[Sideband]

Well it could be the motor. It's the same motor used in all these basic decks....getting hold of one might be a problem.....
As for the control board....they were never repaired back 'in the day' and just replaced. However they couldn't be simpler and I believe are just a basic comparator circuit. The AC127 and 128 are still available if you feel up to changing them....you only need a small soldering iron and a steady hand. The two diodes can be replaced by 1N4148's....they are nothing more than standard signal diodes. There is very little else to go wrong with those boards.

If the voltage to the motor appears to be hunting it doesn't necessarily mean that the the control board is faulty. It MIGHT be trying to control a faulty motor by varying the voltage to it....if the motor is faulty for instance, the feedback voltage from it to the comparator will vary so the transistors will try to compensate. Then again if either of the transistors are faulty but the motor is good, you'll possibly get a similar effect.

Not trying to put you off but just explaining that changing the transistors and diodes may not cure the problem.