I’ve got a Yamaha MODX8 I’m troubleshooting for repair and need some suggestions in where to look.
The issue is a terrible screeching sound on startup from the outputs, both L R and headphone jacks. The screeching can be controlled by the volume knob. I can navigate through sounds and some piano patches come through fine. Switching to organ or electric piano patches sometimes triggers the screeching sound as well as the synth patches.
My guess would be that one of the sound processing chips went bad. I’m unsure how to throughly test the chips besides doing a basic temp check with my multimeter.
I appreciate any and all thoughts and ideas in advance. Picture is of the main board where most chips sat at 80F and these two stuck out.
That’s probably normal working temperature to be honest.
Are there RAM chips on there?
My M1 went bad with similar symptoms and I fixed that with new RAM chips.
The effects processors use RAM to generate the delay/echo/etc.
Might be worth a google to see if anyone else has fixed it this way.
Please forgive my ignorance. I’m not sure what kind of shape the RAM chip would take on. There is a chip that is offered in the MODX8 parts catalogue on Syntaur that looks like this:
Going to prod around a bit more with my multimeter and will post any updates if I come across something.
I’d go through the service manual test points and just make sure all the voltages are correct.
The service manual gives the full parts list for the MODX8 board.
DM if you need more help.
I downloaded the service manual and found to boot the MODX8 board in diagnostic mode you have to hold C#2 F2 and G#2 during power on. I tested the RAM chips and effects chips IC803 and IC804 showed as faulty. I got in touch with Yamaha and ordered those chips since they were only ~6 USD a piece and replacing the entire board is $500+.
I swapped the new chips in but the issue is still persisting.
I’m going to try and check voltages next and report back.
That’s unlucky!
Have you re-run the tests to see if anything else comes up?
I ran the tests shown in the manual, and the voltage readings were all lower than they were supposed to be.
When I tested point 6 to DGND the entire keyboard shut off and I’m worried I shorted something.
Now, with no power on the board, I’m not even getting continuity between the test points and the grounding point. The keyboard still powers on normally and has the same issue since the start of this thread.
/RES is just CPU reset, no harm done there. It resets the CPU is the voltage goes low.
With no power, there shouldn’t be continuity between the rails and ground. This is a good thing!
What were the resistance readings for the rails BTW?
A short can bring down one of the rails. Sometimes a partial short on a main rail such as 5V will bring down the others because they are fed from it.
