27 Oct
2017
27 Oct
'17
8:40 a.m.
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. 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?
Dave Wade
27 Oct
27 Oct
12:13 p.m.
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.
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.
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
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
1:13 p.m.
On 2017-Oct-27, at 12:13 PM, Ethan Dicks via cctalk wrote:
On Fri, Oct 27, 2017 at 11:40 AM, Dave Wade via
cctalk
74S is faster than 74LS, 74LS was combining low power techniques (which result in loss of
speed) with Schottky technology to get some of the lost speed back, to get a better
speed-power product than 'normal' TTL.
The chronological order of development was 74 --> 74H & L --> 74S --> 74LS
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
1:26 p.m.
On Fri, Oct 27, 2017 at 4:13 PM, Brent Hilpert via cctalk
<cctalk at classiccmp.org> wrote:
74S is faster than 74LS, 74LS was combining low power
techniques (which result in loss of speed) with Schottky technology to get some of the
lost speed back, to get a better speed-power product than 'normal' TTL.
Right. My bad there.
The chronological order of development was 74 -->
74H & L --> 74S --> 74LS
I do recall that progression. I've even seen a couple of 74L parts
(and there are more than a couple 74H parts in a 1968 PDP-8)
-ethan
12:53 p.m.
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.
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 )
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
2374
days inactive
2374
days old
6 comments
3 participants
participants (3)
-
bhilpert＠shaw.ca -
dave.g4ugm＠gmail.com -
ethan.dicks＠gmail.com