Oberheim OB-8 Resonance Issue

Pulling out your hair? Don't know what to do or where to go? Ask in here.
Forum rules
READ: VSE Board-Wide Rules and Guidelines

If your Help request has been solved, please edit your first post in order to select the Image Topic Icon to let others know your topic has been solved.

Oberheim OB-8 Resonance Issue

Postby Svanis » Tue Nov 21, 2017 9:44 pm

Hello,

My OB-8 has a voice that doesn't respond to the resonance knob, i.e it's always stuck at 0% resonance no matter what, this is only in 4-pole mode. It works fine in 2-pole, and adjusting the trimpot on the circuitboard named "2-Pole Res" (T304) only works for the 2-pole mode, and isn't of any help here. At least on this particular voice. Is this normal?

I can't find a to adjust the 4-pole resonance in a similar fashion, any guess guys?
Svanis
Newbie
Newbie
 
Posts: 8
Joined: Tue Jun 13, 2017 2:39 pm

Re: Oberheim OB-8 Resonance Issue

Postby recordbot » Fri Nov 24, 2017 1:05 am

open that OB8 up and figure out which voice has the issue, betting you have a busted IC or one of them got installed upside down
recordbot
Junior Member
Junior Member
 
Posts: 128
Joined: Fri Nov 26, 2010 7:01 pm
Location: MA

Re: Oberheim OB-8 Resonance Issue

Postby madtheory » Fri Nov 24, 2017 1:33 pm

These synths are complicated. It could be any number of things causing the problem. The filter chip itself could be fine. It could be a problem with the logic or CPU, anywhere between the front panel and the DAC generating the CV. IIRC the CV for these is multiplexed, but I don't know where.

So do you have:
1. A logic Analyzer and/ or a two channel scope?
2. The service manual, plus the corrections?
3. A DMM?
4. A signal generator?
5. All IC data sheets?
6. Experience in fixing vintage polysynths?

If the answer to any of these is "no", take it to a synth technician (make sure they have a logic analyzer and can handle vintage computer stuff like this).

You might get lucky, and someone here that has more time than me will go through the schematic for you...
User avatar
madtheory
Supporting Member!
Supporting Member!
 
Posts: 4946
Joined: Mon May 01, 2006 12:45 pm
Location: Cork, Ireland
Real name: Tomas Mulcahy
Gear: Novation KSR, PT12, VCZ, Redmatica, MIDIQuest, Casio FZ, SK5, Komplete, M1, Theremin, Digi Vocalist, Quadravrb+, Kaoss Pad, JV, SPD, Cyclone, Drummer.
Band: madtheory

Re: Oberheim OB-8 Resonance Issue

Postby Svanis » Mon Nov 27, 2017 10:34 pm

I was hoping someone could name the components involved so I could check them with a multimeter. I couldn't find a schematic of the filter circuit. It's two circuits running through one CEM3320 filter and the OB-8 switches between the two sets of components to generate a 2-pole and 4-pole mode. I'm doing this with my father and we've had great success repairing old synths and equipment before, I just want to save some headache before I present him with this issue. He's experienced but doesn't know how a synth works like I do.

Here is the documentation available, maybe I'm missing something completely obvious.

https://rubidium.dyndns.org/~magnus/syn ... /oberheim/
https://electricdruid.net/cem3320-filter-designs/
Svanis
Newbie
Newbie
 
Posts: 8
Joined: Tue Jun 13, 2017 2:39 pm

Re: Oberheim OB-8 Resonance Issue

Postby synthchaser » Fri Dec 01, 2017 2:48 pm

You can always swap the socketed Curtis filter chips to a different voice and see if the problem follows the chip or stays with the voice. I recommend rebuilding the voice cards with my kit before trying to do any troubleshooting, it can save a lot of time and headache:

http://synthchaser.com/product-category ... eim-parts/
synthchaser
Newbie
Newbie
 
Posts: 42
Joined: Fri Jun 17, 2016 2:49 pm

Re: Oberheim OB-8 Resonance Issue

Postby Svanis » Sat Dec 23, 2017 6:21 pm

I did what synthchaser said and the issue persists, so that confirms there's nothing wrong with the 3320 filter chip, at least. I'm thinking there must be a compenent or something that stores the (max?) value for the resonance in 4-pole mode, that might be malfunctioning. The 2-pole mode filter resonance can be adjusted with a simple trimpot.
The noise was once malfunctioning on this very instrument, but that was as simple as replacing a bad transistor.
I'll check with a thermal camera as soon as possible. :)

Here is a photo of the lower voice board. The issue is on Voice 3 (7). The 2-pole res trimpot is the one colored red on the inside. https://imgur.com/a/Tpjgj

I can tell an immediate difference in the circuit of that voice, a big ceramic capacitor soldered between a resistor and another point on the circuit board in the middle right of the voice's circuit, however this is mirrored on the upper voice card, so this is correct. (seen here https://imgur.com/a/0OmRQ). If you can see any obvious discrepancies between the voices or voice cards that shouldn't be there, or have any other ideas, please let me know. :)

(on a side note, synthchaser's power supply rebuild worked wonders on my OB-8, so I can recommend that to any owner.)
Last edited by Svanis on Thu Feb 08, 2018 7:45 pm, edited 1 time in total.
Svanis
Newbie
Newbie
 
Posts: 8
Joined: Tue Jun 13, 2017 2:39 pm

Re: Oberheim OB-8 Resonance Issue

Postby synthchaser » Sat Jan 06, 2018 4:01 am

First I'd check the resonance control voltage, ahead of R349 (33k resistor going to pin 9 of the curtis chip) and see if it moves with your resonance knob and matches the voltage on the other voices. Just to be sure.

Next you might try swapping U305 and U307 with a neighboring voice. Of everything involved in the filter circuit, those 2 chips are most likely to fail, IMO.
synthchaser
Newbie
Newbie
 
Posts: 42
Joined: Fri Jun 17, 2016 2:49 pm

Re: Oberheim OB-8 Resonance Issue

Postby Svanis » Thu Feb 08, 2018 2:25 am

U307 was the culprit; a CD4053BE chip. I moved it to another voice and the issue followed, so I bought a new one for less than $5, and the instrument is now restored. Thanks everyone for the help and advice!

On a side note, I'd like to ask synthchaser or anyone else equally knowledgable if the following factory ECO alters the sound in any way, and if there's any point in having the modification done. I'm not sure I understand the description under "reason for change".

Image
Svanis
Newbie
Newbie
 
Posts: 8
Joined: Tue Jun 13, 2017 2:39 pm


Return to HELP!

Who is online

Users browsing this forum: No registered users and 12 guests