The photo seems to show a black mark on top of LF1, if that is a burn mark then maybe LF1 has a short. Also, is the hightlighted resistor burnt or just black in colour?
If all good then my next step would be to desolder the switching transistor (and remove it from the heat sink), then switch on the power and see if the fuse pops or not. If it does, then it is likely you have a fault somewhere from the AC input to the 170V rail.
If not, then most likely the switching transistor (yellow circled in photo above) and/or associated circuitry is shorted out.
Hi Will. You mentioned the previous owner had connected it to 240v instead of 120v, and that had blown the fuse. I think it likely that some other component has been fried before the fuse went. I checked the failure mode of X2 and my money is now on that:
When a Class-X capacitor, also referred to as an "across the line capacitor"—the capacitor placed between line and neutral—fails because of an overvoltage event, it is likely to fail short. This failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open.
I’d test for a short across X2’s pins next...it might be new looking but you didn’t replace it, right?
Yeah I'm pretty sure at least 3/4 of the vintage computer people (myself included) are more on the hardware side than software. I mean if you only want to use old Mac software you'll just download mini vMac or Basilisk or Sheepshaver or whatever emulator, not futz around with ancient finicky mounds of increasingly brittle plastic and metal that take up loads of space and require regular maintenance to continue operating and that cost tons of money to fix when they inevitably break (or if you want something like an original XCeed card, for which most people would have to sell one or more children for medical experiments to be able to afford). So here we sit, heads filled with grand ideas and fully realized hardware mods and nobody to write anything that uses it. Tragic. Good programmers are hard to find.
People love these old things but so far they've basically been limited to the original mid-80s experience (read: slow and monochrome) or a mod (several people have gutted classic Macs to use the cases to house iPads or minis with small LCDs). Once this project is released an unintended side-effect may be to drive up the cost of SE/30s since now you'd be able to do modern things at a reasonable speed, all in the comfort of your authentic vintage Mac. I'm just glad that I bought six of them before your case project was released to the world.
It doesn't matter if they're running on an '030 or '040: basically the legacy IIfx IO chips are intelligent to the point where they can do a number of things without the CPU micromanaging them, which allows the CPU to keep working on other tasks while serial transfers or disk activities were happening in the background. The most basic chips didn't require reworking for the new '040 bus but the memory, video, and NuBus controllers were revamped to work with the new bus protocols. In the case of the Q700, it received none of the IIfx/Q900 IO chips, but the RBV of the IIci was scrapped and it received basically the same video chip as the Q900 with its dedicated VRAM SIMMs. The Q650 shared many of the Q700's chips, though arguably the 650 was a better machine in that it was faster, with an extra NuBus slot and a built-in CDROM, but the downside was that you got the ugly case.
If you compare a Q900 and Q700 for basic software benchmarks (CPU, FPU, RAM, video), there isn't going to be a huge difference. However, if you're also running IOPs in the background and/or are benchmarking IOPs, the Q900 will clearly outperform the Q700.
In a nutshell: the IIfx IO chips help you get data into and out of your Mac faster but that's about it; if you're just doodling on a local file in Illustrator they're not really doing anything for you until you try to move that file somewhere else.