Turntable Repair - Stopped Spinning

[kapnKronik]

Hi All,

I am attempting to repair an old turntable that stopped spinning. I put a brand new belt on it, and it worked properly for about 30 mins, although on the slow side - I had to turn the speed adjustment all the way to its fastest. It then started to gradually get slower, until it finally came to a stop completely. I attempted to restart it by turning manually, but it would just coast to a stop. The timing light still lights up when it is powered on and I manually spin the platter to function the mechanisms, and it switches off properly when hit the 'stop' button and manually spin the platter to function the mechanisms.

I looked up the service manual online and my first hunch was that the 'Drive IC' was bad. But, I tested the voltages of various components against the ground and am now unsure how to interpret them to determine which component(s) should be replaced. I am attaching images of the service manual pages containing the schematic and troubleshooting. I will happily provide any more information needed, and can do any further multi-meter testing as needed. I am listing below the voltage readings I obtained. Any guidance would be greatly appreciated.

Also, in the troubleshooting it stated there should be 14VAC between FC1 terminals 2 and 3, but on the schematic there is no terminal 3, so I was confused and simply measured the voltage at the secondary of T1.

T1 Primary: 120 VAC
T1 Secondary: 9 VAC

IC1 Terminal 1: 9.2 VDC
IC1 Terminal 2: 6.1 VDC
IC1 Terminal 3: 0 VDC

IC101 Terminal 12: 3.1 VDC

Q101 Base @ 33RPM: 5.1 VDC
Q101 Collector @ 33RPM: 9 VDC
Q101 Emitter @ 33RPM: 4.5 VDC

 

[kellys_eye]

You should check the motor feedback signal from Z601. This may be a hall effect device and its position relative to the motor (pickup distance) is quite critical.

 

[kapnKronik]

I'm sorry, I forgot to mention that I am a novice. To check the feedback signal I would need an oscilloscope? Could I check the conduction between terminals on CN101 as suggested further in the troubleshooting to determine if Z601 or the motor is defective instead? Or would that not really give me the same info?

Also, what do you make of the incorrect voltages in the power source and drive circuit?

 

[debe]

I would start with the motor, Unplug the motor conector & feed 6v dc to the motor & see if it runs ok & not partly seized. If motor ok Check R108 & 109 these seem to be a current sensing circuit

 

[Bluejets]

With these old turntables, a bit of oil in the right spot might go a long way also.

 

[kapnKronik]

Ok, thanks, I will try oiling the motor and checking it out of circuit. What kind of oil should I use? I assume there are usually holes on the motor to drop the oil in?

I will be out of the country for a week and a half, so I won't be able to try anything or reply until then, but if anyone has any other ideas I appreciate it. Especially if someone could explain the voltage readings I got, I'd like to try and understand more about circuits and how a failed component can affect it. Thanks everyone!

 

[dorke]

Remove the belt and try spinning the table,it should move effortlessly.
Is the belt you have put in the exact correct size,is it too small, too tight?
With the belt out, try spinning the motor pulley/shaft,again it should move freely.
Don't oil the motor if it turns freely.

Then,
Start with what @debe suggested.

If you have a scope it will be a great help.

Out of curiosity,what make and model is the turntable?

 

[Bluejets]

I was referring more to the turntable bushing and thrust bearing along with any " auto change linkage" it may have.

 

[73's de Edd]

Sir kapnKronik . . . .


If the drive motor spins freely by finger power, I would skip initial suspicion of it.

Looks like the static voltages of the power supply circuitry are being within reason.

Looks like your motor is in the power loop to ground thru the Q101 power transistor and it having full supply voltage on its collector, and MORE than adequate turn on voltage on its base.

BUT it has too much voltage on its emitter, which should be pulled on down to being less that a volt.

With only ~1/2 watt dissipation being shared by the 22 ohm emitter resistors , that should be WELL below any burn out level or even a scorching by virtue of the wattage of resistors that they chose to use..

Go back and measure the emitter again and then probe the mid junction of the resistors and see if the voltage is halved, if not then masure at the opposite lead of R109 and see if 0 voltage is present.
If not check its connectivity into the ground buss.

Last . . . . coming in at a 10% suspicion . . . . might be the timely demise of capacitance in the main storage electrolytic caacitors C4 and C5.
Lastly, see if the commutator noise supression electrolytic across the motor as C107 has developed leakage.

Thaaaaasssit . . . . .

73's de Edd