Apple II no video display

[Dave Wade]

> -----Original Message-----
> From: Ethan Dicks [mailto:ethan.dicks at gmail.com]
> Sent: 27 October 2017 20:14
> To: Dave Wade <dave.g4ugm at gmail.com>; General Discussion: On-Topic
> and Off-Topic Posts <cctalk at classiccmp.org>
> Subject: Re: Apple II no video display
> 
> On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via cctalk
> <cctalk at 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