Hello. I have a problem with this machine, while I think about writing the query I think that maybe I have the answer in front of me, but I don't know. I changed all the belts (4 in total) and pinch roller, I adjusted the speed with a cassette with a recorded 3KHZ signal and after about 10 minutes the speed began to decrease until it reached about 150Hz less (2850Hz), I replaced several motors, all with internal speed regulation (I clarify that they are not new motors) and the symptom is exactly the same without differences between them, the supply voltage is fine in absolute value and ripple (unregulated voltage), all the parts that rotate do so with Total smoothness, the consumption of the motor without cassette is about 90mA (the motor always rotates when turning on the machine) and rises to about 130mA by dragging the tape. As an additional detail, if I adjust the speed as soon as I turn on the machine, and leave it with the motor running for a while but without the cassette, the result is the same, that is, the speed decreases. I also tried cooling parts with freezing spray without success. Everything seems to indicate that the motor is the problem, but do all the ones I tried do the same thing? Am I missing something? Thank you so much.
The voltage varies because it is not regulated, the loss of revolutions is sustained once it reaches that new revolution regime. If I disconnect the equipment and wait, let's say an hour, it starts at the correct speed and then drops. The voltage is fine, around 12V, the speed regulator is built into the motor, I believe it is fed back by current, with that it maintains the revolutions, I insist, I tried several motors and the problem is identical.
I do know certain types of those old motor regulator IC's go bad in exactly this way. But several motors with the same issue? Not likely, still not impossible either. Did you try cooling spray on the internal motor PCB? Easy test to rule things out.
What happens with the motor current over time as it slows down?
What happens with the motor current over time as it slows down?
It is often possible to pry off the back cover of the motor. Desolder the internal pcb from the motor terminals, remove it, reconnect it to the motor with some short wires so you can put the back cover on again with the pcb outside. That allows you to run the motor while having full access to the regulator pcb.