Music and Synth DIY

Effects, Eurorack, Schematics, Synth DIY, Through hole

Marantz PMD-221 pitch modification

Marantz PMD-221 pitch modification for tape echo.

Marantz’s three head cassette players are ideal for tape echo. The PMD-221 already has adjustable pitch via a potentiometer, which is vital to control delay time.

Problem is, the pitch only works in playback mode. That’s no good for an echo device, which requires the signal to be recorded to tape and then played back via the tape monitor function.

Same same but different

There’s a mod documented on AI Synthesis’s page, and they also helpfully have the service manual up as well. (I built their Eurorack tape echo interface to drive the Marantz).

However, my PMD-221 is different – the layout and the component reference numbers are not the same as the one modded on the AI page, nor listed in the service manual. Presumably there are different revisions of the main PCB.

So, for anyone with my version of the PMD-221, here’s how to modify it so the pitch pot works in record mode…

Finding the switch to bypass

This is from the PMD-222’s manual. The pitch pot in my PMD-221 is referenced RM51, like this one, not RM14 as it should be. Here, you can see how the circuit works: part of the multiple pole record/playback switch, SJ01, switches the pitch pot in and out of circuit. In record, it connects the transistor’s emitter to a fixed centre tap, which is one of the outer pins on this four-pin pot. In play mode, it connects to the pot’s wiper.

Disconnecting the trace to the fixed centre tap, and connecting the emitter to the wiper, stops the pitch pot being switched out.

This can be done without removing the PCB. Unscrew the six screws holding the back plate on, and cut the trace outlined below to the fixed centre tap of the pot.

Then solder a wire from the switch middle pin to the trace leading to the wiper. I searched for continuity between the pitch pot and the switch to identify the correct pinout on SJ01 – again, the service manual says something different about the pinout. If you do that, bear in mind the position of the pitch pot has an impact on the continuity reading (had me scratching my head for a while).

Pin five of SJ01
The wiper trace for pitch pot RM51

Increasing the range

I didn’t try this, but if you wanted to drop the speed down lower, I imagine you could add a pot or resistor between these points instead of just the wire. Something like a 500 ohm should do it, before the motor stops.

Disclaimer: follow this at your own risk. It works for me, I’m not going to be responsible if it doesn’t work for you, nor for you screwing up your tape deck if you mess it up…

This is the number at the top of the PCB: