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Diagnosing sick Turbo XT clone board

I don't have a schematic for this board. I'm using the IBM 5160 technical reference and it was close enough to follow along
I've successfully used the 5160 schematics to diagnose two different turbo XT clone motherboards, so in my experience they tend to change very little (mainly just various hacks in the vicinity of the 8288 clock generator for the turbo mode).

On the 74LS245 between the D bus and XD-bus--it seems the DIR is stuck low. DIR is fed from output 3Y of a 74LS27. Input 3C to the 74LS27 appears to be stuck low, and comes from output Q1 of 74F20.
I had a dead 5150 motherboard where the issue was a bad 'LS86 chip (quad XOR) which had an *input pin* internally shorted to GND.... that was a hard one to diagnose!
 
Then simply find all ICs connected to this point and desolder them one by one. The moment the 22 Ohms are gone, you found the bad one.
 
It was indeed the 74F20. Besides one bad 64Kx1 RAM chip, and repairing more damage I had done (ugh--A18 line had been severed somewhere on its way to the expansion slot) it's now good to go.
This would have been a 10 minute repair had I skipped all the buffers/latches and been able to go directly to the 74F20...
 
It was indeed the 74F20. Besides one bad 64Kx1 RAM chip, and repairing more damage I had done (ugh--A18 line had been severed somewhere on its way to the expansion slot) it's now good to go.
This would have been a 10 minute repair had I skipped all the buffers/latches and been able to go directly to the 74F20...
Great news! Yes, sadly just figuring out what went bad (chip, trace, capacitor, etc.) is at least 9/10ths of the entire battle when repairing these things...
 
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