Apple II no video display

Ethan Dicks ethan.dicks at
Fri Oct 27 14:13:33 CDT 2017

On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via cctalk
<cctalk at> 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

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


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