Oberheim OB-8 Resonance Issue

[Svanis]

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?

[recordbot]

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

[madtheory]

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...

[Svanis]

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/

[synthchaser]

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/

[Svanis]

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.

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 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.)

[synthchaser]

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.

[Svanis]

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".