Oberheim OB-X

Here are some images of my Oberheim Ob-X. The work done on this Ob included replacing the Bushings on the keyboard, installing an Encore midi kit, which gave the instrument it's 32 Original memories back and 120 Memory locations in total and a lot of additional control through midi. I also re-caped some of the instruments boards. A few months after purchase one of the filters started to act up. I got that fixed after a lot of fault searching.

Here it is in all it's glory. Some people say you can not hear difference between the Ob-X, Ob-Xa and the Ob-8. I guarantee you the Ob-X is the fattest of them all and sounds closest to the SEM module. The Ob-Xa sounds like a cross between the Ob-X and the Ob-8. Not as raw and fat as the Ob-X and not as nice and friendly as the Ob-8. Feature wise the 8 is most complex and the Ob-X the most simple. But the sounds of this machine is in another league completely.

Funny that you can use the Ob-X as a mono synth through the CV inputs. The Ob-X was under constant development some of them look a little different cosmetically and you might also notice some electronic differences to the different revisions of them.

Here are the midi inputs that I added to the machine. I chose the Encore kit for it's simplicity to install and for it's many features making this synth really complex.

Here's the Encore kit inside my Ob-X. The really nice thing is that it replaces some of the memory IC´s and you get to pull three of the original Flash ROMs out, so this card actually replaces the "brains" of the Ob-X. This means that in some cases it might bring back your Ob-X to life if your ROMs have suffered damage. This Midi kit only requires you to solder two wires to the main boards of your Ob-X. All the green one's are soldered to the midi in/out/thru sockets. The Z80 processor of the Ob-X is lifted over to the Midi kit board and the big ribbon cable is seated where the CPU used to be as seen in this picture.

Here are the cards that make's the sound of the Ob-X. Mine is an 8-voice model. Tuning these cards is not hard but a hell of a job since you need to do it over and over until you find the right tuning. Recapping the entire synth is not the only thing recommended when it comes to the Ob-X. It is actually good if you replace all the trim pots as well.

The upper tier of voice cards, notice that one is lighter in color than the rest ? This card seems to have been sent as an replacement card as the date is another than the rest. Here I was finished with the recapping of all voices. The Ob-X was done in several restoration "waves" as the synth showed some problems before I started doing the work, I needed to solve these first or it would have been a waste of money to restore all those voice cards.

Here you can see five new caps and three of the new trim pots that I exchanged. The mint green caps are low leaking electrolytic ones that replaced the blue tantalum caps that used to be seated here. The two black axial caps are the caps that "clean" the main plus/minus 19 Volts that the cards require to make sound. Typical problems when these are bad are Filter tuning problems, the filter will not let it self be tuned but sounds false no matter how you turn the trim pots. In this image you can also see the CEM 3310 Envelope generator IC. This is the only CEM IC used in the synth, all the rest is built on discrete semiconductors, which is one reason to why this synth sounds as it does.

Here are some more of the voice cards, notice the second card and look at the upper most black axial cap. Can you see that the PCB is black underneath it ? That's the caps that had blown and made the filter of that card sound false. I discovered this by probing and following the PCB trace back to this component from the filter, since a Oscilloscope probe acts as a 1M ohm resistor the problem disappeared when I probed the -19Volt lead and came back when I removed the probe it was easy to follow the problem to this component.

I also did some work on the PSU of the Ob-X. I recapped the whole PSU and exchanged the trim pots, de-soldered the original multi connectors which really was just pins and the cables directly soldered to those pins. I wanted to be able to easily attach and detach the PSU from the rest of the synth. I chose MTA type connectors and replaced the wires also as the old wires were to thin for these types of connectors. The bender and modulations section also got new wires and MTA connectors, in this image they go above the PSU and down to the main board of the Ob-X. On the yellow MTA going from the PSU to the Main board there is one pin that you need to be extremely careful with as it requires that it gets contact from the lower side of the PCB to the upper side (The PSU PCB is double sided). There are two 1N4148 Diodes on the back of the PSU PCB, be extremely careful that these two are seated correctly if you are forced to remove them. Seating them the wrong way will lead more Voltage to the pin that drives the Logic circuits of the memory board making them brake (Or at least one Logic IC will break for sure).

The Ob-X is one massive synth and it does show this in it's weight. There are still some work to be done on My Ob-X as I wasn't systematic enough when doing this machine. Some tantalum caps remain in the synth. And I would really need to replace the DIP switch that turns each voice on or off as it is starting to show it's age.

... More pictures will come up as the work continues ...