Mdk — Mb-17 W Schematic
| Component | Value | Notes | | :--- | :--- | :--- | | | 10uF / 25V | Tantalum (Replace with Electrolytic for warmer tone) | | Feedback Loop (R7) | 47k Ohms | 1% Metal film | | Biasing Resistor (R12) | 2.2M Ohms | Crucial for JFET drain voltage (Target 6.5V) | | Op-Amp (IC1) | 4558 DD | Do not use TL072 here; slew rate mismatch causes oscillation | | Vactrol (Vactec VTL5C3) | (Obsolete) | Substitute: NSL-32SR3 (Requires resistor mod on LED side) | The "Three Fixes" for the MDK MB-17 W If you are trying to repair a unit without a schematic, follow these three steps. They solve 90% of the issues on this specific board:
Unfortunately, finding the official factory schematic for the MB-17 W is like looking for a ghost. But don't reach for the desoldering iron just yet. We’ve done the reverse-engineering legwork.
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 "W" version uses a mechanical 3PDT, but the input pull-down resistor (R2) is rated at 1M. This is too low. Change R2 to 2.2M to stop the DC pop without losing high end. Should you build a clone? Because the original MDK MB-17 W schematic is vaporware, many DIYers have asked if they should just build a clone. Be cautious. The magic of this unit is the mismatched JFET pair (2SK30A and 2SK117). If you try to build this on vero board with modern SMD components, you will lose the "sag" that makes the MB-17 W desirable. Final Verdict Until an original scan surfaces from a retired service tech in Osaka, our community-sourced schematic trace is the best map you will get. If you own an original MDK MB-17 W, treat it gently. It is a weird, beautiful dinosaur of solid-state design.
If you’ve landed on this page, chances are you have a dusty, non-functioning on your workbench, or you are a vintage gear hunter trying to verify the provenance of a "white label" unit. The MDK MB-17 W is one of those elusive late-70s/early-80s solid-state units that sits right between "pro audio" and "boutique experimental." | Component | Value | Notes | |
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.
Posted by: The Circuit Bench | Guides & Debugging We’ve done the reverse-engineering legwork
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.