27 Oct
2017
27 Oct
'17
1:24 p.m.
-----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 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