When you state "the string is OK." are you referring to the rubber belt that connects the servo motor to the pulley?  If you are then check that the motor is actually not spinning as the pulley may be loose. If it is not, then are you capable of performing electronics troubleshooting and repair as there is likely a faulty part that a tech will need to replace?If you have a meter you can start by checking whether the servo motor is getting 15V across it as you use the controls to move the arm.

Thanks for the reply Mark,

With ''string'' i mean the... 'wire' that connects the pulley with the sliding chassis. But the rubber belt you are referring to is also OK. Good hint on that pulley being loose but not in my case unfortunately.

Basic electronics troubleshooting are within my reach so throw your suggestions. I metered  the servo motor and it gets 0.5V when i operate the controls or when the tonearm gets deflected. That doesn't look good, does it?

By the way, the servo motor has a label on that says "max.8V" are we sure its supposed to receive 15?

I’m not sure about the Schematic annotation of 15v, however you should see 7V when pressing either “<“ or “>” across the motor, so that is you issue, Pin 21 and 22 provide the signals for the controls which should be +5v when the respective one is high. If it is then I would look at the inputs and power supply to IC2. BTW, if the servo motor was pulling down the voltage, then you could confirm that by measuring the 7V with it disconnected.  In any case I would investigate that circuit area.

OK you are right the motor was puling down the voltage. With the motor disconnected i measure 3.6V from the board and not 7... Do you think it could be a bad capacitor?

First, I would check that the motor works using a 9v battery. It should change direction with polarity. As to the unloaded measurement, you did not mention whether its polarity switches with the arrow and I’d also check if it is higher with >> and <<. I would also check the overall power supply voltage and the power pins on the OpAmp as per the service manual.  If those are low, then I’d go after the electrolytic caps. Overall my bet is on a bad motor. 

JimT wrote:

Thanks for the reply Mark,

With ''string'' i mean the... 'wire' that connects the pulley with the sliding chassis. But the rubber belt you are referring to is also OK. Good hint on that pulley being loose but not in my case unfortunately.

Basic electronics troubleshooting are within my reach so throw your suggestions. I metered  the servo motor and it gets 0.5V when i operate the controls or when the tonearm gets deflected. That doesn't look good, does it? link

By the way, the servo motor has a label on that says "max.8V" are we sure its supposed to receive 15?

 

Thank you for your kind sharing

Thanks Mark,

Yes the motor is spinning and changes direction with reversed polarity (i gave it 5.6V and is works nice). The unloaded measurement switches polarity with << or >> and gives the same 3.6V either way. I will check the OpAmp and will keep you posted... If it is the caps i was considering starting from C28 and then moving to C30-34-35.

JimT wrote:

[...] The unloaded measurement switches polarity with << or >> and gives the same 3.6V either way. I will check the OpAmp and will keep you posted... If it is the caps i was considering starting from C28 and then moving to C30-34-35.

hmmm...What test points do you measure when you say 'unloaded measurement'? CPU outputs 21, 22 toGND or between opamp outputs 1 and 3 with motor disconnected? 3.6V seems strange at either of these points.
Those caps you mention are unlikely to be the source of the problem. What dou you read on the 5V and 12V power supply output. And what's the voltage at pin2 of IC2? The latter should be around 9V.

-------------

Oops! After actually reading the whole thread instead of just skimming it I realized that you're measuring 3.6V across the motor terminals -- and that's surely off. My recommendations above still stand (and Mark has suggested the same steps already).
Additionally I'd measure both motor terminals to GND instead of measuring just across the terminals. This way you can see whether the problem lies with one of the opamps or the zener diode or power supply.
The schematics show the nominal values:
upon pressing " < " : P1-2 = 7V, P1-1 = 0.15V
upon pressing " > " : P1-2 = 0.6V, P1-1 = 7.5V

OK further to your suggestions the CPU outputs 21 and 22 give 5.15V to GND, the same goes to opamp 7 & 8. Opamp outputs 1 and and 3 give 3.51 and 3.62 to GND respectively.

Pin2 of IC2 to ground outputs 3.64V

The 5V and 12V supply output give 5,15V and 12V.

upon pressing " < " : P1-2 = 3.51V, P1-1 = 0.15V
upon pressing " > " : P1-2 = 0.15V, P1-1 = 3.62V

So the power supply looks OK. Zener diode OK. That leaves us with the opamp? or maybe the caps?