Apple II no video display

[Dave Wade]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Brent
> Hilpert via cctalk
> Sent: 27 October 2017 20:53
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: Apple II no video display
> 
> 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.

Its just not something I have seen. It was the higher loading that was
concerning me...

> 
> 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 )

There are more usable schematics in the Apple II Redbook, but they are
spread over several pages so working out fan-in and fan-out.  
I am sure I got mine from applelogic.org but it appears to be down. There is
a copy here:-

http://www.classiccmp.org/cini/pdf/Apple/Apple%20II%20(Redbook)%20Reference%
20Manual%2030th%20Anniversary.pdf

but as it works on another monitor I suspect the one I was using does not
like Apple video...

Dave