Help save and ensoniq eps 16r

Have an EPS 16+ Rack that has some motherboard issues.

I’m pulling my hairs out now trying to figure out what the heck it is.
Problem is no audio outputs.

What I have done:
• Confirmed the power-supply voltages; All seemed good and within spec.
• On the Analog Test page; I can’t get any analog values, ie: “Volume” or “Mr. Knob” stuck at zero.
• Replaced suspect ICs with no luck.

I’ve tested the pots, they are good, i’ve tested the conductors from the front to the mainboard, all good.
I pulled the motherboard to look for any visually obvious issues. It appears to have been recap’d but all that seems okay, although they used radial instead of axial caps, but didn’t see any issues. All values are correct. While I had the motherboard out I removed U33 4053 multiplexer and U34 TL072 Opamp, installed sockets and replaced these IC’s with new. The Service manual leaves a lot to be desired, but by tracing schematics, I assumed it must be a bad U35 OTIS IC So I ordered one from SYNTAUR. Installed the replacement IC, Problem still persisted. At this point I’m about 4 hours into this machine and left scratching my head, and pulling my hair not knowing where else to look.

I REALLY WANT TO MAKE THIS THING WORK! Anyone have any advise for me?


I thought maybe I should test the voltage regulators on the motherboard, there are a few of them.
Checked the -15 analog supply L7915 @ location VR5, has -25v going in, and -.035v coming out and is HOT as the surface of the Sun! So it seems like that is dead. His polar opposite neighbor L7815 @ VR4 is humming right along pumping out +14.95V. Gonna order an MC7915 (higher amperage) and see if that is the issue.

Would be dumb if that was it. I’ll follow up when I find out.