Ah, frustrating. No need for apologies.
My first thought is that it looks like there's a short or an intermittent connection, but impossible to say where - could be the interconnect cable, the display cable, the power jack, or even the HDD cable. First, as always, double check your work (you're welcome to take a pic of what you've done and share it here, unless you're confident in it), and you say all connections are sound, so next I would test to see if the machine will boot with the top and bottom cases separated, like this:
This is how I assemble and disassemble PB's, partly because I find it faster but also because I have found that sometimes unexpected things happen when the machine's all buttoned up and it's easier to troubleshoot this way. So now when reassembling I always do a series of three test boots:
- first separated like above,
- then together but unscrewed (if it POSTs then I will press machine together in different spots to see if anything happens - sometimes the location is important information),
- then if all good I'll screw it together and do a final boot. Sometimes it's just the wrong amount of pressure in the wrong place and this sort of thing can happen.
If the machine boots with the case separated, then that narrows down the possible causes quite a bit.
Otherwise, you will need to return to troubleshooting basics and work up from a baseline.
But before that, some other initial thoughts:
- is the HDD installed? Try booting with the HDD cable disconnected from the logic board
- try booting with the floppy drive cable disconnected from the logic board
- as mentioned before, troubleshoot without the RAM expansion
- did you verify the display cable is fully intact (no splits/cracks, common near the hinge) and securely seated at both ends? (the display end is kinda tricky imo)
- is the power jack secure? Does ground and power have good continuity with the rest of the board?
- triple check your connections, that they're secure and the right way around, any pins are straight, etc: there aren't many, and they're hard to mess up
Someone else PLEASE chime in as well!