turntable troubleshoot

[djaaz]

Hi guys,
I've been fighting with a direct drive turntable almost as old as me that is suffering from fluctuating speed and it's been kicking my ass.
It's time that people who actually understand electronics take a look at it.
Any chance you guys want to help me with that?
Before i upload photos and schematic (hitachi ht-356), i thought i'll ask.

[mauman]

Sounds like fun!  We can't hurt it, and we just might help.  Bring it on!

[djaaz]

Thanks!

Schematic is joined and here's the link to he service manual:
http://djaaz.free.fr/Hitachi-HT354-tt-sm.pdf

The turntable has strong fluctations of speed (33rpm +/- 2%)

What i did so far:
Clean the mechanical bits, lubricate the spindle ball bearing.
Changed all electrolycs (except the 4,7uf / 160V)
Changed the three sc458 transistors and replaced them with 2sc1518 (saw a youtube videos where it seemed to solve the specific issue)

In the service manual, they indicate that speed should be fixed by turning the trimpot on the board to get  3V on pin 10 of the ICO1. What i have there is a fluctuating voltage that is somewhere around 3V.

My go to move when i don't know what i'm doing is to change everything that could be at fault and i was about to change the rest of the transistors and try to find this obsolete IC but i there might be something smarter to do than to overspend on this old thing.

Aprreciate any help, guys.

[mauman]

IC01 is an OKI MSM5819.  Here's view inside it, from another turntable that uses the same chip in a similar way to control the speed, although voltages are different (Yamaha PX-3).
 
It looks like pin 10 is an output of the MSM5819, nominally 3VDC, adjusted by R101.  If R101 is clean and working OK, and you have a wiggly voltage on pin 10, it's likely coming from the chip.  Inputs to this pin include the 6.144 MHz oscillator on pin 17, and pins 1, 4, 11 and 12.  I would:
1. Check the oscillator frequency which should be 6.144 MHz.  If it's wiggly, replace it.  If it's OK,
2. Check the voltage on the other input pins that are called out on the schematic: 1, 4, 11 and 12.  If any of these are off (unlikely), trace back to the source.  If OK,
3. Replace the MSM5819.