Hi, folks,

This is my first post. I'm something of a newb to electronics repair (trying to follow in my late uncle's footsteps) and I've hit a wall with this motor.

It's a Papst 38.09 hall-effect motor from an early 80's HK ST-7 turntable.

It's original issue was poor speed stability. I did some research and came across what seemed like a promising fix:

-replace electrolytic cap on motor board
-replace tantalum caps on motor board
-replace two transistors on motor board outside motor case
-replace the coarse speed control pot.

I completed those, but the situation became worse. Now, when it starts, the motor over-revs for 3-5 secs.
and then slows to close to correct speed. It is supposed to draw 30 vdc, but it is drawing about 17vdc
during the over-rev. When it slows it draws 29 vdc. I can get it close with the speed pots, but it
still runs the platter at about 35-37 rpm. The belt is new and not slipping.

I double checked the caps, all the resistors and the coarse pot.

There is one component I can't identify which is the round one with the
yellow-black-brown color code next to the rotor. It is #29 on the schematic-a rectangle with a
diagonal line through it. Is this a varistor? Could it be the problem?

Here are shots of the motor board and a copy of the schematic:



Any thoughts or help are greatly appreciated.

Many thanks,

John

 

Welcome to AAC!

A right reading schematic with the component in question marked:

It's a non-standard symbol, but, from the position, I'd say it was a variable resistor.

t is supposed to draw 30 vdc, but it is drawing about 17vdc
during the over-rev. When it slows it draws 29 vdc. I can get it close with the speed pots, but it
still runs the platter at about 35-37 rpm. The belt is new and not slipping.

This doesn't make much sense. Are you talking about voltage or current? We talk about current drawn, but not voltage.

 

Hey, Dennis,

Thanks so much for taking the time to look over the schematic.

You're right about the current/voltage error. I should have known the difference. So, what I meant to say is
when the motor is over revving, the voltage at the supply is ~17.3 volts. When it slows down,
the voltage is ~29 volts.

There is also a Zener diode in the circuit. I wonder if it, or the varistor, are part of the problem,
since they both change behavior at a given voltage?

Thanks,

John

 

That looks like an astable multivib, with current control on the two bases of transistors 1,2. The voltage fluctuation is due to the excess current draw on the motor coils.

You can replace this circuit with a 555 and two transistors outputs.

 

That looks like an astable multivib, with current control on the two bases of transistors 1,2. The voltage fluctuation is due to the excess current draw on the motor coils.

You can replace this circuit with a 555 and two transistors outputs.

This seems an overcomplicated circuit just to drive a motor – there is some feedback from the motor drive circuit to the astable multivib circuit that may be compensating/correcting for a changing motor load.

I would tend to agree with Dodgydave; if you can get hold of a twin channel scope and have a look at how transistors 1 & 2 are switching the motor windings – you could replace the circuit with a 555 timer and a couple of transistors.

It appears that there is a switch within the circuit – I guess to change between 33/45 rpm.