does the MainBoard of a Mac Classic work on a Mac SE? (the connector is the same)I tried to replace the (working) M/B of a Mac SE, with an M/B coming from a Classic Mac (which I don't know if working).
The monitor displays this image: is the Mac Classic's MainBoard broken?
Sadly, the SE and Classic are fundamentally different in just about every respect (except the CRT), so the board cannot be swapped.
What you see is a pretty common symptom of failing/leaking capacitors or damage on the logic board. Your best bet here is to remove the logic board and inspect it for battery damage. If there's no damage, one thing you can try is soaking the board in vinegar, then washing it in soapy water, rinsing it with distilled or deionized water, then letting it dry for several days (I'd recommend using a hair dryer or air compressor to force water out of all the connectors and IC pins). Try the board again after it is COMPLETELY dry.
If the board is still not working properly, replace the capacitors. Note that there may be trace damage caused by electrolytic fluid leakage, so depending on your skillset or desire to learn, you may require the assistance of someone experienced in electronics repair.