Recently started having some issues with my ESQ-
- distorted sound, gets worse shortly after startup
- data slider, pitch and mod wheels are not responsive
- with my multimeter I probed at J12- there is +/- 12VDC at pins 1 & 3 the mod and pitch wheel control voltages are changing when I adjust each wheel (pins 2 & 4)
- ran Analog Input Test from service manual (READ + COMPARE) everything reads 255
- ran Filter Tune Test everything reads 000
- start up message saying battery has low voltage (it was replaced it 4-5 years ago - reads 2.9V with multimeter)
- PSU voltages all test within spec
Working from schematics on buchty.net I traced the mod & pitch wheel control voltages and battery voltage to multiplexer 4051 (U44 on the schematic, seems to correspond to U33) & the opamp (U40 on schematic / U32 on my schematic).
I am able to do some basic repairs, trace circuit, etc, but I was hoping someone more knowledgeable than me might be able to point me in the direction of what to test or try next?
There are apparently some other voltages that are created for the analog circuit.
And my plastic body ESQ1 seems to follow the SQ80’s schematics in this section:
Here are the voltage readings I got-
Pin 3 (output): -5.1V
Pin 7: -5.1V
Pin 16: 2.6V (voltage should be -7V here)
I am suspecting perhaps C74?
Start with hitting READ + COMPARE to get into test mode, then repeatedly hit read as you move the data slider, pitch wheel, and mod wheel. Make note of what it does.
Filter tune should be between 140-160. If it’s not, then you’ve got a mainboard problem. Where to go from there depends on what you see in the first test. It shouldn’t be all 255.
To clarify, the wheels & data slider still read 255 when repeatedly hitting READ. I assume U33 must not be outputting correctly.
I think you were correct on your initial suspicion. Something is affecting the +7A voltage at the Q6 transistor. That voltage feeds the 4051 multiplexer. and the sample and hold chips (SSM2300).
Remove all 4 of the sample and hold chips and test the +7A. If it restores the voltage normal then re-insert the chips one at a time until you find the bad chip. If the voltage is still low then the next component i would look at is the multiplexer. The transistor will probably be running hotter than usual due to the larger voltage drop across the emitter/collector junction.
Thanks Pat - what you suggested is exactly what I planned to try next.
Strangely, when I reseated the SSM2300’s one by one I was measuring 6.8V on pin 16 at the +7A points. Then I had all 4 chips in and was still measuring 6.8V. I guess a dirty connection? Or perhaps an intermittent internal issue? I’m going to go ahead and order a spare SSM2300 knowing the problem could return.
For now I’m not complaining. The synth sounds great, no more distorted output, and the wheels & data slider are working as they should once again.