This mac had been left overnight multiple times in a row on an old window sill in quite an old house, and in cold weather. This had allowed condensation to build up in the mac, until it killed it. Cleaned up, corrosuion removed, a few caps and resistors replaced solved the most of it, but it still wasn’t powering up as it ought.
Further inspaction took us to the final picture with the large square IC, the big square QFN IC is responsible for power to the Mac. It takes quite the trained eye to see what was not right. Top ‘before’ picture, top right hand corner, can see, barely, but can see the smallest bit of blueish greenish corrosion under the chip. Chip taken off, cleaned up, soldered back on and this Mac is back to being 100% is it ought be, all at a fraction of the cost of Apple.