Could the fact that as soon as I turn on the ignition, I have 12V on the TACH terminal and all the way to the MS at pin 24. Is that supposed to happen? Could that have caused the failure of the input circut?
No, this is normal. you have 12v because the coil is litterally just a long piece of wire, so when you turn on the ignition you get 12 to the positive side and it runs clear through to the tach or negative side.
nothing happens until the HEI module sees a signal from the VR sensor, then it momentaraly ( like 4-6 millisecinds ) grounds the tach side, creating the magnetic feild that collapses when it un-grounds it. it is the ungrounding that creates the spark.
so... if you are seeing 12v on pin 24 the wiring to the MS is good. If you are getting spark chanses are verry likelly that you are getting a valid signal to pin 24 at the least.
As I said, A test light will verify this to pin 24, and on the board a logic tester ( LED with a resistor )
should be able to tell you if the opto circuit is working.
when firing direct from the coil, R10 can get hot, and both diodes should be active, if you had them jumpered you may have dammaged the opto, also it is a good idea when firing from the coil to remove XG1 and run a separate ground direct to the block.
The tach signal from the coil is extreamly noisy, that is why XG1 was put there.
Joe