On Wed, 8 Oct 2008 20:13:26 -0700, "james" put
finger to keyboard and composed:
There is an IC that implement the power switch toggle effect. That is
where
the problem lies. The front panel power switch is connected to the clock
input of this flip flop via some RC network but for some reason it doesn't
want to flip. If I manually connect the +12V to the set or reset pin of
this
flip flop, I can turn the monitor on / off at will. So I can add a toggle
switch to do this; it would just look a little ugly.
So you are talking about IC30, the MC14013B dual D-type flip-flop?
You're a walking IC encyclopedia? 
4000 series CMOS logic has been around for decades. Anyone from the
old school would recognise that chip. BTW, I think that the R and S
inputs should be active high, not active low as I've drawn them.
AFAICT, the circuit looks like this:
|----------------------|
| |
| |-- R -- Vcc |
| | |
| ____o___ |
Vcc | | S | |
| | | | |
R |---| D Q*|-- R94 --+
| | | +_|_ C11
+----+---|Clk Q |-- ___ 10uF 16V
| _|_ | | on/off |
/ SW ___ |___R____| |
| | C o _|_
_|_ _|_ | =
= = |--- R --- Vcc
R94 and C11 effectively debounce the switch. If the chip is OK, then
I'd suspect that C11 is open. Try connecting a 10uF cap in parallel
with C11.
Close enough. I tried assuming each of the component is bad and did a patch
test (e.g. short the cap to Vcc or to gnd) but still couldn't make it work.
I hooked up a scope on the Clk and it shows a pulse when the power switch is
pressed. If I use a frayed wire to short the switch (this creates many
pulses) then it sometimes work after many attempts. Increasing or decreasing
the voltage by 1.5V doesn't help (use an ohmeter Rx1 range to short the
switch).
If you short the cap to ground, then the D input is always 0, which
means that Q will always be a 0. If you tie the cap to Vcc, then Q
will always be a 1 ... if the chip's clock input is working.
I don't know if it's a good idea to try to replace this chip. It's surface
mount and the chip is small (about 1cm) and the leads are even smaller.
Practice on a scrap board.
I'll play with this until I get tired of it. If I can not figure out the
definite cause then I'll just glue on an additional switch.
Being CMOS, and the fact that the chip is connected to the real world
(ie the switch), it may be that the clock input has been damaged by
ESD. (???)
I hope other LCD manufacturers are as nice as viewsonic in providing
circuit
diagram. Has anyone ever obtained repair manual for other brand of LCD
monitors?
Mitsubishi Electric Australia will sell me a manual for mine, without
an NDA. LG supply manuals for their LCD TVs, without an NDA. The FCC
database has many circuits for LG monitors and other manufacturers,
all of which you can download ... without an NDA.
Thanks for the info. I didn't know FCC has circuits in their database.
Neither did I until recently. Most circuits appear to have been added
after 2000. Not only that, but there are user manuals, block diagrams,
operational descriptions, and internal and external photos. They're
not always good quality, but they might be better than nothing.
One major downside is that the "searchability" of the database is very
poor. For example, I'd like to be able to locate all LG LCD monitors
for which circuits have been submitted. Or I'd like to find all ADSL
modems which use a Globespan Virata chipset. Maybe the FCC should let
Google manage their database.
- Franc Zabkar
--
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