No Screen (Amiga 600)
Presuming you have checked you have good power rails entering the system, and there are no shorts (5V, 12V and -12V are all still present when turned on and connected to console), then there are a few common causes of no screen.
If you get a black screen, white screen or any other output, this is not No Screen.
No screen refers to absolutely no output over the monitor, as if the console was never even connected.
A common fault with no screen output can be the video encoder (U12, CXA1145P) dying, or having rotted traces near it, or the delay lines dying.
Use an oscilloscope and probe the RGB in pins of the Video Encoder IC (U12) for activity.
If you see 1V signals sitting on top of a DC offset (usually about 2V) then the Denise (U4) is running fine and outputting RGB data to the video encoder.
If you do not see RGB signals then check further back in the system like power, BIOS and main processor chips are all good.
If you see RGB signals coming in ok, check you have a valid XO In/Out (crystal) which should pulse at around 4.4MHz and 1.3MHz.
Next check the RGB Out pins have output similar to the input signals.
Finally, check the Chroma Out and Luma Out pins are outputting chroma and luma signals.
If all the above is ok, your video encoder chip is likely ok. If you have RGB In and XO In/Out but no RGB out signals or Luma / Chroma Out signals, your encoder chip could be bad.
If you have valid Chroma and Luma Out signals, but Chroma and Luma In pins are not receiving the same signals, usually the Delay Lines (or traces to them) are bad.
You can short them out by bridging the Luma Out to Luma In and Chroma Out to Chroma In with a wire. This will cause the Luma signal to be slightly early so colors will be pushed a little to the right, but its a way to get video output without replacing the Delay Lines.