16 Jan
2014
16 Jan
'14
8:09 p.m.
On Thu, 16 Jan 2014, Tony Duell wrote:
As has been said before, one purpose of a schematic is to help you
understand how somethign works (and if you are repariing somethign, in
general you ahve to understnad how it works so you can see what is not
working).
Yeah. I was able to grasp the -12V rail entirely...but not the 555 timer
circuit. :(
Exactly.
Ah, or perhaps Argh! :-). What test equipment do you have? Sicne you
can't detect these signals driectly, you need soem instrument to do
that,. I am tryign to think if there's any way you could find the fault
with just a multimeter...
I have an old Fluke multimeter that is formerly AT&T property. I don't
have much else (yet).
I don't know wh yDEC drew that 555 the way they
did, but it is not easy to
figure out what is going on. It's not obvious at first glance which line
is the +12V power -- or at least it took me a few seconds to figure it out.
Yeah. It's annoying for someone new to this stuff. There are conventioanl ways to dra a lot of
sub-circuits, and it helps --
a lot -- if they are used. The way that 555 is drawn, wit hte timing
compohnets above it, whi;e electrically correct, is not clear.
Yeah. It doesn't help me translate it to the board very easily.
I wonder... If a smoothing capacitor had dreied up, there migth be
excessiveripple on one of the power lines. This might cause the output of
the comparator stage -- the trigger input of that 555 -- to be toggling
at twice line frequency. At which point the 555 would be contiulaly
retrigggered, but maybe occasionally it drops resrt for long enough for
the CPU to do something.
That would explain why the behaviour is sporadic and unpredictable.
Have you put a 'scope on the power liens to check for ripple? And again,
I would look at that trigger input of the 555.
Don't have a 'scope yet. The one I was going to get needed repaired.
A 'scope is vertainly soemthing worth obtaining. Of course, if you get a
non-wroking one, you can have another project of fixing the 'scope. Which
often needs a working 'scope :-)
I have a line on one...but it had a fault that needs repaired.
-tony
--
Cory Smelosky
http://gewt.net Personal stuff
http://gimme-sympathy.org Projects