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If you have a 6V supply (either a bench PSU, or even a 6V battery), connect it directly to the logic PCB with the machine apart.
The -ve side goes to the -ve terminal of the largest capacitor on the board, the +ve side to the commoned end of the 2 piggybacked diodes in the top left corner of the PCB (as you look at it).
If the machine will power up then, the fault must be in the flexible PCB in the I/O assembly, or cracked posts (etc) which are preventing it from making good contact with the logic PCB.
If the machine doesn't power up when the logic PCB is connected to the PSU, then there's a fault on that board. Could be just about anything -- 'on' switch contacts, CPU, diplsya hybrid, passive components.
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The first thing to try is a "master reset". Short the leads together for a second or so of the 470uf (the largest) cap on the left side of the board and retry. Many times the processor will be in la-la land and will not respond otherwise.
If that doesn't do it, Tony's suggestion is spot on. The current consumption will help with a fault finding direction. If it powers up with a direct connection to the diode and capacitor, it is most likely a broken trace on the battery connector. This usually happens only with a battery leak.
Normal operation will be between 30 and 50 ua, standby will consume about 1 ua.
My money's on a master reset. Half-nuts are pretty darn hard to kill.
Edited: 12 Sept 2003, 7:49 p.m.
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Tracy, you have a Belgian name. Are you from Belgium ?