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 Forum index » DIY Hardware and Software
OB8 Warbly VCO2 troubleshoot
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j.dilisio



Joined: May 19, 2009
Posts: 192
Location: baltimore

PostPosted: Sat Nov 08, 2014 6:19 am    Post subject:  OB8 Warbly VCO2 troubleshoot Reply with quote  Mark this post and the followings unread

I'm having a very specific problem with my OB8.
All four VCO2's on one of the voice boards are unstable/warbly.
All other VCOs are perfectly stable.

Here is a sound sample of just the four OSC2's on one board..
http://jdilisio.com/ob8/OSC2unstable.mp3
As you can hear, they all warble the same which means there's a problem with the CV going into those sections, right?

It seems like the problem would be in the Samples/Holds and Latches section of the voice board but before I start replacing caps I wanted to check with the experts.
Schematics.. http://rubidium.dyndns.org/~magnus/synths/companies/oberheim/

Any help/advice is appreciated.
Thanks in advance!



Also, this is a problem that has gone from occasional/barely noticeable to worse over a couple years and much worse in the last couple days (think exponential curve).

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j.dilisio



Joined: May 19, 2009
Posts: 192
Location: baltimore

PostPosted: Mon Nov 10, 2014 8:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Here's a question regarding the schematics..
Looking at Voice board schematics page 2, there are four paths that lead to VCO2.
1- VCO2F (Does the F stand for frequency?)
2- CV1-CV4
3- TR1B-TR4B
4- PW2

-The CV paths split and go directly to both oscillators. Since both VCO's track perfectly fine I'm guessing this is not the problem area.

-The triggers (TR) are all working. The led's light up on cue.

-I'm guessing PW stands for pulse width? This doesn't seem to be the problem.

-VCO2F feeds all VCO2s from one place so that is where I'm focusing.

Here's the schematic for that section..
Posted Image, might have been reduced in size. Click Image to view fullscreen.

It looks like there are just two capacitors for that section so I'm going to start by replacing those.

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j.dilisio



Joined: May 19, 2009
Posts: 192
Location: baltimore

PostPosted: Mon Nov 10, 2014 11:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Just checked the power rails on both voice boards.
They're at +-6 and +-18 instead of +-5 and +-15.
Is that normal for an OB8?

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j.dilisio



Joined: May 19, 2009
Posts: 192
Location: baltimore

PostPosted: Tue Nov 11, 2014 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

I just replaced C10 (.022uf polystyrene) with a modern polyester aaaaaaand,..
SUCCESS!!

It's nice when the electronics gods smile down on you every once in a while.
I'd like to thank everyone for their generosity and help,. just kidding.

Hopefully someone will stumble across this someday and it will save them a headache or a trip to the shop.
Cheers!

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Blue Hell
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Joined: Apr 03, 2004
Posts: 20685
Location: The Netherlands, Enschede
Audio files: 156
G2 patch files: 318

PostPosted: Tue Nov 11, 2014 1:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool great you solved it and documented it too!
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