Boss DD-3 digital delay problem

[beatnik]

I have a problem with a Boss DD-3 delay. It is the older version made in Japan with through hole components.

The delay is actually happening, but the sound is very loud and distorted, even with the mix knob very low. The feedback control is also very wild, starts self oscillating into a distorted mess within first quarter of rotation.

When the pedal is in bypass, with mix knob at minimum there is clean bypassed sound, but if i turn the mix knob up to about half rotation the kinda same distorted sound can be heard like a gated/buzz effect when i hit string harder.

The first thing i did was to replace all electrolytic caps but that didn't solve the problem.

Can you make suggestions what to check next ?

Here is the schematic

https://www.dropbox.com/s/caj4tfinqqtyoih/Boss%20DD-3%20schematic.jpg?dl=0