Please don't confuse the issue by throwing suggestions without a full understanding of what is happening. The pitch control we are talking about is not simply a board mounted micro-pot for tape speed calibration. In fact, tape speed calibration is performed on the big ben by rotating an entirely different pot, mounted within the motor and accessed via the traditional motor rear access hole. The control we are speaking of is a true pitch control FEATURE that allows the user to vary the speed of the tape deck EXTERNALLY. It has a front mounted knob like the JVC RC-M75. The fact that the knob has no effect already indicates that there is a problem within that circuit which is where we should be focusing initially. We all know that an improper sized pulley will alter the tape speed but it would be a STATIC change and won't affect the ability of the control to vary the pitch. In my experience, a wrong pulley is perhaps one of the least likely scenarios. Pulleys simply don't go bad, certainly not at the rate of bad motors, so why change it in the first place? The only reason I can see would be that the motor was swapped which is possible but then the technician would be able to find another matching 4-pin motor? And with a giant pulley to boot and not bother to swap it?hopey said:
After all, let's analyze your 2 suggestions, shall we?
#1, you've concluded that if the pitch pot is faulty so the work around solution to fix it is to replace the pulley? And how many different combinations do you think it will take to eventually calibrate it so that it is just right? And do we just ignore that the pitch control feature is not working? I'm sorry, that solution is shade tree if ever I saw one. Dave has repaired far more complicated circuits than this. Why not first investigate repairing the current failure before entertaining disabling a feature and doing some kind of work-around? Also 4-wire replacement motors are available.... I think Caution might even have one available that might work.
#2, you state that if the motor is faulty, it would run too slow.... how do you know? Not every motor fault is identical. Internally, there is a motor speed control circuit that continually varies the power to the motor to account for varying loads. The heart of this speed control circuit is a small IC and it relies on feedback. If you grip the motor spindle and introduce friction, the torque will actually increase to account for it and maintain the speed. If this fails, it's not a mechanical failure but a failure nevertheless. However because it is internal to the motor, and not really intended to be serviced, failure of this circuit technically still means that the motor has failed and normally would be replaced as an assembly. Only because we are trying to repair obsolete machines do we now consider disassembling the motors to repair these boards. Normally, on 2-pin motors, it is designed to run at a single speed only. On 4-pin motors, the extra 2 pins is intended for external speed control circuitry INDEPENDANT of the normal adjustment used for calibration.
