LCD monitor won't turn on

[james]

I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
turned on even though the adapter is supplying a healthy 12V DC (measured
with load).

I opened the case and measured a pin labeled +12V and found only 2V when the
power switch is held down, and goes to 0V when released.

The front panel power switch itself is functional -- I short out the two
contact points with a wire and got the same 2V as above.

This could mean the circuit that controls the main power (probably a power
transistor) is bad, or there is a short circuit dragging down the voltage.
Since there is no smoke when I held down the switch, I'm hopeful it's the
first case.

The power supply (external adapter) is 12V 4A. The wires leading to the
front panel power switch is very thin, and the switch itself is a push
buttom with momentary contact. So I figure the power switch is not what's
directly turning on the power; it must be controlling a semiconductor that
turns on the full 12V 4A to the entire unit. However, I did not find any
power semi-conductor that looks like it can switch on/off 4A of current.
Here's a photo of the circuit board:

http://www.smugmug.com/gallery/6159236_bYvW4/1/388058044_TTcnS
(put mouse on the photo and click "original" to see full resolution)

Any idea which chip/transistor controls the 12V 4A to the rest of the
system?

To further debug the problem, I need a circuit diagram. Any idea where/how
to get a circuit diagram of a viewsonic monitor? I looked on viewsonic's web
site and the phase "cicruit diagram" or "repair manual" returned zero hit.

Without circuit diagram, the only thing I can think of is to solder a wire
directly from the input 12V DC to one of the +12V lead. If this works, it
means the unit cannot be turn on/off via the front panel switch. If it
doesn't work, it may generate some smoke...

 

[lurch]

*SNIP

http://www.smugmug.com/gallery/6159236_bYvW4/1/388058044_TTcnS
(put mouse on the photo and click "original" to see full resolution)

*SNIP

Don't know if it helps but Google the number stamped near the top of the
board (3150-0122). Got several hits and at least one source for a
replacement board ($80).

 

[Adrian C]

http://www.smugmug.com/gallery/6159236_bYvW4/1/388058044_TTcnS
(put mouse on the photo and click "original" to see full resolution)

Any idea which chip/transistor controls the 12V 4A to the rest of the
system?

IC18 to the left of the power socket? I can't read the type number other
than it's made by International Rectifier..

 

[Franc Zabkar]

I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
turned on even though the adapter is supplying a healthy 12V DC (measured
with load).

I opened the case and measured a pin labeled +12V and found only 2V when the
power switch is held down, and goes to 0V when released.

The front panel power switch itself is functional -- I short out the two
contact points with a wire and got the same 2V as above.

This could mean the circuit that controls the main power (probably a power
transistor) is bad, or there is a short circuit dragging down the voltage.
Since there is no smoke when I held down the switch, I'm hopeful it's the
first case.

The power supply (external adapter) is 12V 4A. The wires leading to the
front panel power switch is very thin, and the switch itself is a push
buttom with momentary contact. So I figure the power switch is not what's
directly turning on the power; it must be controlling a semiconductor that
turns on the full 12V 4A to the entire unit. However, I did not find any
power semi-conductor that looks like it can switch on/off 4A of current.
Here's a photo of the circuit board:

http://www.smugmug.com/gallery/6159236_bYvW4/1/388058044_TTcnS
(put mouse on the photo and click "original" to see full resolution)

Any idea which chip/transistor controls the 12V 4A to the rest of the
system?

To further debug the problem, I need a circuit diagram. Any idea where/how
to get a circuit diagram of a viewsonic monitor? I looked on viewsonic's web
site and the phase "cicruit diagram" or "repair manual" returned zero hit.

Without circuit diagram, the only thing I can think of is to solder a wire
directly from the input 12V DC to one of the +12V lead. If this works, it
means the unit cannot be turn on/off via the front panel switch. If it
doesn't work, it may generate some smoke...

It took my browser forever to render your X3 image (in fact I gave up
waiting for it), but I finally worked out how to link directly to the
photo:
http://photos.smugmug.com/photos/388058044_TTcnS-X3-1.jpg

The fact that the something is happening when you press the on/off
button suggests that the uP is alive, and that the +5V(?) supply is
OK. I suspect that one of the AmTRAN chips (AM30 or AM100B) controls
the power to the backlights. It appears that Q15, the PNP (?) smt
transistor near the lamp connector, switches the +12V power to an
off-board inverter. I'd monitor the +12V supply on its Emitter (input)
and Collector (output) pins. The Base should be getting an on/off
signal from the uP via a resistor. Perhaps the inverter has a short
circuit which has taken out Q15 and/or a fusible resistor.

BTW, the Mitsubishi M52743BSP chip is a "I2C BUS CONTROLLED 3-CHANNEL
VIDEO PREAMPLIFIER", so I don't think it handles power control:
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-pdf-11/117670.html
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-26/DSA-519289.html

FWIW, a Nokia LCD 510L monitor appears to use the same chips, so maybe
your Viewsonic is a rebadged unit.

See http://www.elektroda.pl/rtvforum/topic1015418.html

- Franc Zabkar

 

[Andy Cuffe]

I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
turned on even though the adapter is supplying a healthy 12V DC (measured
with load).

The first thing I would check are all those electrolytic caps. About
half the LCD monitors I see just need caps.
Andy Cuffe

[email protected]

 

[james]

I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be

turned on even though the adapter is supplying a healthy 12V DC (measured
with load).

I opened the case and measured a pin labeled +12V and found only 2V when
the power switch is held down, and goes to 0V when released.

Forget what I said about 2V. I measured the wrong pin.

Surprisingly viewsonic gave me the repair manual, but I have to sign a NDA.
So I can't reveal too much here.

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.

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?

 

[Franc Zabkar]

Forget what I said about 2V. I measured the wrong pin.

I thought you may have been measuring the +12V at the lamp/inverter
connector. It may have helped if you were more specific.

BTW, does Q15 switch power to the inverter?

Surprisingly viewsonic gave me the repair manual, but I have to sign a NDA.
So I can't reveal too much here.

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?

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.

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.

- Franc Zabkar

 

[james]

So you are talking about IC30, the MC14013B dual D-type flip-flop?

You're a walking IC encyclopedia?

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).

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.

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.

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.

 

[Franc Zabkar]

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.

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. (???)

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

 

[Franc Zabkar]

Can you give me details of the fcc site that has
circuit diagrams ...

This is it:
https://gullfoss2.fcc.gov/oetcf/eas/reports/GenericSearch.cfm

Grab the FCC ID off the label on your device and type the characters
into the Grantee Code and Product Code boxes.

Note that most products will not have circuit diagrams because their
manufacturers often choose to hide behind confidentiality provisions.
I still managed to find about 100 circuits for LG LCD monitors and
TVs, though.

- Franc Zabkar

 

[[email protected]]

Forget what I said about 2V. I measured the wrong pin.

Surprisingly viewsonic gave me the repair manual, but I have to sign a NDA.
So I can't reveal too much here.

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.

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?

Could you please provide me Viewsonic VG150 repair diagram or manual?
I also have a broken VG150.