So I have this weird problem with my WM-D6C Walkman; Since shortly after I replaced both the idler tires, capstan ring and the main belt, it developed a 'crackle' and uneven volume when played in any position other than with the cassette door facing up. I have included two clips of the same section of a cassette, recored from the line out of the d6c into the line in of my computer. The first clip, D6C_FlatPlayback, was recorded when placed flat on my desk, while the other was recorded when placed on my table so line out is facing upward. At approx 1:12, I picked up and rotated the Walkman in order to stop it, hence why the sound improves for the last second. The behavior is the same if placed or held so the volume slider faces upward. I have a higher quality (24 bit 192000 Hz) recording if that is helpful.
I would start by checking the grounding throughout the unit. There is a grounding star point on the main circuit board, near the -ve battery terminal, where there are a lot of black wires connected. The black wires connect to grounded points throughout the WM-D6C. There are four screws which hold the main circuit board to the metal chassis. Make sure these are all fitted and tight. The only screw which is connected directly to 0V ground is the one nearest to the record level rotary knob. This is also connected to the spring tab, which connects the ground to the metal back panel via the hole in the plastic lining sheet inside the back panel. Some parts of the PCB are grounded from the other screw positions, via the metal frame.
Okay, so I checked the tightness of all four screws on the pcb, and they should all be tight now. It reduced the amount of noise, but did not eliminate it. As a test, I used a multimeter to test the continuity between the tip of the 6v dc in and screws on both the base panel and upper panel, and found that when held or placed flat, there is a constant connection, but the connection is spotty or non-existent when at an angle. Would this be from the spring tab not making proper contact? I also tried the same test using the battery contact with the same result.
It sounds as if you have found the problem. The spring tab is the only place where the casing makes contact with the 0V.
