Check the Power Supply board first. The manual highlights "C107" and "C109" (Nichicon capacitors near the rectifier). These are known to leak. The manual gives you the voltage ratings (50V, 1000µF). Replace these before doing anything else.
Connect an oscilloscope to the "FE" (Focus Error) test point. Insert a pristine CD (a known good, reflective disc). Turn VR103 very slowly. You are looking for a perfect "S-Curve" on the scope. The manual shows you exactly what the S-Curve should look like. If you can't get the S-Curve, the pickup is dead.
You cannot calibrate a Tascam CD-401 MkII without putting it into "Test Mode." The service manual provides the key combination (usually holding specific buttons while powering on) to disable the tray lock, spin the motor without a disc, and access the potentiometers for laser alignment. Tascam Cd-401 Mkii Service Manual
This is not the "User Manual." The user manual tells you how to press "Play." The Service Manual tells you how to bring the dead back to life.
Here is what is exclusively inside the Tascam CD-401 MkII Service Manual that you cannot find on Reddit or YouTube: Check the Power Supply board first
If you are searching for this manual, you likely have one of these three issues.
The Problem: The tray opens 2 inches and stops. The Manual’s Solution: It identifies the "loading belt" part number (Tascam part # 5711392400) and provides the 12-step disassembly to reach the motor without breaking the front panel ribbon cable. The manual gives you the voltage ratings (50V, 1000µF)
The Problem: The player randomly skips on warm days. The Manual’s Solution: This points to the "Focus Offset" drifting. The manual provides the exact oscilloscope setup (using the "Eye Pattern" on pin 4 of IC101) and which trim pot (VR104) to adjust while the disc plays track 1.
The Problem: "Disc Error" on 20% of CDs. The Manual’s Solution: The laser pickup (Philips CDM-1/10) is failing. The manual gives the exact resistance values for the laser diode (typically 45-55mA current draw) and how to check it using a multimeter without destroying the diode.