> -----Original Message----- > From: Ethan Dicks [mailto:ethan.di...@gmail.com] > Sent: 27 October 2017 20:14 > To: Dave Wade <dave.g4...@gmail.com>; General Discussion: On-Topic > and Off-Topic Posts <cctalk@classiccmp.org> > Subject: Re: Apple II no video display > > On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via cctalk > <cctalk@classiccmp.org> 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 > screens.
That was the problem. I was trying an LCD TV that I had used successfully with other vintage equipment. When I tried it on another screen (small one, also LCD for in-car reversing camera) I get a display. I was a bit surprised as there is a modulator and that wasn't detected either. > > 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. > No I don't have a working Apple II. Well I do now! I did compare with a CoCo 3 and the sync pulses on the CoCo3 looked much bigger. > > 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 intentionally. I was wondering if it needed more drive than 74LS could provide, but as it works, apparently not.. > > -ethan Thanks for the suggestions Dave
