I'm at a point now where I'm running out of ideas.
I do have access to an oscilloscope in a local hacker space, but I haven't used any oscilloscope yet, so I would either require someone to help me or learn how to use it on my own. But that might be an option if everything else fails.
You absolutely need to use an oscilloscope to diagnose this sort of thing. You'll probably want to get someone who knows what they're doing to help you out with this.
Fortunately the U4/Denise operation is fairly simple and easy to check. For starters:
1. Verify you've got a valid AC waveform on all 4 clock/timing inputs on pins 32, 45, 35, 36 - expect 15kHz, 7MHz, 3.57MHz
2. Check there's constant bus activity on all 8 address inputs
3. Check there's constant bus activity on all 16 data I/O lines
4. For each colour group (4 bits each of red, green, blue), expect to see 5V waveform activity on all of those outputs when the machine should be displaying the 'insert disk' screen, see the schematic for details
5. Check there's ~15kHz on the horizontal sync out (RGB connector pin 11)
6. Check there's 50/60Hz on the vertical sync output (RGB connector pin 12)
7. Check the 5V video supply is there at U40/U41 pin 20
8. Check for about 4.5V on pin 21 of the video hybrid, HY1
Another easy way to do it when you're not familiar with the circuit operation is compare various signals against a working A500.
You might like to check for video signal on the composite output as well as the RGB output.
And don't rule out intermittent IC sockets causing problems. Visually check each contact with the IC removed. If the IC is overly easy to extract from the socket, then replace it.
