Black / No Screen (Intellivision)
If you have checked all your power rails are good, while under load (console turned on) including the red flying lead all the way to the middle IC power, and you are getting no screen or black screen, there are some checks to do next.
With the age of the console it is good to re-do all the wire board-to-board connections. From the power board to the motherboard, from the composite video board to the motherboard and the red flying lead between the motherboard.
Any one of these loose or broken can cause issues.
As the RF module often fails over time and is much harder to consistently make sure if there is a signal or not, it is worth doing at least the basic composite video mod by soldering a composite video lead to the Composite pin of the composite video board.
The console will display nothing on video out if there is not a game inserted.
Make sure you clean your game pins with a pencil eraser, then IPA.
Clean your cartridge slot on the console with fine sand paper then IPA.
An absolute must for all Intellivision is to replace the IC sockets that most of the IC chips are connected via. They are very low quality and even with heavy cleaning with IPA or similar, the sockets are often not good enough for a successful boot.
Replace all sockets with brand new ones.
If all the other tests are done, or if you have the means, it is always good to swap the chips into a working console to test if they are good.