Boss DM-2 Delay pedal schematic diagram

The Boss DM-2 analog delay pedal was sold from 1981 to 1984 and is still sought after thanks to its warm, natural sound. Produces a delay time ranging from 20 to 300 milliseconds, and has 3 knobs (Repeat Rate, Intensity and Echo).

Fig. 2: Boss DM-2 Delay circuit board - component and foil side

According to this page on BossArea.com, there are at least 3 different versions of this circuit. The first DM-2 version was using the MN3005 BBD and the MN3101 Clock Driver IC (PCB marking: ET5214-510).

Later they changed the clock driver to MN3102 and the BBD to the low current version of the same circuit, the MN3205 (PCB markings: ET5214-510A and ET5214-510B). The MN3205 is a little noisier, but it generates less distortion (only 0.8% compared to 1% for the MN3005).

• Controls: Repeat Rate, Echo, Intensity
• Connectors: Input, Output, AC Adaptor
• Input Impedance: 470kΩ
• Output Load Impedance: over 10kΩ
• Residual Noise Level: less than -100dBm (IHF-A)
• Delay Time: 20ms-300ms
• Recommended AC Adaptor: ACA Series
• Current Draw: 11mA (DC 9V)
• Dimensions: 70(W) x 55(H) x 125(D)mm
• Weight: 450g

ICs, transistors and diodes used in the Boss DM-2 Delay:

• IC1: uPC4558C or JRC4558DD (dual operational amplifier)
• IC2: NE570N (compandor IC)
• IC3: MN3005 or MN3205 (4096-stage BBD)
• IC4: MN3101 or MN3102 (BBD clock generator/driver)
• Q1-Q4: 2SC732TM-GR (NPN silicon transistors)
• Q5: 2SK30ATM-Y (N-channel JFET transistor)
• Q6, Q7: 2SC945-P (NPN silicon transistors)
• D1, D2, D5: 1S2473 (switching diodes)
• D3: RD5.6EB3 (5.6V Zener diode)
• D4: RD11EB3 (11V Zener diode)
• LED: SLP 135B (red LED)

See also the DM-2 adjustment procedure (GIF) from an old service manual (ET5214-510 version).