Emulator iii distortion on voice 8

[chrisr710]

Hi, I was hoping for some help from the experts on an emulator iii I am working on. The unit has distortion on voice 8, which progressively gets worse with heat. I tried all the obvious things... cleaning the various seated IC's and memory, connectors, etc etc. I then expected there to be one of the DACS, the op amps, the curtis chip. or one of the resistors or caps around the audio outputs for channel 8. I have replaced all of these, or swapped them with another channel, and the problem persists. Because of the card cage design, it is a bit difficult to signal trace these units. I began soldering small wires on the boards, then inserting them, turning the unit on, and then attaching my scope to the wires. Needless to say, this is tedious.
I am running the 315 hz sine wave from the UST disk.
Although on the other channels the sine wave is clean, on channel 8 the sine wave gradually thickens and distorts. Sometimes it seems like an "octave down" effect.  The frequency remains constant. I have attached an image of the schematic with the results of what I have done. The distortion is present coming right out of the DAC, and also there is a dc voltage at pin 17 of the DAC, which is not present on any of the working channels. I have replaced all the chips around the DAC, all the caps, all the resistors and op amps, the multiplexer 4053. and swapped the DAC for that channel with another channel. Nothing changes. I now think  that the problem is further upstream, and I was hoping for some advice on what to check next, as I am believe the problem is more in the digital circuitry. ANY advice would be appreciated!! Thanks so much. This forum is wonderful.

[dr.c]

What happens if you swap the two boards ? Does voice 8 become good and voice 16 gets crappy ?

Keep in mind that, on the schematics , voice one is voice zero, voice 8 is voice 7 and voice 16 is voice 15. Are you fixing the right one ?

[dr.c]

Did you chage IC36 (feedback) ?

[esynthesist]

Hi there.

I think it's very brave of you to do all this effort Smiley
My EIII has probably the same problem: a sine wave which is perfectly shaped on all other channels is distorted on voice 5. I don't remember where my technician has soldered the probing/measurement wires but I know he also got frustrated by keeping seeing the distortion while not being able to solve it even after replacing all surrounding components (capacitors, resistors, filter chip, ...).
After a few weeks of investigation his efforts were stopped and the machine returned to me "unrepaired"... And I can't say its due to the inexperience of the technician, because he's known to be one of the best in the area (Belgium/Netherlands).

I'm living with this EIII problem for 2 years now, which sounds more awful than it actually is because I simply disabled voice 5 on the EIII since then.

I hope you (or Dr C) find a solution !

///E-Synthesist

[dr.c]

Lets hope !!!!

[chrisr710]

I think I have finally located the source of the distortion. It has turned out to be an ssm2300 chip. Although I had swapped them around and noticed that the distortion stayed on voice 8, if you scrutinize the schematic you notice that swapping the chips around on the same voice board is not a useful diagnostic tool. This is because the same pin on each IC goes to the same voice. So, I moved all 5 of those chips to the other board, and the distortion moved to the other board. VOILA. Although I thought the distortion was coming right off the DAC, which would be upstream from the Curtis chip and therefore also from the SSM200's, that was my fault. When I attached my scope probe to the DAC, I had created a tiny break in the solder, so the distortion I saw there was not related to the root problem.  Hopefully this helps somebody else: Moving those 2300 chips around on the same board  will still keep them related to the same voice!! I bet you somebody else was baffled by making that same faulty assumption. Thanks for all that responded.