I could you some of your vast experience here
Back in Oct 2018 I bought a capacitor kit for my Falcon. It was lying and moving around with me for the past 3.5 years until I finally decided to put it in action (I've also wanted to finish one huge point in my TODO list, CosmosEx internal installation&guide which requires me to take out the PCB).
So fine, I went ahead and recapped the beast. I had to wait for a couple of weeks for the last remaining caps to arrive because the package turned out to be incomplete. So I could verify the result only now, when soldered the last pieces. I powered the Falcon up and saw the Atari logo, yay. Switched it off and next time, it wouldn't even initialise video (i.e. produce any kind of sync signal, not even the infamous "out of range"). Better said, it would like in 1 out of 20 retries (pressing reset didn't have any effect on it). Tried with diag cart, same. Not in a happy mood I rechecked the board the next day and to my relief, I found out that one cap, C181, was soldered the wrong way (yes, what a shame!) The cap was all right (it was actually rated for 100V, so no worries here), verified with multimeter, still 1uF. Swapped it, soldered back, powered the machine up and ... success! It would boot.
Switched on, off, tried the diag cart... everything worked. Then I connected my IDE DOM and ... it wouldn't start anymore (same symptoms -- not even video sync). Pretty pissed I inserted the diag cart again and ... it works. And it's like that to this moment. I insert the cart - everything (video, audio, keyboard, all internal tests, RAM, IDE, ...) works. I remove it -- no boot at all.
So far I have tried:
- verified all values and orientations of the caps
- cleaned the PCB on both sides as much as I could to remove any random metallic parts
- pushed all ICs, cleaned all sockets
- resoldered all the caps (I even replaced the opposite one with a brand new, just to be sure)
- stripped it completely naked except (even without FPU) except RAM and NVRAM
Looking into TOS 4.04 boot in Hatari, there is not that much happening before the video init kicks in -- except the ROM it accesses the memory controller and PSG. ROM obviously works (otherwise the cart wouldn't boot) so I don't understand why diag cart doesn't show any errors for PSG/memory then?
I have one thing to verify -- that "short strokes of luck" maybe weren't totally random. My feeling was that it always happened after a longer period of time, i.e. like when a certain cap would discharge. (this would be also supported by my finding of the opposite cap -- the Falcon was lying on the desk overnight, i.e. I would switch it on after a longer pause). This could maybe (?) imply that over that 3.5 years some of the caps went bad but then again, why that bloody cart makes it all work like as if nothing is wrong?