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The levels are TTL but I gather there would be a buffer on the signal lines similar to LX200's keyboard.
In which case the most likely place to have the trouble you experience is there. LX200's use 74LS74 hex buffers that are the first things to go if you get 12v on the wrong line. They eventually put snubber diodes on them to handlle problems.
I don't know what is used on the EQ drive boards but tracing the tracks on the PCB should reveal it. Have you put an ohm meter on the signal lines. If so what is the reading.
Barry
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