This Nexstar mount is left untouched after an accident four or five years ago. The hand controller was suspected to be faulty and sent back for repairing but it's lost eventually. Until a few months ago, I managed to buy a replacement from astromart, but the problem persisted, so I concluded that it's not the controller but something in the main circuit board inside the mount body.
The symptom is, the ALT motor will slew in both directions without problem, but the AZ motor will only slew one direction but not the other one. I swap the controlling circuit by swapping the plugs and the problem migrated to another motor. I can also reverse the polarity of the driving circuit of the same motor to enable the mount to slew in another direction.
I carefully trace the circuit board this time, and I found the driving circuit (orange), the decoding circuit (blue, to read the signal from the encoders of the motors), and I also found that each functions are controller by two sets of four identical chips.
So far after some examinations, I concluded that a certain chip is faulty and I circled it in purple. I shall see if I can order replacement chip.
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