Oberheim OBXa early model (series 2)

Synth Repair
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Oberheim OBXa early model (series 2)

All 8 voices are continuously sounding. Each are receiving 5v on pin G1. Interestingly, autotune causes them to stop sounding and they cycle through the autotune routine. The second this has finished they all start sounding again. I removed MC14099B, the addressable latch for the gates, and interestingly the voltage at C3 dropped to 1.1V, but all the voices still sounded. I have another OBXa here and have swapped over the Z80, the RAM, and the ROM and none of this makes a difference! Help.

PS It’s got audio, its just the system wouldn’t let me post without another tag and there wasn’t an appropriate tag so I selected ‘no audio’

Best wishes,

Craig

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Hello,

Synthchaser.com specializes in Oberheims, repairs, mods and parts kits.

He’s the best to ask on that.

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Thanks. I’m aware of him, however I’m in the UK and he would probably want me to send him the synth so I’m not sure it’s worth my while. What do you think?

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No he’s helped other customers we’ve referred to him.

He may know of someone closer to you who can help also.

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Thanks. That’s very helpful. I think I’ve narrowed it down to faulty address line. Now what’s left is for me to find out where the fault is. There is very low resistance (10k) between the A3 address line and ground strongly suggesting a short somewhere. One of the issues with this particular OBXa is the boards are none conformal and there’s oxidation on the traces… wish me luck finding the short since I’m going to need it!

Best wishes, Craig

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Hi, I think I ended up at a dead end with the fault finding and in fact I have two OBXa’s here and both have the same fault! On turning on they make random noise - all voices are active meaning gates are open voice card lights on- they are irresponsible to the controls , however, when selected they will run the autotune routine, but then revert back to their garbled noise. Help, I’m running out of ideas!