Oberheim OB-Xa noise generator problem

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Spyros12
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Oberheim OB-Xa noise generator problem

Post by Spyros12 » Sun Sep 23, 2018 2:53 am

Well I get a strange sound from the noise generator. The problem is probably the MPS5172 transistor where the noise gets generated. There is a socket where the transistor applies. If I’m not mistaken a good replacement for that chip is 2n3904. I really don’t understand how the legs get into the holes. For example does the first foot of the transistor gets into the second or to the first hole of the socket etc.
It would be nice if an OBXA owner would take a picture from the upper left corner of the lower control board where the transistor is located so i can see in what row the transistor legs are placed in the holes.
Every kind of help would be appreciated.

synthchaser
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Re: Oberheim OB-Xa noise generator problem

Post by synthchaser » Mon Sep 24, 2018 12:09 am

MPS5172 is the same as 2N5172 which is still available. 2N3904 should also work but it has a different pinout (EBC vs ECB).

Bleppe
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Re: Oberheim OB-Xa noise generator problem

Post by Bleppe » Tue Oct 23, 2018 11:10 am

Hi Spyros12,

Did you solve noise issue?
I have also a strange sounding (distorted ringmodulated noise) noise generator and swapping out the transistor didn’t make any difference for me, nor did swapping out the following OP Amp.

Spyros12
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Re: Oberheim OB-Xa noise generator problem

Post by Spyros12 » Mon Jul 29, 2019 10:41 am

The problem was a transistor at the lower mother board. It is located at the upper left corner. This transistor has a green socket replace it with another. Watch out, you will need to cross two of the transistor legs. If I remember correctly the first one with the second.

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