On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via cctalk <cctalk@classiccmp.org> wrote: > Folks, > > I have an Apple II that produces one beep at power on, but my monitor says > "no video present". A scope on the output shows frame sync but at low > levels.
Are you using a vintage CRT or a modern LCD with composite-in? I have found numerous examples of 1970s video not being acceptable to modern screens. Do you have a working Apple II to compare the scope trace to? It's possible you have a defective component, but I would try to line up some comparisons between good and bad examples to narrow down the differences if any. > The odd thing is that ic A9 appears to be a 74S151 not a 74LS151. I > wouldn't expect this to work, but I assume it was working at some point in > time! I don't think that should be a problem. 74S is often a sub for 74LS when the higher speed of 74LS is not required but the higher drive of 74S is (and the higher power consumption of 74S is irrelevant in-circuit). There are many times when you can put a 74S where it calls for a 74LS but often you can't sub 74LS for 74S where it's there intentionally. -ethan