74161 investigation

I did some further investigation into the problem with the 74161As. The problem is limited to location D-8, the other 74161 locations can be populated with 74161As without issue. Here is what I have found out about the problem with D-8 so far.

The high level of the input to D15-9 (preload) is just at 2 volts. There is a small glitch below 2 volts just before the char rate clock(CLA) raises. D15 responds by lowering VINH and things go south from there. D15 should preset VINH to high, but apparently the floating preset input drops a bit at the same time, resulting in a preset to zero. Apparently a 74161A at D8 is fast enough to occasionally see VINH drop and counts on the nearby raising edge of CLA, putting the state machine into a mess. Apparently the 74161 doesn’t have enough setup time to count at this point. The time between early VINH drop and the next clock edge is about 20ns, but that is just about at the limit of my old, uncalibrated scopes resolving power, so consider this an estimate. This appears right on the edge of the 74161 setup time spec. Apparently enough for an A part, but not enough for a non-A part, which ignores this glitch and continues on nicely.

Cassette Interface

When I first tried the cassette interface, the input signal after the AC coupling cap looked rather nasty on the scope. I have learned through some efforts to build a regenerative RF receiver that AC coupling caps should be good quality. Testing this theory, I replaced the original cap, which came from a scrap box with a new monolithic cap. Sure enough the signal cleaned up considerably and I am now able to read from the tape correctly. The only thing not working, is the READ LED. I’m going to see if I can put this small circuit into my spice simulator and see how it should work, as there is nothing obviously wrong with it, though the input signal level is pretty small. It’s possible I just need a lower power LED. I have to run one additional test on the cassette interface, prior to declaring victory on this front. This is involves typing Basic into memory, WOZ style, then saving it and seeing if I can load it back in again.

So What is Left

I’ve got a few things left to do prior to taking orders.

I should be able to complete this effort before the end of Feburary. Those who are on the wait list will get the first shot at kits. If you asked to get on the wait list, I’ll send you an email when I’m ready.