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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: 200
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: 200
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: 200
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: 200
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: 24079
Location: The Netherlands, Enschede
Audio files: 278
G2 patch files: 320

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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Jan
also .. could someone please turn down the thermostat a bit.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
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teecee



Joined: Sep 29, 2009
Posts: 7
Location: London

PostPosted: Thu Nov 10, 2016 4:20 am    Post subject: OB8 warbling oscs
Subject description: Synth calibration / repair
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Hi there - thanks for taking the time to post your findings on this thread. It's great that the community helps out in this way.

So, last night I switched on my OB8 for first time in a few months and noticed that some voices were warbling. I thought this was probably due to oscs needing to warm up and stabilise... and, indeed, after a while, all seemed ok.

However, later in evening, when I came back to the unit, I noticed more warbling.

I decided to give it a full calibration, just to make sure. Am pleased I did this as many of the oscs were in need of calibration.

All the oscs successfully passed calibration process, but the warbling persisted.

By this time it was 2am, so I gave up. I need to check later which voices are warbling and if they are on same board.

If so, I will check the capacitors. So thanks for your pointer there.

One last thing - the components on my voice boards are not marked, no silk screened component codes. This made calibration interesting as I had to work out which trimmers related to each function. I'll also have fun locating C10.

Any further pointers appreciated...

Thanks again

TC

Last edited by teecee on Thu Nov 10, 2016 4:30 am; edited 1 time in total
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teecee



Joined: Sep 29, 2009
Posts: 7
Location: London

PostPosted: Thu Nov 10, 2016 4:30 am    Post subject: Re: OB8 warbling oscs
Subject description: Synth calibration / repair
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teecee wrote:

One last thing - the components on my voice boards are not marked, no silk screened component codes. This made calibration interesting as I had to work out which trimmers related to each function. I'll also have fun locating C10.


I've just noticed that the Magnus Oberheim page has a diagram of the component layout. I wish I had noticed that last night. Doh.
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j.dilisio



Joined: May 19, 2009
Posts: 200
Location: baltimore

PostPosted: Thu Nov 10, 2016 5:26 am    Post subject: Reply with quote  Mark this post and the followings unread

It sounds like you're having the same problem I did.
Check those polystyrene caps. They tend to go bad over time.
I'm not sure why they were used in the first place.
The polyester replacement cap functions perfectly fine in it's place.

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