Conrad Johnsen mf2500 problem - diyAudio
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Old 17th April 2017, 08:31 PM   #1
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Join Date: Feb 2003
Location: West Coast of Norway
Default Conrad Johnsen mf2500 fault

Hi, have had this for a while but have never tried it
Anyway - replaced all fuses (3 were blown) just to test. On fuse on the back and 2 on the pcb.
Connected a DMM on each loudspeaker terminal and fired up.
Left channel showd 72Vdc - Right channel ~100mV.
After some seconds a bit of smoke appeared somewhere on the board.

It is late here now and bed time... I will take some pictures on a later occasion.
Any initial typical faults that I could be aware of? Blown transistors maybe?

Looks as one or two resistors are blown but I guess it is a reason why that has happened?
I know this is not much to go after....

Last edited by bambadoo; 19th April 2017 at 09:20 PM.
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Old 18th April 2017, 08:24 AM   #2
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Join Date: Feb 2003
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Some pictures.

First is a overview of the amp.
Second is the area which I saw smoke. However I cannot visually see a problem there.
Third I noticed afterwards that what seems as a cap or resistor has been toast.

I will take everything apart on a later occation
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Old 18th April 2017, 08:51 PM   #3
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Does anyone by any chance have schematics for this amp? I would be very grateful. Please pm with details.
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Old 19th April 2017, 09:19 PM   #4
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Great service from CJ. They provided schematic after I made an enquiry.
Fault analyzis will be easier.
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Old 21st April 2017, 10:47 AM   #5
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Location: West Coast of Norway
At least Q12 (mpsu60l), Q23(nsdu57) and R12 (100r) minimun are broken on left channel. Waiting for some equipment so I can du some more digging..
Probably more that is broken.
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Old 21st April 2017, 11:01 AM   #6
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Join Date: Oct 2012
it would be a good idea for you to build a "dim bulb tester" before you dig deeper into your amp.
had you had this before your initial power up you could have determined there was a fault without causing additional damage.
there's info here at DIYaudio as well as google links about it.
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Old 21st April 2017, 05:59 PM   #7
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Location: West Coast of Norway
Yes of course. I think the damage was already done when I bought it. It was sold as defunct.
Some power transistors are also toast. At least 1 2sc3281 toshiba. Havent desoldered the 2sa1302 yet but at least one is fried.
On fuse holder was also bad and broken to the pcb.
Bastard work dissasembling of this amp.
Waiting for a Variac also.
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