Oberheim OB8 sound issue

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calidude49
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Oberheim OB8 sound issue

Post by calidude49 » Fri May 24, 2013 6:40 pm

I have an OB8 that has started having issues. When I hit the keys sometimes they work and sometimes they don't. If i hit it a couple of times then it will work. If I hold the key down it works then stops then has a scratchy sound and studders. I did clean the contacts for the keys but this didn't help. Often when I hit the key I can hear the sound very quitet through the amp but it is way off in the distance. Anyone have any ideas on how to fix this? I plan on selling the keyboard and would rather it worked to get a better price. Any suggestions are appreciated.

Bob

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Re: Oberheim OB8 sound issue

Post by V301H » Fri May 24, 2013 7:35 pm

Check the connectors for the keyboard and circuit boards.. Unplug, clean and re-seat each one. Or before doing that narrow it down by applying gentle pressure on the connectors as you play to see if it affects or temporarily corrects the problem.
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Re: Oberheim OB8 sound issue

Post by synthparts » Fri May 24, 2013 8:25 pm

Before you do that hit a known good key 8 times in a row and see if you get the same sound every time (with Unison mode is off). It could be a problem with 1 or more of the voice circuits not the keys. To find a good key put the synth in Unison mode (which will stack all 8 voices on each key). If your OB-8 has MIDI also try it from another MIDI keyboard...
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Re: Oberheim OB8 sound issue

Post by calidude49 » Sat May 25, 2013 12:47 am

Ok, I tried the test with Unison and I have discovered a couple of things. When Unison is on all keys play well. With it off there are issues. Also, if I split the board the upper higher keys don't work. This is telling me the keys are not the issue but then how do I figure out what is.

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Re: Oberheim OB8 sound issue

Post by synthparts » Sat May 25, 2013 6:04 am

Right the keyboard is not the issue, it is voice problems. On Page 2 the patch number buttons let you turn on and off the 8 voices 1 to 8. Switch all of them off except each one by itself to tell which voices are working.
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Re: Oberheim OB8 sound issue

Post by Bitexion » Sat May 25, 2013 11:25 am

You probably think all voices are working in unison mode, but it could be that you're hearing 6 or 7 voices instead of 8, it's quite impossible to judge by hearing. One or more voices are probably bad.

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Re: Oberheim OB8 sound issue

Post by radek tymecki » Sat May 25, 2013 1:06 pm

go to page 2 and you can manually turn off selected voices

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synthparts
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Re: Oberheim OB8 sound issue

Post by synthparts » Sat May 25, 2013 11:02 pm

Just curious, was my last post invisible? :)
synthparts wrote:Right the keyboard is not the issue, it is voice problems. On Page 2 the patch number buttons let you turn on and off the 8 voices 1 to 8. Switch all of them off except each one by itself to tell which voices are working.
Vintage Synth Parts - http://www.synthparts.com

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Re: Oberheim OB8 sound issue

Post by neanalog » Sat May 25, 2013 11:06 pm

It sounds like one of your voices isn't functioning. The OB8 usually has LEDs indicating which voices are being triggered. You'll need to open the lid and look at the 2 voice boards while playing single notes. Once you find the offending voice, first check things like the filter inputs, outputs, vca in/out, and the envelopes using an oscilloscope. You will be able to see the signals that are present and you can pinpoint the location using this method. Since many of the chips in this synth are socketed, these sockets can cause connectivity issues. These need to be checked when you find the offending spot on your voice board. Having socketed chips does make swapping out bad ones a lot easier though. I have fixed dozens of these OB8s, so if you get stuck, feel free to contact us at New England Analog.
-Rob
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