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Oberheim DX drum machine

2/4/2003 3:49 AM
Paul M.
Oberheim DX drum machine
I have a friend who wants me to fix his Oberheim DX digital drum machine. (It's big, blue, and it was manufactured in 1983, according to the date stamp on the PCB) Some things aren't working properly...  
I know one chip is bad for sure, but the circuitry is too complicated to try to follow without some help.  
Does anyone have a copy of the schematic or know where I can get one? Is Oberheim still in business?  
Any help appreciated.  
2/4/2003 10:28 AM

The Oberheim name is still going though the company that bult that machine is long gone. The new Oberheim does not support any of the older stuff.  
Smartparts might be able to help with service literature.  
What is wrong with it? WHat chip is bad and why do you think so? I have not been in one of those in many years.
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2/4/2003 3:39 PM
Paul M.

Well, I know for sure that the hi-hat chip, marked as an MBM 2764-30 is bad. Whoever installed it in the socket messed up some of the pins. The one place that I found that claims they have that chip sent an email saying they don't have any. (Emar International)  
Also, the cymbal buttons don't do anything and the bass drum has very low output.  
I'm just wondering how the signals get routed, if that one bad chip can effect some of the other functions.  
2/5/2003 11:02 AM

The 2764 is just a ROM. The 2764 was common back them. As a ROM it comes blank and has data programed into it. If the chip is electrically OK, we might save it. If it was not originally in a socket - and I bet it was - mount a socket there. Most DIP sockets can be stacked. That is you can push the legs of one socket into the holes of another. If legs break off a valuable chip, we stick it in a socket, then the pins of the socket become the new pins for the chip. For the pins that are broken off, there is usually a little nub left. We take a piece of thin wire - if you are anal enough you can flatten it even - and push it into the new socket where the missing pin would have gone. Now lean the wire up against the nub and put a drop of solder there to make a connection. Do this to replace any missing pins. Now you have an old IC in a new socket. Just plug this whole thing into the old socket where the chip used to live.  
If that is not a clear description let me know. I have saved may program ROMs for stuff like this and for Arcade video games over the years. No one sells those chips cheap, so they are worth the effeort to save.  
THis is a ROM, and the sound is stored digitally. This chip is not in the signal path. What happens is when you hit the button for say SNARE or the sequencer triggers SNARE, the progran initiates a sound window for that event. The CPU then looks to the ROM - its memory - for the sound info to put there. If you found a ROM labelled SNARE and one marked GONG and you swapped them, yep, the sounds would trade places on the buttons. The CPU would never know the difference.  
SO other than if the chip were bad enough to load down the power rail or something, the ROM should have no effect on anything else but its own sound.  
My experience with these drummer is that if sounds are absent or very low, it is more likely programming than failure. I can't count the number of times I had Yamaha RX series drummers presented with the complaint missing drums sounds or poor levels. Every time I would find the sound for the "missing" drum was panned over to the other side. Outputs are stereo but they used only one side in most cases. Every thing about the sound is a parameter. Pan, level, etc.  
If ANY of your sounds work, then the signal path is OK, since they all come from the same place. Your CPU generates the sound digitally from memory ROM. It sends this through the DAC, and on out through standard audio citcuitry - couple op amps or something near the jacks. If your sounds are missing it is then due to missing memory or a problem addressing that memory, or the misadjusted parameters.  
If this thing has a battery for its RAM, check it.  
This may not be as screwed up as you think.
2/5/2003 3:47 PM
Paul M.

Thanks, Enzo.  
That is a big help and gives me some direction. Hopefully I can save this chip. A couple of the pins broke off clean right at the body of the chip. I'll see what I can do.  
2/6/2003 7:04 AM

Carefully grind away some of the chip body material around where the leg came out if you have to. That will expose a bit more for solder to get to.
2/12/2003 3:49 PM
Paul M.
Good news!
I am somewhat surprised, but I fixed it! I successfully soldered the pins back on the chip.  
All I really did is figured out that some of the chips were in the wrong sockets and relocated them, got rid of the layers of dust inside the thing. Sprayed some contact cleaner inside the stubborn buttons that didn't want to work.  
Now it works correctly, and reliably.  
All I have to do now is replace the 3V lithium battery, which I'm sure being from 1983 is not good anymore.  
Never did find a schematic...  
Thanks for the help Enzo. :)  
Paul M.
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