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If you block the fork, what do you mean by "nothing happens" - does it speed up, slow down, or keep constant speed? It *should* speed up, since it is getting information that indicates the disc has stopped. If it doesn't speed up, there is some problem with not having enough control range and/or not having speed trim set correctly (or possibly other circuit errors, of course). Then, when the fork pulses are applied to the chip, it should slow down and eventually lock. If you have a spare chip, you might consider substituting it. It is possible to blow out some chips if you disconnect the power supply from the chip but still have full voltages on the input pins (which I think you said you tried once).
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