Viewsonic 15 inch LCD hard to turn on

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OK, I got a 15" Viewsonic VA550 LCD monitor for 99 cents at the thrift
store.

Very occasionally, I can get it to turn on. Like 1 out of 100 presses
of the power button. About 1 out of 20 times, there is a quick blink of
the power light, but usually nothing. A strong light reveals that it is
not just the backlight, there is no picture.

If I hold the power button down while plugging in the power supply,
about 1 in 3 or 4 times it will turn itself on. When on, it looks
great.

I tested the power supply, and with the back off I can see a green LED
on the back of the video board is on when the power supply is plugged
in.

Once it is on, it stays on.

I've checked the power button contact resistance, it seems fine, and
shorting the pins with a metal object doesn't turn it on any more
reliably. Reseated the 2 ICs in sockets, and reseated all connectors.
Close visual inspection, don't see any visible cracks or damaged
components, no fishy smell or strange miscolorations.

Any ideas? I've never worked on an LCD monitor before. I've already
found a few references online stating that Viewsonic refuses to provide
parts or service manuals.

Thanks,
Steve Greenfield

 

[James Sweet]

OK, I got a 15" Viewsonic VA550 LCD monitor for 99 cents at the thrift
store.

Very occasionally, I can get it to turn on. Like 1 out of 100 presses
of the power button. About 1 out of 20 times, there is a quick blink of
the power light, but usually nothing. A strong light reveals that it is
not just the backlight, there is no picture.

If I hold the power button down while plugging in the power supply,
about 1 in 3 or 4 times it will turn itself on. When on, it looks
great.

I tested the power supply, and with the back off I can see a green LED
on the back of the video board is on when the power supply is plugged
in.

Once it is on, it stays on.

I've checked the power button contact resistance, it seems fine, and
shorting the pins with a metal object doesn't turn it on any more
reliably. Reseated the 2 ICs in sockets, and reseated all connectors.
Close visual inspection, don't see any visible cracks or damaged
components, no fishy smell or strange miscolorations.

Any ideas? I've never worked on an LCD monitor before. I've already
found a few references online stating that Viewsonic refuses to provide
parts or service manuals.

Thanks,
Steve Greenfield

Once it's been on for a while can you shut it off and turn it right back
on or does it still act up?

 

[James Thompson]

OK, I got a 15" Viewsonic VA550 LCD monitor for 99 cents at the thrift
store.

Very occasionally, I can get it to turn on. Like 1 out of 100 presses
of the power button. About 1 out of 20 times, there is a quick blink of
the power light, but usually nothing. A strong light reveals that it is
not just the backlight, there is no picture.

If I hold the power button down while plugging in the power supply,
about 1 in 3 or 4 times it will turn itself on. When on, it looks
great.

I tested the power supply, and with the back off I can see a green LED
on the back of the video board is on when the power supply is plugged
in.

Once it is on, it stays on.

I've checked the power button contact resistance, it seems fine, and
shorting the pins with a metal object doesn't turn it on any more
reliably. Reseated the 2 ICs in sockets, and reseated all connectors.
Close visual inspection, don't see any visible cracks or damaged
components, no fishy smell or strange miscolorations.

Any ideas? I've never worked on an LCD monitor before. I've already
found a few references online stating that Viewsonic refuses to provide
parts or service manuals.

Thanks,
Steve Greenfield

I would suspect the power supply, as in a switch mode that is shutting down
from turn on surge. It may be a sensing resistor that has changed value and
the sense circuit now thinks it draws too much current and shuts it down. Or
an IC is actually starting to draw too much current possibly after a small
surge from a storm. If it will run a while and turn off and on correctly,
Try hitting diff IC's with freeze to find which one is weak.

 

[[email protected]]

Once it's been on for a while can you shut it off and turn it right back
on or does it still act up?

It still acts up. I have the back off, but have it laying back down so
it still gets warm. Leave it on 30 seconds or several hours, shut it
off, it still won't turn on again. I can hold the power button down
while plugging the power supply in, and most times it will then turn
on, as before.

Steve Greenfield
Polymorph Digital
http://www.polyphoto.com

 

[[email protected]]

I recently fixed a VS 17" LCD monitor and found a number of bad SMD
electrolytic caps in the power supply. You can usually spot them
leaking on the PCB. Replace those first and see if it works better.
Scott

 

[[email protected]]

I recently fixed a VS 17" LCD monitor and found a number of bad SMD
electrolytic caps in the power supply. You can usually spot them
leaking on the PCB. Replace those first and see if it works better.
Scott

I didn't see anything obviously leaking or smell the typical fishy
smell.

Looks like I need to build an ESR meter.

Thanks,
Steve Greenfield

 

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Steve,
I built my own ESR meter using a function generator and a fluke DMM. I
set the generator to 10kHz and about 10V RMS. I then placed a 10k
resistor in series with the output so it will be sourcing 1mA RMS. Take
the other end of the resistor and the ground from the function
generator and connect these to your DMM on the AC mV range. The meter
should read overrange. Connect a 10 ohm resistor across the points and
adjust the output of the function generator until the meter reads 10
mV. Now your ESR meter is calibrated. Connect the two points across
various caps in the circuit. If you find any that read more than a
couple mV (Ohms), they are most likely bad. Note, a 1uF cap at 10kHz
should read 15.9 ohms, a 10uF should read 1.59 ohms and so on.
Scott

 

[[email protected]]

Steve,
I built my own ESR meter using a function generator and a fluke DMM. I
set the generator to 10kHz and about 10V RMS. I then placed a 10k
resistor in series with the output so it will be sourcing 1mA RMS. Take
the other end of the resistor and the ground from the function
generator and connect these to your DMM on the AC mV range. The meter
should read overrange. Connect a 10 ohm resistor across the points and
adjust the output of the function generator until the meter reads 10
mV. Now your ESR meter is calibrated. Connect the two points across
various caps in the circuit. If you find any that read more than a
couple mV (Ohms), they are most likely bad. Note, a 1uF cap at 10kHz
should read 15.9 ohms, a 10uF should read 1.59 ohms and so on.
Scott

Clever. The high voltage means it isn't well suited to in-circuit. Any
reason not to use a higher frequency to minimize the affect of the
capacitive impedance? Most ESR meters seem to use 50KHz or 100KHz.

That's good though, clever, quick and dirty. Actually, if you put a
couple of shottkey (sp?) diodes across the leads, the output can't go
high enough to trigger a silicon junction but it shouldn't affect the
ESR reading.

Steve Greenfield

 

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Steve,
I forgot to mention that. I used a pair of 1N4007 inverse paralleled to
clamp the open circuit voltage. 10kHz was a good compromise between
keeping the Xc low while being at the top end of the bandwidth of my
Fluke 87. It is worth experimenting with though if you have a better
meter.
Scott