I'm sure someone with true expertise like pimpmaul will chime in eventually. In my personal direct experience, it could be:
- a bad STIC - debugging procedure is to pull a STIC from a known working unit and swap it into the bad one and see if it works
- cracked connections on the boards
- bad connection within the RF box mounted on the motherboard
- bad RF box
- some other bad component / connection on the motherboard (this is where someone like pimpmaul really stands out!)
The sucky part of all of this of course is cracking open the shielding around the main board.
The STIC is typically in a DIP socket, and with a modicum of caution, you can remove it and put it into another unit without damaging anything. It may be that the STIC is directly soldered to the mainboard in some units (Intellivsion II maybe?), which makes that debugging a much greater PITA.
But once you've cracked open the unit, I think you can open the RF box to see its guts w/o too much trouble. Plug in your cable that connects to the TV and observe how the center connector of the cable fits. Ensure that it makes contact with the signal connector within the box. Use an Ohmmeter to test continuity from the signal in the RF box to the male end on the other end of the RF cable. If that checks out, then the signal itself is bad, which means it's one of the other problems. If continuity test fails, it could just be fatigue on the connector in the RF box, and bending it back to make solid contact could solve the problem -- but it could also just delay the inevitable, or simply break.
The RF modulator swap itself is not too tough, and IIRC they're pretty cheap. I think intvnut even has a supply on hand for a few dollars each. I've done this on a couple units.
Your post also reminds me: I think I have 4 or 5 'BSOD' (Black Screen of Death) units to debug, myself.
Edited by intvsteve, Mon Jan 6, 2014 2:51 PM.