Thorens TD104 Turntable Motor Control Help

[plumsolly]

I've got a Thorens TD104 here, with a motor that will occasionally speed up to its fastest speed (much faster than 45 rpm). There is a service manual with schematic and circuit descriptions online, and I've tracked the problem down to the motor control section.


I can recreate the problem by shorting T103. The problem is that when this condition is created (either artificially by me or naturally during operation) T103 continues to conduct until you bring the base to ground or do something else that knocks it out of that state - Even cycling the power to the unit will not undo it. In other words, it seems that if T103 conducts for more than a certain period of time, it latches, causing the motor to increase its speed up to its mechanical limits. This would all be fine if it never got "tripped" in this manner, but it was happening during normal operation.

Can anyone offer any insight as to why its latching, and how to prevent it?

Thanks,

Ben

 

[PRR]

I would wonder what state/voltages are at T102 and C6.

 

[JohnRoberts]

I would put a scope on tachometer signal.

JR

 

[plumsolly]

Thanks for the help. Here are some voltages as requested, both when its working and when its not:

Working Mode
Base of t102:  7.1V negative-going spikes
Collector of t102: 16.5V negative-going pulses
R111 side of  C106: 7.4V negative-going spikes
opamp side of  C106: 9V positive-going spikes
Tachometer: 185mv pk-pk sine wave, 545Hz

Failure Mode
Base of t102:  scope won’t trigger
Collector of t102: see pic

R111 side of  C106: 7.4V scope won’t trigger
opamp side of  C106: 9V scope won’t trigger
Tachometer: 110mv pk-pk sine wave, 3.4KHz

The working readings are inline with the manual:



Thanks again for your insight on this,

Ben

 

[plumsolly]

Any thoughts?

 

[CJ]

found this>

I have recently solved the same problem with my Td-105. The first op-amp of LM3900 input has no signal from tacho-generator coil. Use multimeter and you should read resistance between two pins of tacho about 1 kohm. If infinite, very small gauge copper wire is somewhere in break, usually at two connection pins. If so, yo should try to replace complete motor. It is not easy to find but if you are experienced and highly patient, try to carefully dismantle tacho cap from the case of motor and to repair and recover connection ends of coil. Don't forget thereafter to readjust R130 and R132 trim pots."

 

[JohnRoberts]

Any thoughts?

I would put a scope on the tachometer...

You are looking later in the circuit, go back out to where the tachometer signal originates. Perhaps an intermittent connection.

JR

 

[plumsolly]

I put a scope on the tachometer - see above.

Ben

 

[JohnRoberts]

Your description says photo shows collector of t102... tachometer is somewhere off to the left on the schematic that is cut off.

Does the actual tachometer output change between working and not?

The problem may be in the electronics. You need to probe back closer to the source of the tach signal to see exactly where it stops working. When you find where, then find why.

JR

 

[plumsolly]

I listed the signal of tachometer but did not provide a pic:

Working Mode
Tachometer: 185mv pk-pk sine wave, 545Hz

Failure Mode
Tachometer: 110mv pk-pk sine wave, 3.4Khz

Best,

Ben

 

[moamps]

If there is no AC signal present on pin 5 in the failure mode, and tacho output is OK,
IC may be cause of the problem, please check AC on pin 4 also. Did you check VCC/2 voltage (4,7V) in the failure mode?

 

[JohnRoberts]

I listed the signal of tachometer but did not provide a pic:

Working Mode
Tachometer: 185mv pk-pk sine wave, 545Hz

Failure Mode
Tachometer: 110mv pk-pk sine wave, 3.4Khz

Best,

Ben

OK, so perhaps tach is valid and turntable is running 6x faster.

I woud trace the signal back between the tachometer and Q102

The tach signal is getting lost somewhere between them.

JR