From my experience with the IIx machines I own (2 complete units and a spare motherboard) is that the capacitors do tend to leak and the ROM SIMM sometimes doesn't have a good contact in its holder because of oxidation (you can apply slight pressure to the side and the machine bongs). In my case I just recapped all 3 boards and cleaned the edge connector of the ROM SIMM and all work great (with new batteries).
Kind of reminds me of a LC III I had that booted up fine and worked when I got it (for about 10 minutes when there was a noise coming from the speaker) and the machine never booted again. This was from capacitors finally going bad when I turned it on, recapping that board made it work like new. You can tell when the speaker bong volume is very faint the caps are about dead, same in compacts like the SE/30.
Kind of reminds me of a LC III I had that booted up fine and worked when I got it (for about 10 minutes when there was a noise coming from the speaker) and the machine never booted again. This was from capacitors finally going bad when I turned it on, recapping that board made it work like new. You can tell when the speaker bong volume is very faint the caps are about dead, same in compacts like the SE/30.