Ah bugger sorry got my port layout confused, didn't want to drag my 840AV out of the cupboard (even if it is 1metre away from where I type this).
My 840AV exhibited similar problems when I got it (a few years ago, in bad condition) - it wouldn't start up every time, or not chime/black screen etc. Leave it be for a few hours and it'd boot up fine again, could use it for hours, but if I had to restart it'd play dead.
I was elbows deep in fixing two SE/30 boards at the time, so ended up replacing the caps on the 840AV as well with Tantalum replacements. I found replacing the caps on the 840AV a bit more fraught - they seemed more fragile to remove - but was able to do the job OK and the beast works fine now. I'd also check/clean your RAM contacts, etc.
JB
My 840AV exhibited similar problems when I got it (a few years ago, in bad condition) - it wouldn't start up every time, or not chime/black screen etc. Leave it be for a few hours and it'd boot up fine again, could use it for hours, but if I had to restart it'd play dead.
I was elbows deep in fixing two SE/30 boards at the time, so ended up replacing the caps on the 840AV as well with Tantalum replacements. I found replacing the caps on the 840AV a bit more fraught - they seemed more fragile to remove - but was able to do the job OK and the beast works fine now. I'd also check/clean your RAM contacts, etc.
JB