Sony WM-BF58 inside View

The Sony WM-BF58 has a well-designed and engineered circuit that continues to perform well. In order to save space, they mounted the CX20111 IC on the trace side. This Walkman has the same type of transport mechanism as the WM-B52, with the same dual drive belts. To change the drive belts, undo the two screws that secures the circuit board. The motor and leaf switch wires are barely long enough to allow lifting the board to gain access to the motor pulley and flywheels. You should see a pair of standard thickness drive belts, where one belt is the main one that drives the capstan flywheel and the other belt provides fast-forward and rewind functions. I had a bag of assorted belts bought from China and it had the precise two I needed!

Repair

This unit came to my lab for repair and the symptoms were that it would play for a few minutes, sometimes as long as twenty minutes, the speed would go wobbly for a few seconds, and then the motor would stop completely. During this condition, the servo IC LA5523, would heat up.

The best way to diagnose the fault is to measure the voltage across the motor terminals when this condition arises. It will read 0.1 V or thereabouts indicating a short. However, we do not know if it is the servo IC that is short or the motor, so the next step is to desolder one of the motor wires, and then measure the voltage the servo IC is producing. On my unit, it was around 3 V indicating that the servo IC was operating correctly. You could do a further confirmation test by connecting another motor and seeing if it spins, but not necessary.

"It’s all elementary by dear Watson!" I said. A worn out motor usually goes short at random periods and therefore the servo IC heats up! Luckily, I found an exact replacement from my spare parts collection, and replacing the motor solved the fault. It was then a simple matter of calibrating the speed using my WS-48 Sony Test Tape, and it was good to go.

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