Pioneer M25 power amp bias & offset adjust ? - diyAudio
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Old 20th November 2005, 10:44 AM   #1
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Question Pioneer M25 power amp bias & offset adjust ?

Hi,

just repaired a Pioneer M25, the failure was in the rail regulators.

On power up there was an initial offset of 0,5V on one channel that faded away after warm up.

Now one heatsink gets slightly warmer than the other.

There are 4 pots each channel and I do not know what to do with them...

Anybody have an idea or schematic please ?

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Old 20th November 2005, 03:29 PM   #2
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hi,

I had saw the M25 sch.IIRC,each channel of M25 has two pots which is 100 Ohms for adjust the protect current,and one pot for adjust the offset .....like the KRELL KSA100 sch.

If you research the KRELL KSA100 sch,and trace the M25...I think you can easily find the pot for adjust offset.
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Old 20th November 2005, 05:06 PM   #3
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What has the M25 got to do with Krell ??? Same ?
There are 4 pots...
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Old 20th November 2005, 10:22 PM   #4
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My scanner is worse....I just check my paper,the 4 pots is :

VR1(470 Ohms) for adjust offset
VR2(220 Ohms) for adjust the idle current of power stage
VR3 and VR4(all 100 Ohms)for adjust the limited current of pretect circuit.

I can't see your attachment.there are some M22 pictures which the brother of M25,M22 is class A,M25 is class AB.

good luck

X.G.

M22 pictures

http://amp8.com/sub/tr-amp/pioneer/pi-m22.htm

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KSA100 schematic

PCB layout for Krell KSA100

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Old 21st November 2005, 07:41 AM   #5
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Thanks,

VR1(470 Ohms) for adjust offset
VR2(220 Ohms) for adjust the idle current of power stage
VR3 and VR4(all 100 Ohms)for adjust the limited current of pretect circuit.

This is from M25 ?

M25 board looks different from M22.
Also it seems M22 does not use ringemitter transistors, 4 different output 2sa / 2sc devices...

What I have read is that M25 is class A up to 30W and B up to 120W.
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Old 21st November 2005, 08:35 AM   #6
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Quote:
Originally posted by Bernhard
Thanks,

VR1(470 Ohms) for adjust offset
VR2(220 Ohms) for adjust the idle current of power stage
VR3 and VR4(all 100 Ohms)for adjust the limited current of pretect circuit.

This is from M25 ?
yes,what I said is according to the M25 sch
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Old 21st November 2005, 01:08 PM   #7
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Ok, I will try.

As you have the schematic... There is a zener diode in the regulators, it was dead and I can not identify it. Can you see the type ? 30 V ? 33V ?

Thanks.
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Old 21st November 2005, 02:09 PM   #8
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A picture of the left channel pcb. No electrolytics in the amp section Click the image to open in full size.
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Old 21st November 2005, 02:35 PM   #9
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Left and right are identical.
Attached Images
File Type: jpg m25board2.jpg (9.2 KB, 435 views)
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Old 21st November 2005, 02:41 PM   #10
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Quote:
Originally posted by jacco vermeulen
Left and right are identical.
Amps yes, pcbs no. Right channel has rectifiers, left channel has protection.

Not sure but I think it is like marked now in the picture.
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