If your Bally early solid state MPU LED doesn't light, or just stays on, you
could have a serious problem, but most likely you do not.
When this happens, the MPU is not booting properly. It's not even getting
to the point of self testing. Chances are that you've got connectors that
are not right, or that one of the socketed chips is not seated properly.
With the machine off, reseat each connector on the MPU and the power
supply (on early machines the power supply is on the right side of the head,
and in the center of the main cabinet on later models). Then, ground
yourself (just hold the wire grounding braid) and press on each socketed
chip on the MPU to make sure each is well seated. Be careful not to flex
the circuit board too much or you may break or split traces.
Turn the power back on and see if the problem has gone away. If not, you
may want to wiggle some of the connectors on the MPU and power supply to see
if that makes any difference while the power is on. If not, then it's time
to check the test point voltages on the MPU and the power supply.
All voltages are with all boards present and connected with the machine on.
Voltages within 10% are probably okay.
If all these check out okay, then you will need a schematic. You will need
to check traces, continuity, and components. This gets very ugly and you
may just wish to send the board out to get repaired. Hopefully, you won't
need this, and it is not the most likely scenario. Most problems like this,
by far, are just connection problems.