repairing old computers

Circuit diagram/service manual and a logic probe and pulser. Scope is borderline useful.

Many repairs can be accomplished with nothing but a multimeter and a good idea of what to look for. As someone else said, a schematic and logic probe can be very useful as well.

Logic probe with audible indicator very useful with schematic and service manual. Oscilloscope useful for one very important test, to verify that the digital signal on any given line is a clean 5 to zero volts with no roundness or excessive ringing on the line.