As one who has often resuscitated Macs, most of them unknown to me before our first meeting, may I suggest a process similar to this:
1) Put the 'new' Mac on a suitable work surface, and ground it only, with a power cable. Open the case.
2) Remove/blow out the accumulated detritus/dust/hedgehogs/stray camels/whatever, and verify the presence or absence of PS, MLB, CPU, RAM/VRAM, drives, cache, expansion cards and whatever you fancy, and reseat/reconnect them all.
3) Replace the PRAM battery with at least a known good one (>3.3V in-circuit), and preferably with a new one. <2.8V in-circuit is potentially more inhibitory than no battery. Reset CUDA, if there is one. You know, a priori, that many Macs will not boot without an adequate battery, so why waste time discovering that the new one is one such?
This saves a lot of delay in getting the Mac going.
de
1) Put the 'new' Mac on a suitable work surface, and ground it only, with a power cable. Open the case.
2) Remove/blow out the accumulated detritus/dust/hedgehogs/stray camels/whatever, and verify the presence or absence of PS, MLB, CPU, RAM/VRAM, drives, cache, expansion cards and whatever you fancy, and reseat/reconnect them all.
3) Replace the PRAM battery with at least a known good one (>3.3V in-circuit), and preferably with a new one. <2.8V in-circuit is potentially more inhibitory than no battery. Reset CUDA, if there is one. You know, a priori, that many Macs will not boot without an adequate battery, so why waste time discovering that the new one is one such?
This saves a lot of delay in getting the Mac going.
de