Transistor Problem?

[Catenary]

I'm learning by trying to repair a NAD cassette tape deck. When first powered on, the capstan motor (M701) runs for 1/2 second or so then shuts off. If I turn it on again less than 5 seconds later there is no response. Waiting at least 5 seconds, however, allows the motor to run for 1/2 second again. I don't believe the motor should run at all unless I press the PLAY button (Contact 4 in connector MP701).

With power on and after the 1/2 second run time, if I press PLAY, the motor runs for three seconds then dies. No amount of waiting will allow it to start again unless I de-power the whole unit for the five or more seconds mentioned above.

Can someone explain what is going on in the transistors Q275 and Q276? If the problem lies there, I have a hope of repairing it. If the problem is in the MECHANICAL CONTROL chip, then I might as well use the unit for experimentation because I can't find it online.

Many thanks for any help to this newbie.

 

[Tha fios agaibh]

I'm a newbie too but I'll give this a shot.

Q725 pnp has 9v on its emitter. When the base voltage falls .7v below the emitter It causes a current from the emitter through the base and a larger current from the emitter through the collector. This current forward biases Q726 which has its collector hooked to pin 5 of motor. When Q726 is forward biased it pulls pin 5 to ground from collector through emitter.
So when pin 19 of IC701 goes low, the motor should turn on.
Not a perfect explanation, but it's how I think of it.

 

[Harald Kapp]

I agree.

Do you have an oscilloscope? Then it were easy to monitor the timing of the signals. At the times stated by you, some measurements can be done with a multimeter, too.
To check whether the behavior of the capstan motor is due to one of the mentioned transistors or due to action of IC701, measure the voltage from IC701, pin 19, to ground. If there's a switching action on this pin and the motor reacts accordingly, the transistors are o.k. and the observed behavior is due to IC701.

Check switches S703 and S704 for correct connection and operation. Check also the connection MP704/MC704 for good contact.


The next point to look at is IC704, connectors MP703/MC703. IC704 is a hall effect sensor. It is likely supposed to generate a square wave signal when the cassette spool rotates. In case the cassette stalls, the motor is turned off to prevent 'tape spaghetti'. Pin 3 of IC704 is the output. Do you measure a changing signal there when the motor runs or it the output static?
The problem may be electrical (a defect of the IC) or mechanical (check for the position of IC704 and look for a magnet on the axle or wheel that's being monitored by IC704. IC704 may simply have been bent too far away from the respective magnet.

 

[Catenary]

I'm a newbie too but I'll give this a shot.

Q725 pnp has 9v on its emitter. When the base voltage falls .7v below the emitter It causes a current from the emitter through the base and a larger current from the emitter through the collector. This current forward biases Q726 which has its collector hooked to pin 5 of motor. When Q726 is forward biased it pulls pin 5 to ground from collector through emitter.
So when pin 19 of IC701 goes low, the motor should turn on.
Not a perfect explanation, but it's how I think of it.

Thank you for this explanation. Now I'm going to spend the afternoon learning about transistors.

 

[Catenary]

I agree.

Do you have an oscilloscope? Then it were easy to monitor the timing of the signals. At the times stated by you, some measurements can be done with a multimeter, too.
To check whether the behavior of the capstan motor is due to one of the mentioned transistors or due to action of IC701, measure the voltage from IC701, pin 19, to ground. If there's a switching action on this pin and the motor reacts accordingly, the transistors are o.k. and the observed behavior is due to IC701.

Check switches S703 and S704 for correct connection and operation. Check also the connection MP704/MC704 for good contact.


The next point to look at is IC704, connectors MP703/MC703. IC704 is a hall effect sensor. It is likely supposed to generate a square wave signal when the cassette spool rotates. In case the cassette stalls, the motor is turned off to prevent 'tape spaghetti'. Pin 3 of IC704 is the output. Do you measure a changing signal there when the motor runs or it the output static?
The problem may be electrical (a defect of the IC) or mechanical (check for the position of IC704 and look for a magnet on the axle or wheel that's being monitored by IC704. IC704 may simply have been bent too far away from the respective magnet.

Funny, I was just thinking about an oscilloscope yesterday. No, I don't have one, but I can see some effects on my analogue VOM.

These are all excellent suggestions and I appreciate them. In checking the switches with my magnifiers on I found some evidence that my problem may indeed be mechanical. You've given me a splendid troubleshooting guide.