The answer is blowing in the fuse.

A very frustrating and worrying night last night.

I re-stacked all the G boards and fixed the broken -26V wire. But as soon as I powered up the fuse blew again.

So, I started removing wires and measuring impedance and trying things but after another couple of fuses had gone I gave up for the night, dejected.

It seemed at one point that I had identified which rail on which G board was the problem. But, when I left it disconnected and reconnected some of the other boards I was back to square one with another blown fuse.

From that it appears that no single board is the problem alone. When a few are connected simultaneously the overall impedance drops and the current rises, killing the fuse.

The voltage is lower than -26 but it's always been like that. Back in this post I measured -23.6V

And if the impedance is above around 2K ohm all is well. When I keep adding rails though, it drops to around 1K ohm and the fuse is blown.

Luckily I work in an electronics company and after a quick chat with some hardware engineers (I'm software) a bench power supply was tossed my way and I have another way to investigate.

With a bench supply I can just use this to power the boards investigate and save blowing more fuses. I can set the maximum current to 500mA, or less, and then add rails till it clicks off. I can also measure the current being used for each board and try and work out which is taking the most to help trace the fault. My plan is to test each board in isolation first, checking the current as I go. I should find one that is taking more current than the others - although they will not all be equal, so I have to use some judgement. I was also advised to check for heat. Anything getting warmer than expected is probably the source of the trouble. I can also test all the ICs on each board and hope and pray that none are blown [shudder].

This is all very annoying. I seem to have precipitated this fault when fiddling around with the noise problem which was a silly mistake and should have been diagnosed without all the messing about I did. It's also worrisome because if I've blown one of the custom Yamaha chips the replacement might be costly and difficult, or even impossible.