newbie building a scratch-built computer

Well done! This is true, and it's one reason for knowing how to use simple test gear effectively. I have debugged a lot of things that way, watching the brightness ot the LED to see if siagnal is stuck high, 50-50 (possibly a clock waveform) or something else :-) On the other hand, I once spend a sunday afternoon tracing a non-fault in the memory arbitration circuitry of a Whitechapel MG1, using a simple logic probe. I nearly went sane (!). The next day I sent off almost all the money I had and bought a LogicDart.... The reason I san non-fault is that the thing reported a multi-bit DRAM failure, and it appeared the CPU wasn't finding any RAM at all. That's why I thoguht it was a control/arbitration problem, not a memory chip failure. In fact, there was no fault at all. The boot ROM needed more RAM than was on the mainboard -- IIRC it needed a pair of memory expansion boards too. Nowehere was this mentioned... Why? I would have thought having known-good connections and low-impedance power and ground connections (neither of which tends to be the case on a breadboard) would be very helpful in only having to find the _real_ faults.