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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Viewsonic 15 inch LCD hard to turn on
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 |
#2
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Viewsonic 15 inch LCD hard to turn on
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#3
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Viewsonic 15 inch LCD hard to turn on
wrote in message oups.com... 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. |
#4
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Viewsonic 15 inch LCD hard to turn on
James Sweet wrote:
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 |
#6
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Viewsonic 15 inch LCD hard to turn on
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#7
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Viewsonic 15 inch LCD hard to turn on
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 wrote: wrote: 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 |
#8
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Viewsonic 15 inch LCD hard to turn on
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#9
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Viewsonic 15 inch LCD hard to turn on
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 wrote: wrote: 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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