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 Forum index » DIY Hardware and Software
Oberheim OB-X resurrection
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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Wed Apr 24, 2013 9:03 am    Post subject: Oberheim OB-X resurrection
Subject description: How to restore a vintage analog polysynth
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Brought this beast back from the dead - it's making great music, still more restoration work to do.

http://www.retrosynth.com/~analoguediehard/studio/keyboards/oberheim_obx/index.html

Sounds from the beast

http://www.retrosynth.com/~analoguediehard/studio/keyboards/oberheim_obx/oberheim-obx-sound-patches.mp3
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sneakthief



Joined: Jul 24, 2006
Posts: 531
Location: Berlin

PostPosted: Wed Apr 24, 2013 10:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Nice work! I appreciate the time you put into describing your workflow.

So you have 4 other voices to repair and calibrate? And they need a 2nd motherboard to drive them?

I can totally relate after just having finished restoring a Prophet 600 whose midi wasn't working - at one point I even killed the whole thing when I blew a CMOS chip that prevented the MUX from cycling through the analog inputs Mad Then I also blew another CMOS that buffered 2 of the bits 16 bits going to the DAC... I noticed something was amiss when the keyboard couldn't scale properly and used a logic probe to check which of the 16 DAC inputs weren't pulsing.

After being totally frustrated, I sat down and calmly decided it would be a learning experience and everything proceeded well after that point.

I think the key to working on this vintage of synth is a lot of patience and tracing out the logic workflow carefully to try to understand exactly what is happening.

Good luck with the rest of this restoration! So nice to see one of these old classic beasts resurrected.

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Sneak-Thief - raw electrofunk
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The Real MC



Joined: Jun 20, 2008
Posts: 60
Location: Painted Post NY

PostPosted: Wed Apr 24, 2013 11:03 am    Post subject: Reply with quote  Mark this post and the followings unread

The processor board generates the signals for all eight voices, with connections in two sets to cover two motherboards each of which can hold four voices.

I have the extra motherboard to accomodate four more voices. Last time I plugged it in, it pulled down the power rails. That's a future repair job.
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elmegil



Joined: Mar 20, 2012
Posts: 1534
Location: Chicago
Audio files: 14

PostPosted: Wed Apr 24, 2013 7:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
After being totally frustrated, I sat down and calmly decided it would be a learning experience and everything proceeded well after that point.


That may be the single best piece of advice for troubleshooting synths I've seen Smile

I know that method tends to work for me, though often it also takes a couple days of doing something unrelated to forget the detail of the frustration, otherwise I tend to get hung right back up in it again.
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mph



Joined: Aug 25, 2007
Posts: 67
Location: France

PostPosted: Fri May 03, 2013 6:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi

thanks for the information!
I did the same kind of work one mine ( a four voices upgraded to six voices) last year and I still have some problems here and there.

I have two issues I can't solve:
> voice boards pitches are shifted randomly when cross modulation is activated, so if the same note is repeated several times it plays an arpeggio
> SH LFO amplitude is too weak, and I can't see what IC does the job.

Could it be related to the 4051 you're pointing in your description?
I don't see what 4051 it is exactly, the SH part of the shematic is really not clear to me.

Thanks in advance for your help. And sorry for asking: information on this synth is too rare to miss a chance to learn more.
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radek tymecki



Joined: Mar 22, 2009
Posts: 77
Location: PL

PostPosted: Sat May 04, 2013 3:34 am    Post subject: Reply with quote  Mark this post and the followings unread

few weeks ago i was restoring my ob-8... i'm still impressed that in early 80's you were able to save envelopes voltages etc during calibration procedure...
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