Sometimes the MPU board is damaged by battery corrosion so hardly that even
are all parts in the clock section are replaced, the MPU won't start
In these cases you could replace the quartz crystal with a TTL Oscillator.
clock section of the system 80 MPU:
Clock is produced via Z3, some resistors and a crystal (3,57... MHz), after
clock is divided by four ( with Z2). After this you should have a clock of approximate 900 KHz
Just replace Z3 with a TTL Oscillator ( data sheet: TTL Oscillator).
Unfortunately, there is no 900 KHz oscillator available, but you could use the 1MHz variant,
and tune your pin up ;-)
Picture of the original MPU, clock section:
Desolder all the parts not needed (Y1, Z2, R3, R4 und R5) and put the oscillator
at the postion of Z2. Because we need the clock at pin 9 of Z2, make a connection from
pin 8 to pin 9 of Z3, and you are done!
Picture after MPU modification:
I used this succesfully on two of my MPUs, but:
NO WARRANTY! If you damage your board, you are on your own!