Hello everyone, thank you for taking interest in this issue.
This is an older E3631A, from '96 with 1.5 firmware version.
After power on the fan tuns on, there is a beep and the VFD lights up then it goes blank. When pressing the recall button, the unreg indicator is shown.
All voltages on the power rails appear good.
From the discussion in various topics on this forum, it appears that this indicates that the communication between the different CPUs doesn't work properly. So the signals from the main CPU was checked. On J4 to the front panel the supply voltages are present but all the signal lines are static. On J3 to the bottom panel some pulses can be seen from the IO CPU (which is passed on from the ASIC to the main CPU) but there is not response to that from the main CPU.
The oscillator of the main CPU is running, there's activity on the address/data lines.
On the reset pin (from the 5V regulator to the main CPU and the ASIC) however, there is periodic reset signal after ~200us. The reset pin of the regulator was lifted and it is stable, so the reset signal must come from the CPU or the ASIC.
It looks like something very basic is wrong here, that prevents the main CPU from properly starting.
Did anyone see this kind of failure? Any tip is welcome on what to check, how to proceed.
The contents of the EPROM was read, it looks valid but I cannot be sure that it is not corrupt. If someone had this firmware and could post it, it would be highly appreciated.
Thanks!