I don't suppose you know what capacitance caps are on the motherboard? I plan on trying to repair a Portable III very soon myself, but haven't yet cracked it open. I tried three and all exhibit the same behavior as the OP, though I haven't opened any of them yet.That sounds like a shorted tantalum capacitor on one of the power rails on the motherboard to me. I’d check there first.
I ordered 10uF 35v for mine. I had three of them pop, one of which was visibly exploded and two that showed no visible signs of damage but showed no resistance during continuity testing.What are the specs on the tantalums you used? I ordered a whole bunch yesterday, but for replacing on IBM 5150/5160 motherboards. I think those are 10uF, 16V.