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aj164 · **Posted:** 27 Sep 2023, 14:35

I found the offending component. I realize I may be talking to myself here since I’m working on a Sony, but this same push-pull circuitry might be in the other brand players as well. Even if you don’t have a dual-sided player, the motor still needs to brake to slow down the disc. Applying the “brakes” is biasing the motor in reverse temporarily.

The circuit diagrams in the service manual are tremendously helpful. The positive motor lead measured around 13V. The negative lead measured -13V while spinning up for Side A. For Side B, the negative lead went positive, but the positive lead never went negative. That narrowed down where to look. Oddly enough, it wasn’t a power transistor but instead a switching transistor that biases the power transistors on the motor positive lead. Q55 is a PNP transistor. It passed the base-to-collect and base-emitter tests, but showed leakage from C to E. Active components rarely fail unless the passive components (capacitors) fail. This power supply was full of bad, leaking capacitors.

The PNP transistor is a 2SA1309A, but it looks like many other PNP will substitute. I used a power transistor from a donor power supply, and it works.