JCM 900 OT problem

Carlos Rajoo

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Hi

I have this JCM900 model 4100 which has its power transformer blown. Upon checking all the components on the board, I dont see any burnt or shorted components. I then checked the output transformer. On the primary side, I get a DC resistance of 17.6 ohms on one side and a 12.9 ohms on the other. I realised that quite a number of marshall amps have a DC resistance of 16 to 17 ohms on either side of the primary winding. Does the winding showing a reading of 12.9 ohms has a partially shorted winding. I did a test by connecting a 12V AC across the secondary and measured the voltages induced on the primary. The side which has a resistance reading of 17.6 ohms shows 45V AC and the side with a resistance reading of 12.9 ohms shows 33V AC. I realise that DC resistance and impedance are different, but passing a AC voltage thru the winding would refer to its impedance across each half of the primary winding. Does this indicate a fault in one side of the primary winding and could have caused the power transformer to blow. Please assist.

Thanks
 

Jon Snell

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Usually if there are shorted turns, as you fear here, after some time the sound goes grainy and weak plus the transformer runs very hot.
The DC resistances are about right and as the transformer is designed, unlike a mains transformer, to work with a load to e lalanced, I am not convinced it is faullty but it could be.
What are your symptoms ... you don't say what the issue is.
 

Carlos Rajoo

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Usually if there are shorted turns, as you fear here, after some time the sound goes grainy and weak plus the transformer runs very hot.
The DC resistances are about right and as the transformer is designed, unlike a mains transformer, to work with a load to e lalanced, I am not convinced it is faullty but it could be.
What are your symptoms ... you don't say what the issue is.

There are no symptoms as the power transformer is blown and I cannot power up the amp. What about the AC voltage test . With a 12V AC applied at the secondary, I get 45V AC on one half of the primary and 33V AC on the other. Doesn't this matter.

Thanks
 

Jon Snell

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There are no symptoms as the power transformer is blown and I cannot power up the amp. What about the AC voltage test . With a 12V AC applied at the secondary, I get 45V AC on one half of the primary and 33V AC on the other. Doesn't this matter.

Thanks
My mistake, I misread your post.

`JCM 900 OT problem'​

Yes the transformer should produce equal voltage. To me your title means Output Transformer problem.
It is faullty.
TAD supply a good range. Look on the old transformer for the part number and order one.

and many more suppliers.

Edit; addition of more data ...
 
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