A question that's puzzled me for years - Why do Jennings 400 boards always fail?
I know it's something to do with the battery, but it didn't seem to make much difference how corroded it was. However, I think I worked it out now. Here's a snippet from the J400 schematic, showing the battery charging circuit. Note the zener diode, circled in red.
Now here's a picture of that circuit on an actual board. Note the zener diode, circled in red. Now note that there's no position for it laid out on the board. It's an afterthought.
Referring to the schematic, where it says "+8V to +12V" it should actually say, "+15V to +18V" because that's what the J400 power supply delivers. In a board with a healthy battery, the battery will hold the voltage down to 4.5V. But if the battery is dead, leaking, or missing, it will pass the 15 to 18V through. Now if that zener diode is installed, it will hold down the voltage to 5V. But if it isn't, and remember it's an afterthought, the CMOS chips get 15 to 18 volts. Being CMOS, they may shrug that off, but all the chips they connect to are TTL, for which 15V is sudden death. Bye bye board.