Oberheim OBX repair > need some help

mph · Joined: Aug 25, 2007 Posts: 59 Location: France

Hello

I'm in the process of repairing an Obx which has suffered from dead caps on the psu and shortcut tantalum cap on the main board.
I've changed all the regulators, the electros, the bridge rectifier, the battery, the trimmers and two resistors on the psu. Say it's brand new.

Then I've changed all the tantalum and electro caps on the other boards (including the voice boards).
But I also need to change the tropical fish caps as they are completely dry and broken.

This leads me to have a very strong doubt about some caps:
on each of the voice boards there are two caps which are noted 1nF on the schematic but appear to be 10nF ( up to down code reading: brown / black / orange / white / red) on the pcbs.
They are around the CEM3310 EGs (page 20 in the service manual):
http://manuals.fdiskc.com/flat/Oberheim%20OB-X%20Service%20Manual.pdf

Also I have a "modern" voice board which has the "gate on" LED indicator fitted which had already been serviced some years ago, and those 10nF caps were replaced by 1nF caps... but I don't know if the "old" voice boards are exactly the same as the "modern" ones and if the value change is a sort of "factory" mod.

I have to mention this synth is one of the very first which were in production at the end of 1979 (it even used to have a proto EEPROM instead of the known references; but it has been retrofitted with the regular EEPROM few years ago).

Please could someone experienced with this synth or CEM3310 implementation give me some information about that.
Thanks a lot.

Here is a detail of a voice board before I change the tantalum & electros; but you can see the bad shape of the "fishes":
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Cheers

mph · Joined: Aug 25, 2007 Posts: 59 Location: France

Hi
sorry the image seems to not load properly.
Please if someone knows this synth enough, what cap should I use to replace the 10nF around the 3310 chips?
The original caps have a different value from the ones drawn in the schematics (which are 1nF). So is the 1nF value an upgrade?
What will it change for the CEM3310 functions?

Thanks

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Obx voice board detail

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The Real MC · Joined: Jun 20, 2008 Posts: 55 Location: Painted Post NY

OB-Xs were built starting in July 1979, not end of 1979.

Without knowing the passive component designator (thank Oberheim for NOT putting them on boards or schematics!!) I can't tell where that cap is used. If it is a power bypass cap then I would not worry whether it is 10nF or 1nF. If it is a timing cap then dielectric is critical as well as value.

mph · Joined: Aug 25, 2007 Posts: 59 Location: France

Thanks MC,
and sorry for the date code; to be clear the serial number is 793301 for this machine.

The caps I'm talking about are on the gate buss and pins 4&5 of the CEM3310 chips. I don't think they are bypass caps but I can't say if they are timing caps...
What do you think?

Here is a detail of the schematics (but as you can see on the picture posted above, these chips are 10nF in "real").
Edit: and a detail of the datasheet.

Obx CEM3310.png

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CEM3310 datasheet.png

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EDIT: well, on the datasheet they put a 3nF in this place... so is it critical to use 10nF?

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The Real MC · Joined: Jun 20, 2008 Posts: 55 Location: Painted Post NY

Ah, much better.

Pin 4 is the gate input, pin 5 is the trigger input. Gate fires all four stages in sequence, trigger is used for multiple trigger and re-triggers the DR stages. For systems that do not use a separate trigger signal (such as the OB-X) you can simply couple pin 5 to pin 4 via a cap. The cap simply converts the gate signal to a short pulse for trigger.

Probably the only difference between the cap values is the lower value cap (1nF) is better for fast playing.

defog · Joined: Aug 24, 2011 Posts: 107 Location: Philadelphia

Hi there,

I recently overhauled an OB-Xa so I'm way too familiar with these synths. Feel free to message me if you need any help.

mph · Joined: Aug 25, 2007 Posts: 59 Location: France

@ Real MC: thanks! so I've stuck to the schematics with 1nF caps.

@ defog: thanks a lot, PM sent.

miserlou · Joined: Today Posts: 1 Location: North Carolina

I turned on my OB-X eight voice a few weeks ago and it's finally taken a bad turn and doesn't work. I would greatly appreciate some advice on where to start trying to diagnose and fix the problem(s).

Here's what's going on:

I turn it on. Some indicator lights work, others don't...this is across the entire panel. The keys produce a little sound, but not much more than a click or really sick little tone. When I try choosing a new Group and Program, there's a different sounding click or noise. Some don't work at all.

I've also noticed that when a press Autotune, the LED lights on each card are not lighting in sequence as they use to...in fact no lights at all.

Any help would be greatly appreciated.

Thanks!
Rod