so i have a broken chumby classic (ironforge 1.8: http://files.blueocty.com/hdwedocs/IRONFORGE_MX21_V1_8_FINAL.pdf ). in some sort of fashion, there was a short in a power rail somewhere (i suspect DC in and ground, but i'm not sure if so, or if that was indeed the root cause) which blew a diode and some buck regulators. i have now replaced all those parts and checked that all voltages (5v, 3v3, 1v8) are correct and the unit does power on however never functions beyond a white screen
spitballing appreciated 
@mavica_again our first thought involves JTAG'ing the CPU to see if it's fetching instructions
@atax1a we do have a j-link but very little idea how to do what with it
@mavica_again the way it works on our cortex m0 with SWD is that we use openocd which detects the chip and serves a GDB remote from which you could, e.g., stepi a few times to watch it execute
@mavica_again to elaborate a little more: if the ROM is busted, we'd expect instruction fetch to get some kind of garbage (all-0, all-1, or floating-bus random values); if the CPU is busted we'd expect the SWD connection to fail entirely, and if the RAM is busted then we'd expect load/store instructions to either fault or go bozotic
@mavica_again
stepidisassembles the next instruction when you issue it, and if it looks like it's fetching code then you've got a good CPU and a good ROM. you could also poke bytes into memory and try and read them back, if you know where that's mapped