The player needs to be modified to receive control voltage from the DAC board. This is as simple as soldering a wire to cassette player ground electrical (GND) and another to the variable speed control potentiometer's wiper, which is labeled VS+ (variable speed positive, perhaps) on the board.
To start, remove the four screws on the back of the player and carefully open it up, being sure not to damage the battery and speaker wires.
Solder to VS+
Solder a 6" length of red silicone covered wire to the VS+ point. This will go to the VA (voltage A) pin of the DAC board.
Solder to GND
Next, solder a 6" length of blue wire to a GND point on the player as shown here. (You can find a GND point with a multimeter in continuity mode testing from a known ground if needed, such as the negative battery contact.)
This wire will go to the GND pad on the DAC.
Run the two wires you soldered outside of the player via the nearby DC jack hole.
NOTE: If you plan to use external power, you can run them through a gap in the cassette well, or drill a separate hole for the wires.
Use some short lengths of heat shrink tubing to neaten up the wiring if you like.
Wire to DAC
Solder the wires to the DAC board as shown:
- blue wire to DAC GND pin
- red wire to DAC VA pin
Plug in the USB C cable to your computer and the QT Py so you can get ready to code it!