I had a similar experience recently on my first classic mac (an SE/30). I recapped, it worked for a while, then a black screen. Turns out chip UG8 went bad - I think the leaking capacitor electrolyte got to it.
I was able to figure it out by purchasing a cheap oscilloscope from Aliexpress and an ATX power supply extension cable, so I could run the machine with the logic board outside the case. I had to learn about reading schematics and using an oscilloscope, and surface mount desoldering/soldering - but in the end I was able to narrow down the one bad chip and replace it, and I have a working machine now!
Oscilloscope I bought - Zoyi ZT-703S with two probes:
https://www.aliexpress.us/item/3256...st_main.5.2ab91802zciZ35&gatewayAdapt=glo2usa
There are other inexpensive, handheld oscilloscopes from Hantek and OWON. I picked this one because many retrocomputing youtubers say its decent for a first oscilloscope, the company seems to offer firmware upgrades for it, and its easy to take screenshots to post on forums (you use usb cable to transfer them to PC). It also has one of the biggest if not the biggest LCD of various handheld scopes.
ATX board extension cable I bought:
https://www.amazon.com/gp/product/B09N9BZRD9/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
This cable lets you extend the connector at J12 to the analog board, so you can position the logic board outside the case.
Here are threads on my recent adventure:
I have an SE/30 that I recapped a couple weeks ago. Its worked fine and i've probably put 20-30 hours on it since recapping playing tetris and exploring HD images via my BlueSCSI v2. Its never once crashed or behaved strangely. Earlier today I was looking at some old documents in ClarisWorks...
68kmla.org
I am going to attempt replacing UG8 on my SE/30. What is the best way to remove SOIC chips like these? I've done some research and Chip Quick (ChipQuick?) SMD removal solder is a possible way, though the "mess" afterwards is a little concerning - looks like you need to use this stuff...
68kmla.org
To my untrained eye that has only ever worked on one SE/30 and with minimal retro repair experience, it looks like you have VSYNC and HSYNC (vertical and horizontal sync) but you are missing the "Video Out" signal. I would be curious if an oscilloscope shows working VSYNC (pin 14 on UG6) and working HSYNC (pin 13 on UG7) but
MISSING VIDOUT signal (pin 13 on UG6). If you are missing VIDOUT then I would suspect chip UF8 the binary counter surface mount chip is bad, which seems like a common chip to fail due to its proximity next to leaking capacitor C7.
Depending on the revision SE/30 motherboard you have, you may have to modify the ATX extension cable a bit as shown in this video:
Video shows a 24-pin atx extension which is really wide, I purchased a 20-pin atx extension since its less wide - it fits without modification on my later revision SE/30 where the CPU is not socketed but instead soldered to the board.