Originally Posted by
Pere Casals
Let me enumerate some possible issues, from general electronics:
as it looks that it "reboots"... or it has intermitent power...
>> One thing I'd check is power suply voltage in the controller board. Check voltage supply in the micro pins, from micro part number you find the datasheet, check voltage in the power pins. It would be good to know what a what the oscilloscope show in the power pins, and in the clock pins.
> Also when a microproessor is "powered on" the voltage is unstable at the beginning, modern devices detects that "brown current" condition and delays the boot until voltage is stable, old devices use a battery (that also may preserve parameters) to run the microprocessor during a few milliseconds until voltage is stable. If that battery is low then the micro fails to start and the watchdog reboots the micro in cycles: Check if a (usually lithium) battery is there, from part number know the original voltage and measure the actual voltage.
> If there are parts (chips) in sockets, unplug/plug them, sometimes a bad contact is there. Do that with all connectors inside.
> Capacitors. In those old boards electrolitic capacitors are a weak link, some start leaking electrolite that would short circuit what in contact. If you see leaks on the board or inflated capacitors this may be repaired by cleaning the board and replacing the capacitor.
> A too high load. If when booting the micro powers some device/function that is is short circuit this may lower the power supply for the micro making it boot again.
If you can locate one of those old/good technicians that were repairing old TVs or industrial electroics perhaps there is a chance you can get it fixed.
In general old digital boards fail from connectivity or from capacitors, the microprocessors are usually fail safe. Other things that may fail are RAM chips and, specially EEPROM chips that store the "program", in that case it would be more difficult to repair, if a EEPROM fails you need an original programmed part.
... so I'd check connectivity, battery, power supply and capacitors, other issues may be more difficult to repair.
I had to try to repair a number of old embedded controllers of similar kind, some were easy to fix, some I wasn't able. Not having the schematics and service manuals is a problem, so you check the easy/common issues that may solve 3/4 of the cases. Old electronics was easier to repair, and thanks to ebay you may even get discontinued parts. Today the higher integration makes it more difficult, you throw away the board or the entire device.
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