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Old 27th January 2013, 04:33 AM   #1
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Default Rewind primary 100v/240v?

I have a Japanese 100v Sansui A909 amplifier that I want to convert to 240v. I have tried a 200v step-down trans but the Sansui trans got hot after about 1 hour or so! Hot enough to worry me,(this is at low to medium volume) I could still hold my hand on it though? I tried a 1000watt step down,same thing. Is there any place I could send the transformer to for a rewind on the primary for 240volts? I live in Geraldton Western Australia 6530.

Last edited by redrooster; 27th January 2013 at 04:36 AM.
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Old 27th January 2013, 08:26 AM   #2
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No surprise that a 200V transformer caused problems, try a 110V transformer.
Let the amplifier sit for 20 minutes with no load an check for excessive transformer heating.
Before we get all the lectures about 110V being too much for 100V transformer, Japanese transformers are usually quite conservative in their design

If you must modify the transformer it would be better to get a new one wound. The primary is usually the inner winding and getting out the first lamination which must be done before a rewind is a major PITA
As an aside the amplifier bias circuitry might be giving trouble and that is the source of the heating.
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Old 27th January 2013, 03:19 PM   #3
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Actually the 1000watt step-down transformer is 110volt!The first one I used was 200watts,110volts. Maybe its a bit too high voltage(extra 10volts) what if I set the bias a bit lower? I cant find a 100volt step-down anywhere.

Last edited by redrooster; 27th January 2013 at 03:24 PM.
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Old 27th January 2013, 03:29 PM   #4
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You'd be better searching the forums for the specs of the transformer and having one custom made. May not be cheap though.

There must be a Sansui A909 service manual somewhere on here that will let you know what secondaries you need.

I doubt whether a company would want to use a second hand core though for liabilty issues.

Nothing to stop you DIY'ing one though - HARD WORK by hand.

Someone might even have a NOS transformer with the correct primary windings. You could search on E-Bay.

Last edited by KatieandDad; 27th January 2013 at 03:34 PM.
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Old 27th January 2013, 03:49 PM   #5
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You can use a Variac, like the one shown in the photo below.
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Old 27th January 2013, 03:52 PM   #6
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Quote:
Originally Posted by Junie View Post
You can use a Variac, like the one shown in the photo below.
You can but they're not cheap.
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Old 27th January 2013, 05:05 PM   #7
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If you have a large step-down transformer, but the output voltage is a little high, think about adding a "boost/buck" transformer.

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Old 28th January 2013, 04:43 AM   #8
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Quote:
Originally Posted by redrooster View Post
Actually the 1000watt step-down transformer is 110volt!The first one I used was 200watts,110volts. Maybe its a bit too high voltage(extra 10volts) what if I set the bias a bit lower? I cant find a 100volt step-down anywhere.
The heating caused by faulty bias is a separate issue from the heating caused by excessive magnetising current, The bias will heat the heatsink the magnetising current will heat the transformer, make sure which problem you have first.
Variacs are often used by Japanese when overseas. Off the shelf 240 - 100V transformers are not common, they are usually supplied with a Japanese machine. As another poster mentioned bucking the voltage by 10V is easy enough and the transformer will be relatively cheap.
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