Apple II no video display

[Dave Wade]

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. 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 have done some googling and can't see any reference to this substitution.
Is it something any one else has seen?

 

Dave Wade