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Pajarico

How to know the ratio of a burnt transformer?

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My audio equipment burnt its transformer. I know it's only the transformer cause I unplugged the supply board (composed of the transformer and some diodes) and started to smell weird, some smoke coming from it, etc :P

To pinpoint the problem to the transformer, I desoldered it and plugged directly the primary to mains with and ammeter in series, and in the secondary there was no load. There were about 1,5A when I unplugged mains just to avoid it being burnt. Unfortunately, with these tests it broke one of the primary windings.

1)How can a transformer become shortcircuited internally when it has been working just fine for years?
2)How can I know the ratio if some of the windings are broken?

Also, after rectification, the voltage of both secondaries goes into two pairs of capacitors to generate a simmetric supply. There are two pairs:
-2x4700uF/35v
-2x1000uF/25v

3)It is safe to choose a transformer with two secondaries, so each one gives a DC of 70v and 50v (actually lesss, just to leave a margin)?.

I attach a scheme of the transformer and the value of the windings in ohms. The pin marked as "BROKEN!!" is where one of the mains is plugged. The windings used on the board are the ones formed between that pin and the two pins at the right. The measured ohms are relative to the "N/C" pin, which is a no connect pin on the board and thus not used (I've included it here just for information purposes).

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1. Age makes the insulation weak and repeated heating and cool of makes things even worse.
2. The only way to determine the ratio (if you cannot find the data elsewhere) is to dismantle the  transformer and count the turns as you unwind the coils.
3. No I don

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Thanks for your reply.

The equipment is a Sony CMT-CP2WA. I have an open thread here asking for a service manual:
http://www.electronics-lab.com/forum/index.php?topic=10796.0

The transformer itself has the configuration attached above. Externally it has a gray chasis, is non-toroidal, weighs a bit, and has a metallic plate perpendicular to the pins side with drilled holes to screw it inside the equipment. Going from the outside to inside I see:
-Gray metallic chassis
-A copper-coloured sheet that is surrounding the core (it is not a winding, I don't know what it is, maybe to capture EMI?)
-The iron core
-One of the winding fits inside the iron core
-Two plastic pieces where the winding is rolled
-And... something else but it's under the plastic piece and can't see it.

Nothing really remarkable, and I cannot take photos.

There is a sticker on the gray chassis:

1-435-386-11
MADE IN CHINA
RD 053 SET


And a molded reference on the plastic pieces that carries the winding:
SP744 2SODK
SET D-1


SP74 42PODK
SET D-1

To me it seems like those plastic pieces are standard made and they build different transformers joining different pieces.

I don't find any logo. I did search google for the part number but no luck.

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My audio equipment burnt its transformer. .........


Basically you may have to study the other circuitry and the audio eqpt specification from the user manual.
1. whether input is 230 or 110V-- depends on your location.

2. you have two options. a. get it wound as it is by a local winding shop/repairer.
b. do it yourself.
If option is (b) ,  you have to count the secondary and primary turns while ripping it and Try to duplicate.
If you know the seconday voltage, as per the electrolytics, i cold feel that secondary may be 15 to 18V or 20V at the wiorst.
But you can't take chances, lest you may end uploosing other parts.

By any means , it appears to judst duplicate it by careful reverse engineering.

Even if the primary is partly burnt, at one place the wires may be jammed, it is still possible to be able to count the turns while ripping.

FInally, if the make is known, transact with the manufacturer and seek a replacement transformer or the tranformewr data-

Some times if you have a service data you may know the unregulated dc a\oltage and even the secondary AC voltage. if so you could improvisea substitute for it.

All the best.
Sarma

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Thanks for your reply Sarma:

Things I did:
-Search for service manual: so far no luck. It exists but all of the pages storing it are pay services.
-Searched for the maker: Hard to know... The references described above yield references in google but it leads me to pages where the part is listed or even available for purchase but no further info, no datasheet, no specs, nothing.
-Searched for a replacement/reseller: There are some in internet but prices are high (60 or 100 euro I have seen), sum the cost of shipping of a piece that weights 2 or more kilos and they will rip my pocket. The cost alone is to much for this, I would consider first buying another equipment...

Things I haven't done:
-Ripping apart the transformer and rewire it... to hard to me I think.

If you know the seconday voltage, as per the electrolytics, i cold feel that secondary may be 15 to 18V or 20V at the wiorst.
But you can't take chances, lest you may end uploosing other parts.

Those voltages are wild guesses or based on my commentary about the caps?

Regards.

PS: From the back of the cassette: "AC230v 50Hz 70W".

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It is OK.

do you find regulators inside or voltage stabilisation using zenor diodes? f so what is / are the regulator(s) / zenors used?

Incidentally you never indicated the make and model Number of the equipment under reference.

perhaps a replacement Tranformer is much cheaper compared to total equipment -- unless the said equipment is very much old-- then better to change-- while so, better go for modern 5.1 audio systems  and try covert all your casttes to CDs progressively.

Sarma

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