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Old 31st May 2004, 12:26 AM   #1
sdoom is offline sdoom  Germany
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Join Date: Mar 2004
Location: Heidelberg
Default Sony XM-5540 SMPS problem

Hi@ all


I have a broken Sony XM5540. I traced the fault to be in the power supply section. One driving transistor for the FET and all 8 FET were burned (2SK736) . As I cant find the part anywhere I tried to test with two IRFZ34 and no load for getting "warmed up".

The clock generator is a MN3102 "CMOS-clock generator". I replaced the broken small driving transistor with a new one and checked the voltage with a scope. I had a square-voltage with exactly 5V peak at the gate-resistors. Frequency is stable running @ 50 kHz. from the driving transistors there are 2 lines going to the FET. 4 FET are attached to each line with each a 10 Ohm resistor . Also there are two 1k-resistors, one going from each of those lines to ground.

As usual, the FET are furthermore connected to ground and to the transformer coils. When I installed two IRFZ34 to see if the SMPS comes back to live the protect-LED turned off and power LED went on. So I added another 4 IRFZ34. The power LED went on again but I had an idle current (12V side) of aprox. 8A which I assume is a little high as the IRFZ34 went up in smoke

I made sure that the amplifier section is ok. Also the rectifier-diodes are ok , caps are also fine. All 4 misic-channels are fine (I attached a +/-30V power supply to test those curcuits) .

I think that the 10Ohm resistors are too small (IRFZ34 has a smaller RDS_on) . I replaced those with 22Ohms, and I also tried 100Ohms. Still the idle current is around 8Amps

What I wonder now is that I think there is something missing. It happens to be that Im not the first person working on this amp-

Usually (at least in most Corea-amplifiers) there are 2W-resistors in series with a 10-22nF capacitor parallel to the transformer coils. Actually there are spaces on the curcuit-board but there are no parts....... ? Does anyone have a XM-5540 or a schematic diagram? Any help is greatly appreciated.


Thanks guys

Steph
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