Alternator fail and consequence for tach

[DaytonaCoupe66]

Hi; 1982 ROW 930 (Canadian) 3.3 stock. Was driving at highway speed yesterday on a rare very hot day and noted the tachometer peg at the red line, bounce a few times, then die and rest at 0. A few seconds later there was the sound of an electrical snap somewhere in the car and the radio died. The radio would not restart but all other electrical functions were fine. I continued 15 minutes to the destination, and the windows were slow to close indicating a weak battery. The engine idle while stopped at traffic lights sounded 2-300 rpms higher than usual. All fuses and relays were checked and confirmed to look good, including the radio. After sitting for 30 minutes the battery would not drive the starter, but the radio was working again.

The car was flatdecked home. When I turn the key the oil idiot light illuminates, but not the charge light. I cannot recall the last time I consciously looked at this bulb on startup. The bulb was checked and is fine. I will remove the alternator for testing. It would have been replaced 4 years ago before I bought the car - seems like a short life.

My question pertains to the tach - have I fried it? Is there a diode or capacitor or fuse in the tach? I'm not keen on restarting the engine until the alternator/regulator have been tested. Probably wishful thinking that the tach will be fine when the alternator/regulator are back to spec (assuming this is the problem)?

Thanks......D