A4000 sudden death

[Castellen]

been looking for some time for a replacement Bridgette.

I have spares of these if you need any, though they're generally untested.  I can also fit the part to the board for you if that helps.

http://amiga.serveftp.net


@McTedson
If any of the various reset lines are being held low, I'd certainly suggest focusing more in that area.  If _KBRST is permanently low, there could be a problem with U150.  If _IORST is being held low, it could be one of the CIAs.  You could quickly swap U300, U350 positions to check them, they're the same part. The individual reset lines are formed from U140 and U141, though it's very unlikely for either of these two gates to fail.  But it's a good starting point in the circuit for tracing reset line problems.

And in fear of stating the obvious, check that _FAIL (U150 pin 56 and power connector pin 1) is at 5V.  From memory, U150 causes the system to be held in reset state if the power supply holds _FAIL low.  Worth checking if you haven't already done so.

[Castellen]

The reset logic can be complex in that any number of system devices can cause a reset.  The outputs of U141 are open collector, so anything else sharing the _EHLT _CPURST or _RESET lines is capable of causing a system reset, including the CPU board.

I'd expect U140 to have a totem pole output, so therefore the lines _FPURST _IORST and _ERST are only driven from U140 outputs.  That's why these lines don't have pull up resistors, while the lines on the U141 outputs do.

Not sure exactly what drives the U150 internal _KBRST logic, no way of easily knowing what goes on inside the gate array.  I'd imagine _FAIL has some influence on the _KBRST line.  Aside from that, U150 is mostly doing address decoding.

If you've measured _FAIL at 5V, it's probably OK.

Quite possible there's something dodgy happening on the CPU board, as it's capable of generating system resets.  The best suggestion is to try a known working A3640 or whatever to isolate the fault to the CPU board or main board.  Else you could be chasing your tail for days.