On 2017-Oct-27, at 8:40 AM, Dave Wade via cctalk wrote:
> 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?
Don't know about the frequency of occurrence of such a substitution in the
apple II but
the relative downside of 74S to 74LS is the higher loading from inputs of S
types (aside from higher per package power consumption).
The worst case on A9 looks to be inputs I2,I4 which are driven by one LS output
which is also driving two other LS inputs.
This should still be within the loading rules.
Can't say I would be expecting the S type to be a problem.
How about using the scope to trace back through Q3 to the resistive inputs of
R6,7,8 where the video elements are resistively combined.
See how the signals and levels are at the input end of those resistors.
If you're seeing frame sync without line sync, perhaps look back towards the
sync combiner which appears is probably C13.
Would have to stare at the logic more to discern what all the inputs there are
doing.
The apple II schematic is terrible (IMO).
(looking at:
http://www.freeinfosociety.com/electronics/schemview.php?id=1472 )
