Pioneer PL-S40 speed fix

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Kashofa
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Joined: 13 Jul 2019 01:26

Pioneer PL-S40 speed fix

Post by Kashofa » 18 Sep 2019 12:41

I have a PL-S40 turntable that did not spin at an even speed. It would speed up and slow down on each revolution with an audible warble while playing disks. It's a quartz direct-drive turntable. I confirmed the platter wasn’t dragging physically against anything and I found 45 rpm was stable. Only 33 was affected. What to do...

First I read a few posts online and followed advice to clean speed pots and the speed selector switch. That didn't seem to help at all. Then I found a service manual and started reading. The turntable uses a Hall effect sensor to measure the speed, which is a way of using magnetic sensing to trigger a switch. I assume the speed sensor is generating a series of timed pulses in sync with the spinning motor using this effect. Then a quartz circuit can generate the precise timing pulses required for stability, and a comparison circuit can use the speed sensor and quartz timing to regulate the motor speed. BTW this was all my reasoning based on the functional description from the manual, I can't read circuits well enough to determine if all of this is perfectly accurate.

I know the quartz circuit and comparison circuit are things I could not fix with my abilities. So I decided to focus on the speed sensor, which must be partially mechanical in operation. Fortunately the turntable is very easy to disassemble. I was able to separte the motor and speed control section as a unit, and then remove just the motor board from the speed control board. A bracket holds the motor together and removing that I could separate the motor halves. Under where the motor is mounted there is a pair of zig-zag electrical traces. That looked to me like the speed sensor, with the motor magnets spinning above to generate the speed signal. There are two sets and the traces can be followed to the 33 and 45 rpm adjustment pots.

The zig-zag traces on the board were very oily, so I cleaned it off with alcohol. It looked like someone had previously used spray lubricant around the motor and the oil was on everything. At that point I didn’t think I’d actually fixed anything but I was finished exploring and reassembled the turntable. And surprisingly the 33 speed was as stable as the 45 speed!

My assumption is that excess oil was distorting the Hall effect used by the speed sensor sensor circuit. If you find a similar problem with this turntable or any design using a similar speed sensor, try checking under the motor for etched sensor traces with excessive oil on them.

lenjack
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Re: Pioneer PL-S40 speed fix

Post by lenjack » 18 Sep 2019 16:22

Very cool :!: I assumed you were in trouble, but you weren't =D> . Great job of troubleshooting. Amazing, the way you figured this out.

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