
I have got a Falcon here, not unlike this one (rare my ass...):
I got it in a "rattling PSU" state. So I disassembled everything I could from that tower, used a PSU which I know it works and I got ... nothing. The only sign of life was the poor internal fan. Not even the infamous "out of sync", "going to sleep", not a single life sign on screen.
Since the Falcon had the Mighty Sonic 32 installed, I carefully removed it (thanks @Badwolf for the hint in viewtopic.php?p=80248#p80248 -- much easier than messing with the CPU legs!), just to avoid another possible source of failure (and the PCB CPU is much easier to diagnose...).
(Un)fortunately, that didn't change anything. I measured all the usual CPU stuff from the Service Guide (RESET, HALT, BERR etc), couldn't see anything suspicious. Combel's input and output clocks are OK, Videl's input +5V and input clocks are OK, too.
After power on, ROM CE (measured on TOS ROM's pin 3) goes from an initial activity to 0V and stay there unless I press the reset button (interestingly, I see a lot of activity while I'm holding the button but as soon as I release it, it quickly ends up in 0V). That is perhaps OK since the memory card it out.
So that perhaps mean that ROM is read but in my case, I get nothing on /ROM3 and /ROM4 (always high) and the guide is quick on recommending of replacing the Combo IC but this seems like a premature conclusion because what if the Videl is dead and is blocking any meaningful operation?
The Service Guide's sections for both halted and non-halted CPU do not give a clear answer what happens if the Videl is dead, is the unit expected to operate to at least execution of code from the cartridge?
I'm mentioning Videl so often because this one looks really dead. I checked its +5V, RESET and input clocks but I get nothing from DOTCK, HINT, VINT, DE ... The only suspicious thing is that VCS (chip select) is raised only upon power on. Pushing the reset button doesn't ever lead to active high (checked against a working Falcon, there it does go to active high). This pin is connected directly to the Combo IC so that could imply something wrong with it but: The COMBO IC is responsible for handling the interrupts generated by the VIDEL IC, and the loading of DRAM data into the video chip’s input buffer. COMBO receives the interrupt signals VINT for VSYNC and HINT for HSYNC. A request for video data to be loaded into the video RAM buffer within the VIDEL IC is generated to the COMBO IC by the VREQ signal, and DRAM data is loaded into the video chip’s input buffer via the VLD signal from the COMBO IC.
So if it doesn't receive anything from the Videl, how should Combo IC behave then?
Do you have any idea what to check next to either confirm or reject the idea of a dead Videl?