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Old 26th April 2012, 06:07 PM   #1
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Default SMPS PSU from Oberheim Xpander not working need help

Hi all,

I have an faulty Oberheim Xpander japanese version. The problem comes from SMPS PSU. I already replaced the switcher transistor T1 2SC3262, bridge D1 and the resistor R1 on the below schematic because these parts where dead. I did replace R1 with a 10ohm 5W as I couldnt read the original value as it has burned. If someone nows a good value for R1?

Now the PSU works with no load, I have the right voltage. When I connect it to the synth boards all the voltage fall down and the Xformer L1 start to buzz a little.
I have removed the PSU from the synth and bench test it.
As soon I try to load the output of the PSU the voltage collapse.
I load a 240ohms on the +12V (or -12,+55) the voltage collapse to 6V.
And 240ohm is not a big load considering all the electronic that is inside this synth.

Can it be L1 that has burned is some way? Or what looks strange to me is the rectified 220VAC (first scope pict)

I took some scope pics from various parts.

Schematic
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Bridge D1 A-B 100V/div 5ms/div
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Across L1 100V/div 5ms/div
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T1 base 1V/div 10us/div
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T1 collector to emitter 100V/div 5us/div
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Pin 11 from U1 MB3759 10V/div 10us/div
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Old 26th April 2012, 07:26 PM   #2
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1) the 10R is only to limit inrush, so the value is not critical.

2) Try to load only the voltage sensed output, in fact the winding that appears to have 45V. In such case, the voltage must be stable. If this is the case, the SMPS needs load proportional in all outputs.

Check base resistors and the IC, appears that it has the driver build in and may be fake. Also, recap all the supply, and use 105deg and low ESR caps.

Keep us posted.

Good luck.
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Old 1st May 2012, 07:28 PM   #3
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Thanks Osvaldo, I did load the sensed output (+5V not 45) with 10ohm and the + and - 12v with 100ohm each and the the voltage is stable. So I think now smps works.
When I connect it to the boards then the -12 drops to -10.5, +12 to 11.5 and the +5v to 4.7V. The +55v is stable and the 9VAC too.
Unfortunately the synth is not working. The three Vacuum Fluorescent Display
are black and no reaction when pressing any knob.
As I dont have the corect schematic (the synth is the japanese version), only for american version avaible, and the service manual does not match to the boards.
Im stuck now until I find a japanese service manual. If someone now where to find it...
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Old 5th May 2012, 08:17 PM   #4
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> Bridge D1 A-B 100V/div 5ms/div

You should have ~300V DC there, not pulsating. Looks like your primary-side smoothing caps are bad or have a bad connection.
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Old 10th May 2012, 10:22 AM   #5
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Many thanks megajocke I did change the cap and now works ok.
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Old 10th May 2012, 08:28 PM   #6
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Yay!
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