27 Oct
2017
27 Oct
'17
12:13 p.m.
On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via cctalk
<cctalk at 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