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Electronics Forum / Repair / July 2008Viewsonic VX715 LCD
I've got two of these with the exact same symptom. The backlight cuts out a few seconds after powering, and stays out until the the unit is turned off/on again. There are two boards in this; the power supply, and signal processing/CPU. I subbed the power supply board with a confirmed good one from another unit and got the same result, ruling out that board as the cause. Also tried disconnecting the backlight cables one at a time, but no change. It appears that a signal from the CPU board is causing the backlight to shut off. Is there any solution other than replacing the CPU board? > I've got two of these with the exact same symptom. The backlight cuts > out [quoted text clipped - 6 lines] >CPU board is causing the backlight to shut off. Is there any solution >other than replacing the CPU board? I saw somewhere that replacing the capacitors by the HV coils for the back light fixes the problem (this works as I have used it to fix the problem). ----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =--- But I subbed that whole board with a known good one, and it did the same thing...... >> I've got two of these with the exact same symptom. The backlight cuts >> out [quoted text clipped - 15 lines] > Newsgroups > ---= - Total Privacy via Encryption =--- >But I subbed that whole board with a known good one, and it did the >> [quoted text clipped - 12 lines] >> back light fixes the problem (this works as I have used it to fix the >> problem). Where do the backlights connect? Is there a seperate CCFL driver board? All that I have seen have one, but I have not worked on a VX715... ----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =--- I dont know if your model is the same as the ones i have been working on but i have fixed every one by replacing the one single capacitor in the balast board > But I subbed that whole board with a known good one, and it did the same > thing...... [quoted text clipped - 18 lines] >>Newsgroups >>---= - Total Privacy via Encryption =--- Assuming you swapped both the main power supply board and the inverter board with no change, you probably got worn or bad tubes. I am dealing with a gateway 18" now with similar issues. Most of these inverters have intelligent controller that shut down the inverter if the lamps do not strike fast enough, are open circuit, or draw too much current. Without hot-wiring the protection circuits, you may not get to the bottom of the problem. bob ----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =--- Is it possible that both sets of lamps are bad? How to I bypass the protection circuit to determine if the lamps are the problem? >> But I subbed that whole board with a known good one, and it did the same >> thing...... [quoted text clipped - 40 lines] > Newsgroups > ---= - Total Privacy via Encryption =--- >Is it possible that both sets of lamps are bad? How to I bypass the >protection circuit to determine if the lamps are the problem? Download the datasheet for the lamp controller chip and check out the application circuit. There should be a feedback pin that senses lamp current. Monitor that with a scope. Here is one example: http://focus.ti.com/lit/ds/symlink/ucc3972.pdf This forum suggests that the 16V 1000uF capacitors may be a common fault with your model: http://www.aplusperfect.com/articles/lcd_capacitor_repair Maybe the "known good" PSU is only marginally good and has trouble striking your worn lamps ??? As a matter of course I would reflow all the solder joints at the transformers and coils. >>> But I subbed that whole board with a known good one, and it did the same >>> thing...... [quoted text clipped - 28 lines] >> >> bob - Franc Zabkar  SignaturePlease remove one 'i' from my address when replying by email. |
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