Updates on repairing Xerox Alto

Provided the PSU is good (and you have checked that, right :-)), I can't see that plugging all the boards in can do any damage. OK, when it doesn't work, you don't know immeidately where the problem is, but it shouldn't be all that hard to identify sections that are working. Somebody mentioned the trick of tracing the microcode with a logic analyser. This is something I've done many times (not on the Alto, since I don;t have one, but certainly on PERQs and HP9800s). Assuming you have a sorce listing of the microcode, this will tell you a lot about what it's doing or not doing. I understand that the Alto doesn't alas, have a test connector carrying the microcode address bus (the 2 machines I mentioned both do). So you have to pick the signals off the backplane If you don't ahve a logic analyser, then another trick that can help is to connect the microcode address lines to one set of inputs of an <n> bit comparator. The other set of inputs comes from a suitable set of DIP or thumbwheel switcehs. Feed the 'equals' output of the comparator to a logic rpboe or a monostabel and LED. The idea is that while you can't trace the microcode, you can at least see if certain routines are being executed (set the address of the start of the rotine on the switches, if the logic brobe is triggered, then the microde is gettign to that address). Does it ever run any of the video routines, things like that. -tony