Apple Disk II speed control board failure

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gjbl's picture
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Apple Disk II speed control board failure

Now, here is something for real Apple2 experts  :-)
Apple Disk II - the diskette spin motor won't start, all other functions seem to be OK, but with floppy not spinning its hard to tell. The analog board looks OK, the "motor on" signal is generated, the motor itself spins when powered. I traced the problem to the speed control board (SCB), initially I suspected Q4 (the power transistor), but it looks OK, BUT ... Q3 is faulty!  There is no visible damage on the board, no signs of overheating, etc. It is rather rare that a low power transistor fails by itself... Working with the SCB is difficult, for example the cables to the motor are so fragile that they broke at solder joints after I had bent them a few times. So, should I assume a single transistor failure, replace it, solder everything up and hope for the best OR is it possible that the source of the problem is somehere else?



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