Hi there!
Some months ago I got a broken GX4000 (untested, you know). It turned out that diode D183 was shorted and input voltage reached the 5V rail, damaging at least one of the RAM chips.
I've seen that the ASIC is generating some of the expected outputs: Z80 CLOCK, Z80 WAIT are there and look OK. The problem is that the reset signal seems to be kept low (active) and I think this is (at least) what is preventing the console to start.
I know that it's quite probable that the ASIC was partially broken because of the over voltage, but I would like to be sure before giving up though.
Is there detailed information about pin functions, startup sequences, … anything other than the schematics that can be found in the Service Manual?
I'm particularly interested into the meaning of the different RESET-related pins (RESET, SRST, BRST,...) whether they are inputs or outputs and how they are expected to behave during console startup.
Thanks a lot!