Drop it in the comments below. We are still missing the revision number for the output transformer variant. Disclaimer: This post is for educational purposes regarding vintage audio repair. All trademarks belong to their respective owners.
If you hear a low-frequency "put-put-put" (1-2 Hz) when the effect is engaged, look at the ground plane around the opto-isolator. The schematic shows a missing star ground. Solution: Solder a 10-ohm resistor between the input jack ground and the PCB ground. Mdk Mb-17 W Schematic
The MB-17 W uses two 100uF filter caps that dry out. If the unit passes signal but sounds like a dying gnat, check the negative voltage rail. Replace C15 and C16 (100uF/16V) immediately. Drop it in the comments below