> -----Original Message----- > From: cctalk [mailto:cctalk-boun...@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@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
