Tandy 1000EX Not Booting - Help

Hey everyone. Brand new here, so please bear with me.

I restored a Tandy 1000EX - cleaned it, retrobright, replace memory chips, upgraded with memory expansion card - and built an ISA adapter for the card. Turned it on and everything worked. Then, I did something incredibly stupid... I connected an XT-IDE ISA card upside-down, turned the computer on, and something went *pop* inside. Now the computer powers on, but does not boot. The fan runs and the disk drive read light is on consistently. That's it.

I opened the computer and substituted all the socketed chips, including the ROM and the video chip, into another EX, and they all work fine. Before I go through and start checking each capacitor, resistor, etc., does anyone here have any idea of what I could have blown/need to replace?

I am not sure if the EX has a fuse in its power supply, but I would check there first. The fuse may not be made of glass, however.

Ouch. Pretty much any IC chip that is connected directly to the ISA bus is likely to have been damaged. In general I'd expect resistors and capacitors to be OK unless they appear physically damaged.

Do check the power supply voltages, and go over the components with a magnifying glass. Some small burning or bulging could provide a clue. There are usually some TTL buffer chips between ISA slots and the rest of the circuitry. A schematic might help you identify those. Those are likely culprits and are probably soldered on to the board.

Thanks so much for these suggestions! I did check the fuse on the power supply, and that is working fine. The mainboard is getting power, and the fan is working. I downloaded the technical manual and a service manual, and I'll start checking the power supply voltages and the buffer chips.

Okay, so let's see what sort of mayhem can be wreaked by reversing an ISA card in its slot:



The biggest danger of damage is going to result from +12V, -12V and -5V hitting the wrong lines. +5 may well not do much damage. My suspicion is that you've blown one or more of the data or address line drivers. But that's just a guess.

I'd start with a 'scope or logic analyzer to see what signals are being asserted; check to see if there's a valid clock signal and generally to see if the CPU is running or is stalled.