newbie building a scratch-built computer

Another one is to used an edge-triggered flip-flop (74F74 or simiar). Reset it buy hand, tie 'D' high, the clock input to the signal you want to look at and an LED + resistor from Q/ to Vcc. That one leaves no doubt that a pulse has come :-) Be aware that such simple testers tell you nothing about the timing of the signals, and that can be very misleading. Recently I was debugging an HP9820 'calculator' which uses a bit-serial processor. Data is shifted into the MSB end of various registers, out of the LSB end (that is normal enough). Anyway, on the M register (Memory address), bits 15...12 were changing (detected with a logic probe type of text), the other bits were all stuck low. Since this is built up from 4 off 7495 chips (each one 4 bits), my first gueess was that the chip handlinh bits 11...8 had failed -- the outputs of that were stuck low, then 0's were always being shifted into the lower 2 chips, so those were low too. So I changed it. No change to the fault _at all_. Not only did the machine not work, sitll, but bits 11....0 of the M register were still all stuck low. It's only after I got out a logic analyser and looked at the timing of bits 15....12 that I realised the timing was crazy, with pulses of half the clock period on the outputs, and the outputs were always low when the active edge of the clock occured. That meant all the lower shift regitster section correctly shifted in a 0 from bit 12. I replaced the high nybble chip (actually using the one I'd taken out from bits 11...8 -- 7495s are not common!), and the machine sprang to life. -tony