, 2019, 06:43:10 pm

There is only one transformer.

Must be a massive transformer - 1000VA or even more?

ArthurDent · « **Reply #37 on:** January 27, 2019, 04:06:24 pm »

SilverSolder - "Must be a massive transformer - 1000VA or even more?"

Check the specs, the power consumption is 500W or 650VA. Output ratings often are wildly different from power input depending on the output power rating method used. Max output is rated from 1200 to 1400 watts.

ArthurDent · « **Reply #38 on:** January 27, 2019, 06:19:11 pm »

The cir mil listings of the two wire sizes used (.9 and 2.1) are 1288 and 6529 or about a 5:1 ratio for current capacity. The link below is to a wire table with the wire’s mm diameter in the next to last column and the cir mil numbers in the 4th column from the left.

If the 240 volt input is 650VA then the current is about 650VA/240V=2.7 amps. That would mean the main secondary current could be up to 5 times that or as high as 13.5 amps. Assuming power out is about equal to power in then 650VA/13.5A=48V. There are also transformer losses and another 17 volt secondary winding that subtract current and would make the 13.5A figure lower. The actual main secondary current rating might be closer to 10A which would make the voltage closer to 65V which would make more sense. The power supply load varies with the audio output levels as well so using a larger secondary wire size than needed would lower the coil resistance and help with the power supply stability.

Another way of looking at it is that the 460 turn primary is for 240 volts input or 1.917 turns per volt. If the main secondary is 280 turns, 280turns/1.917turns per volt=146 volts which gives almost 200VDC peak and this would mean the VA rating of the secondary would be much greater than the power in. Half of that 200VDC (which has to be wrong), is about 100VDC or 73 volt RMS which is within the ratings for the filter caps.

Both of the above are S.W.A.G. (scientific wild-ass guesses) as I have no idea what the design parameters are and I probably couldn’t follow them if given. However the main secondary voltage can’t be as high as 146 RMS or 200 Peak and 50V from the first guess to the 100V from the second guess looks somewhat believable with closer to 100V looking like the best guess. If the turns count on the main secondary were half of what was mentioned as the counted turns then you’d get reasonable numbers.

http://www.ihiconnectors.com/AWG%20wire%20sizes.htm

BurningTantalum · « **Reply #39 on:** January 28, 2019, 10:45:37 am »

Thanks so much Arthur, you are tenacious ! The chart is useful and I have saved it to file.
I am certain that I counted the secondary turns correctly. The first 30 were bifilar, so 30 is valid. The chopped off windings I counted in 10s into a waste bin and marked off on a pad in the traditional '4 strokes and a slash' ie 50 turns. Then divided the total by 2 and added the 30.
I have a couple of 50-0-50 toroids so may lash them up as 100-0-100, plus a small transformer or PSU and check the amp works which I'm sure it does. The output transistors are OK.
What I really need is some kind soul with a working amp to measure the AC from the transformer, but I still don't know what I have done wrong.
I am surprised that the quoted output rating seems to revert back to the days of Japanese 'music power' etc ('both channels instantaneous switch-on thump peak power' as we used to call it...)
Regards, BT

SilverSolder · « **Reply #40 on:** January 28, 2019, 01:08:56 pm »

One way to get an approximate verification on the winding count might be to weigh the copper leftovers on a good postal or kitchen scale? - you can weigh a known number of turns as a reference, or go with an "average diameter" of the winding, to do the math on how many windings would have made sense. This kind of thing should get you in the right ballpark, at least.

BurningTantalum · « **Reply #41 on:** January 29, 2019, 05:35:30 am »

The secondary winding was unwound in one length. I have it wrapped on a cardboard tube and there are two ends at each end. I have paced out the rough length of it, as I had tied it to a verandah post to enable it to be wound onto the tube, and it went round and through the house. It was roughly 45 metres of kinked wire, unstretched laid on the ground. Each turn was roughly 200mm (the inner primary turns are about 175mm). This gives at least 240 turns which is not far from what I counted.
I am going to check the amp works and leave this until I find more info and can source a reasonably priced alternative (maybe a pair of smaller toroids to make x-0-x)
Thanks all.
BT

SilverSolder · « **Reply #42 on:** January 29, 2019, 10:43:27 pm »

That definitely sounds thorough!

fenugrec · « **Reply #43 on:** October 05, 2020, 08:02:16 pm »

Whoa, someone else rewinding a P4500 toroid ?
I just recently found the service manual too, and decided to verify idle current properly and run some high power tests. Decided to check here just for fun; my unit is wired for Canadian / US 120V mains so my information may or may not be useful.

Around 2013 I rewound both secondary layers to repair mechanical damage - the toroid clamp bolt worked itself loose and the thing banged around in the case for who knows how long, severing both secondaries. Primary was luckily intact. I vaguely remember getting a quote of about 4-500$ for a new transformer, so I instead went the manual rewind route. It took me a long time to work up the motivation and the supplies to do so, but it turned out to be roughly a 2-day job IIRC.

Back then I was a bit careless when taking notes, but I wrote "33 turns sec, double wire" (implying centertap of course, not parallel) for the outer layer, measured as 0.43mm dia (with / without varnish ? no idea). I replaced that with 25ga to be safe.
Inner secondary is 159 turns, that "go around" 3 times around the toroid, so to speak (i.e. "36h around a clock"...) Dia ~ 1.25mm, so I went with 16ga (1.29mm).
I wrote "pri, 2x 200t ??" but that is of course only a guess since I didn't touch that layer. Also I don't recall seeing a thermal fuse, which was possibly hidden under the paper wrapping of the primary.

I used a poor-man's version of a shuttle : I took a bike rim, split in one place, measured + rolled the wire onto it, removed the coil by bending inwards both ends of the rim, then folded the coiled wire into a thin oval that just barely fits through the toroid (using a pvc pipe to form a radius at both ends to prevent kinking). Rubber bands at both ends of the "shuttles" to keep them disciplined.

When powered on, at idle, I measure 135VDC on the B+ / B- bus. Filter caps are rated 160V so "it works, mate". I'm fairly confident this is within a few % of specs.

Let me know if you want other measurements or info. If I were to do it again I would get better adhesive tape for the end instead of black e-tape as I did (the clear mylar insulating tape I found was non-adhesive). Other than that, I think it's a viable method for expensive / hard to replace toroids like this with a reasonable number of turns.


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Author Topic: Toroid dual 240V primary (Read 5732 times)

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