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[SOLVED] Servo Motor does not lock on cold start

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hafrse

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Hello,

I have a circuit that controls the speed of capstan motor in a tape recorder, when powered on, the motor goes to the highest speed (unlocked) and then stabilizes to the correct selected speed after about 3-4 sec. If the power is powered +8v at pin 1 to the control circuit but the motor is disconnected and connected again, the servo locks directly to the correct speed.
its seems that only on power-on together when the motor is engaged when selected to Play (pin 9) connected to the motor.

Pin 1 +8V power
Pin 2 squarewave signal from the tachmeter
Pin 3 controls the speed by 4 capacitors, one for each speed, and then to GND
Pin 4,5 GND
Pin 6 is used for max speed through a diode and 47K resitor to +8v
Pin 7 not used
Pin 6 to indicator(not used)
Pin 9 to the motor

What can be wrong that causes the motor to spin very fast and then stabilizes when the circuit is powered by the +8v first time? as I explaned if I move the selector to other position where the motor is not engaged and engage the motor again (worm start , circuit is powered) , it locks directly
Thanks
 

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FvM

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What kind of motor (PMDC?), where ist it exactly connected. Schematic shows only "motor indicator" at pin 9.

The control principle isn't obvious, I guess there are other circuit parts involved.
 

hafrse

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Hello, it is a DC motor, schematics shows pin 9 and 8 , pin 9 goes to the motor pole , pin 8 goes to the indicator if it is available at this model.
Corrected:
Pin 1 +8V power
Pin 2 squarewave signal from the tachmeter, can be from 30khz-60khz depending on speed slected at pin 3
Pin 3 controls the speed by 4 capacitors, one for each speed, and then to GND
Pin 4,5 GND
Pin 6 is used for max speed through a diode and 47K resitor to +8v
Pin 7 not used
Pin 8 to indicator(not used in this particular model I have since it lacks indicator level meter)
Pin 9 to the motor pole, the other pole is GND
--- Updated ---

To explane about the modes in the rotary selector switch involved here:

"Stop" No +8v power DC motor is not engaged
"Play" +8v adn motor is engaged at the same time (used to adjust audio when motor is not engaged)
"Test" No +8v power DC motor is not engaged

Now if I go from -> to:

Stop->Play Motor will not lock the first 3 sec (cold start)
Play->Test +8v present, DC motor is not engaged
Test->Play Motor will lock directly since the +8 is already present from Test mode
 
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FvM

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Doesn't Stellavox provide locking of the capstan to an internal crystal reference oscillator respectively an external time reference? If so, where's the reference injected?
 

hafrse

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Hello,
Not in this unit , it uses 1 capacitor for each speed, capacitor is connected to pin 3, the other pin of the capacitor goes to GND and it uses pin 2 which is the Tacho signal from the DC motor. There are no other circuits related to this one. I am looking what can cause this overshoot on power on, rest of the time after the power on, DC motor is locked
--- Updated ---

Attached a block digram and a picture of the module.
Component involved:

SMC Capstan motor
STF Amplifier for the Tacho signal
SMU Motor control (the schematics attached in previous posts)
 

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