These electronic rev counters are basic, but pretty old, electronic technology driving a standard 'meter' movement. I have just repaired mine. It was rather optimistic showing readings more than double what the speedo would suggest it should!
The biggest problem faced - by miles- is getting the jiblets out of the chrome housing and then back in again - I can't emphasise enough how much care and patience is needed, it must not be rushed else you will have a cracked or mangled chrome housing. They were not designed to be repaired. Unrolling and then re-rolling the rolled over chrome case needs a lot of movement in the metal and fatige failure stalks the hasty.
Before considering a repair I would check it is getting 12v on the supply tag (voltmeter on the rev counter's tag not the much easier to do disconnected wire...), check the case really is connected to ground (resistance meter connectd to rev counter case and to a really good metal chassis point) and that the ignition wire is wound one turn around the little connecting loop (shown in the photo in the link provided earlier by another member.)
Finally check the ignition wire is actually carrying the ignition coil primary current pulse and has not been bypassed (You know: a desperate engineer trying to solve an igniton problem made sure.....).
If all that is good your rev counter needs fixing. Oh, and buying a used one you'd be lucky to get one that hasn't got the 'over optimistic' problem, which will need a dismantle to fix anyway.
Good luck!
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