carus@cbnewsc.ATT.COM (bryce.w.carus) (06/03/89)
This is a follow up to the previous discussion on 1541 drives that don't turn off after power up or resets. The schematics that I've seen on the 154x series don't deal with the speed control area which, based on the complaint noted, is where I think the problem lies. I traced out the circuit and found a straight forward closed loop DC motor speed control. There is too much to try to "draw" here but a quick word description should do... Color codes: green = 12v power input, black/blue = ground, gray/yellow = AC tach output from the motor, orange = motor on/off command line, and red = regulated power input to the motor. Trace the red lead to a power transistor whose base is switched hi/low thru two general purpose transistors. A third GP transistor buffers the motor on/off lead. This is the section where the problem is probably located...as usual, in the higher power sections. In my case, a 2.2u-16v tantalum capacitor was leaky. A 2.2u-35v was it's replacement...better because 16v capacitors are not very conservative in a 12v section. My suggestion is to "work your way out" from the power transistor and substitute electrolytics and tantalum capacitors as you go. If that doesn't work, then substitute transistors and resistors as you go. If you can test the components as I did then you'll zero in on the culprit quickly. The newer "Newtronics" boards don't have as many discrete components but work on the same general principle: the motor's speed is a function of the applied voltage at the red lead. If the motor runs at speed (300 rpm) but won't switch off, then the circuit associated with the orange on/off lead can be traced. The last thing I'd suspect whould be the IC's, the first would be polarized electrolytics with voltage ratings near 12v. Somewhere in between would be high wattage components. This is all pretty standard debugging stuff... Good luck, I hope this info will be generally useful to the NET.