Prepare to be amazed :)
Here is little tutorial how I repaired another one A600 motherboard. I buy this one as not working from ebay from man we all know It was 20 minutes job and more time was needed to publish this online All is done using just voltmeter and ohmmeter. Most important thing everything is filmed in process of investigation. Nothing is prepared and discovered earlier !!!
Here it goes:
1. What I did first is to visually check for cold solder joints and realized that some time ago in this A600 was some kind of accelerator. Since it is not standard option to add accelerator to A600 attaching something on CPU may damage CPU pins. So I solder back pins but it seems that was not the real problem. Again black screen
2. Then I checked voltages on CPU everything was fine.
3. After that I checked RESET and HALT pins of the CPU. Hm something held CPU in reset state. Where is the problem?
4. My first thinking was that maybe KB_RESET found on PIN 63 on the GAYLE is maybe low and that is the reason why GAYLE is blocking main CPU. And yes that signal is LOW so here we have reason why motherboard is not working. But then again why that line is low ?
5. So few minutes later and looking at the A600 schematics there it was possible cause for all the problems, little tiny, tiny thing called resistor. On the back side of the motherboard there it was pull up resistor R511D.