I'm afraid I don't know how it works but I guess the frequency and duration (2 seconds) of the tone is the important factor. Unfortunately recording of the tone and detection of it are just another couple of things that seem to fail on these machines!
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Here's a list of other common problems, just off the top of my head:-
Faulty LDR's
Broken PCB tracks around the push buttons
Dirty relays
Faulty mercury wetted relays
Dried out electrolytic capacitors
Oh and for anybody who are wanting to buy one please don't have it sent through the post. The cabinet is a very crumbly, thin, chipboard and the mechanicals weigh a ton. I would have had all three cabinet styles if I hadn't have had a teak version sent through the post
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Regards
David