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Repairing a turntable motor

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Marky P

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Hi,

I have a Citronic PD-1 mk III DJ turntable that appears to have a motor problem. When the deck is powered up, the platter just rocks back & forth. It can be manually pushed and then it does spin but it's unstable. I'm assuming this must be a motor problem. I've got the motor out, which is on a PCB. There is a large round counterweight that the platter sits on and removing that reveals four coils mounted on the PCB. I've checked the resistance of each coil and they are the same at 30 ohms. When powered up, each coil gets warm, so they must all be getting a current. There is a voltage regular on the board, just wondering if that could be faulty? There are also lots of other components on the motor board, so I suppose any of them could be causing the problem. Any help would be great.

Thanks,

Mark.
 
Do the coils get equally warm? I'm guessing the fault lies in the coil driver electronics.
 
Just want to mention, could it be a fault with the motor drive transistor array (STA431A)? Problem is, can't find a spare one on line.
 
Just want to mention, could it be a fault with the motor drive transistor array (STA431A)? Problem is, can't find a spare one on line.

Quite probably - however, these sorts of components are rarely available as spares, only to the motor manufacturers.

It's a DC brushless motor, with the coils switched in sequence by the IC, and the rotor position detected by hall effect sensors.
 
Thanks. I have tried searching the net for the STA431A but like you said, only seems to be available in bulk to trade. Do the hall effect sensors look like small transistors? There are two things like this on the board, and it looks like each one is connected to two coils. Could these be faulty?
 
Yay! I've fixed it! There was a dry joint on one of the hall effect sensors. Bit of soldering and now all is well! That's saved me a fortune :))))
And thanks for your help.
 
Excellent! Nasty little blighters, dry joints.
 
Excellent! Nasty little blighters, dry joints.

They certainly are. Couldn't see the problem at all. It was only though fiddling about, that the leg came clean off the board. They were mounted on the track side flat on the board.
 
bad solder is always something to look for..... haven't seen a turntable do the washing machine thing in a long time. it was a common failure on early direct drive turntables back in the 70's.
 
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