27 Oct
2017
27 Oct
'17
1:10 p.m.
-----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:
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.
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.
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.
-ethan
Thanks for the suggestions
Dave