Repairing PC monitor Philips 190v7, backlight turns off after few seconds. - Page 1
Forum section: electronics / repair

author: Yig666
author_rank: Newbie
timestamp: December 13, 2019, 01:45:40 pm
content: Monitor: Philips 190v7Power Supply Board: 715g1813-1 ver dMain Board: 715g1767-1Problem: backlight turns off after few secondsA year ago, the backlight of the monitor started to turn off after a few minutes of use, so I haven't used it for several months. I recently started using it again and it worked perfectly for a while until it started to have problems again, now the backlight turns off after few seconds.Recently I decided to repair it, I replaced 8 of 9 electrolytic capacitors on the power board and nothing changed, I didn't change the rest of the capacitors because I didn't find them in the store.The physical size of the capacitors i changed is different from the original one.When i touch tester screwdriver on the metal case it lights up but if i touch it with the finger i do not get electrocuted, i am not sure if this is a problem or not.Electrolytic capacitor list with codes from the power supply board:1. C820 Original Hermei 220uF 25V Temperature 105 C , Replacement Taicon 220uF 25V Temperature 105 C (larger diameter than the original)(In the schematic I found has a different value from what was mounted on the board)2. C952 Original Hermei 220uF 25V Temperature 105 C , Replacement Taicon 220uF 25V Temperature 105 C (larger diameter than the original)3. C956 Original Hermei 220uF 25V Temperature 105 C , Replacement Taicon 220uF 25V Temperature 105 C (larger diameter than the original)4. C936 Original Elite 2200uF 10V Temperature 105 C , Replacement Nichicon 2200uF 16V Temperature 105 C (smaller diameter than the original)5. C933 Original Elite 1000uF 25V Temperature 105 C , Replacement Guika 1000uF 25V Temperature 105 C (smaller diameter than the original)5. C932 Original Elite 1000uF 25V Temperature 105 C , Replacement Guika 1000uF 25V Temperature 105 C (smaller diameter than the original)6. C840 Original Hermei 470uF 25V Temperature 105 C , Replacement Thongx 470uF 25V Temperature 105 C (smaller diameter than the original)7. C927 Elite 68uF 25V Temperature 105 C, Replacement with 2 capacitors in parallel (NRSZ 47uF 63V + Huahong 22uF 63V = TOTAL 69uF) Temperature 105 C ( I replaced this capacitor with two because I didn't find it in the store. )8. C907 Elite 100uF 450V Temperature 105 C (I didn't change it)After i replaced the electrolytic capacitors nothing changed, but next day I tried to start it and it started working again, but sometimes it blinks a little if you pay attention (it is not very visible). It worked for a while until it started to have problems with the backlight again.I have checked for cracked solder joints and i didn't saw anything.I tried to manually turn on backlight by disconnecting the connector from the Main Board and connecting with a resistor the pin 10 ( white wire marked O/F on the board and BL_CTL on the schematic ) with pin 3 ( red wire marked +5V ) and the same problem persist the backlight turns off after few seconds. So the problem is with the power supply board.After that I tried to bypass the Inverter protection by shorting pin 3 of OZ9938GN IC to ground and now the backlight works but this does not solve the problem and it could harm the components in the inverter board.I have the following questions:1.It is a problem that I changed the C927 Elite 68uF 25V capacitor with 2 parallel capacitors (NRSZ 47uF 63V + Huahong 22uF 63V = TOTAL 69uF) ?2.Does it matter what is the ESR of the capacitors I have changed? It will be better to put back the old capacitors?3.It is a problem if i run the monitor with pin 3 of OZ9938GN IC shorted? I don't have the courage to run it for more than 30 seconds.If anyone can help with my problem, it would be of great help because i don't have money right now to buy a new monitor. Keep in mind that I'm an electronics noob.English is not my first language, so I'm sorry for my grammatical mistakes.Thank you!I put below a link with a schematic i found on the internet for the power supply board and some photos.Schematic:https://drive.google.com/file/d/18S_IqUCv7c2KDkHt5WVZPdGODE-h5kXi/view?usp=sharingThe photos I took1.https://drive.google.com/file/d/13DPnF9kGm1r9sF2F0MHHtTZ3Oje-AnVC/view?usp=sharing2.https://drive.google.com/file/d/16Z9lzym5er4A176W2tgWUCvXO8V90b4S/view?usp=sharing3.https://drive.google.com/file/d/1d-xuwC9kBiePjbT_TYJJVrsksvQIfwRi/view?usp=sharing

author: george.b
author_rank: Frequent Contributor
timestamp: December 13, 2019, 01:54:34 pm
content: Sounds like a worn backlight lamp to me. As for your questions:1. No, that should be fine.2. Does it matter? Well, yes. Does it matter enough to make a difference in this case? Probably not.3. Quite possibly, yes. Current limiting is there for a reason. We don't know what margins you're working with there.

author: Yig666
author_rank: Newbie
timestamp: December 13, 2019, 02:32:19 pm
content: Thank you for the answer, I think the best option will be to try to replace the CCFL backlight with some led strips with a separate power supply.

author: george.b
author_rank: Frequent Contributor
timestamp: December 13, 2019, 04:52:42 pm
content: Yeah, that is a good option, especially if you can't find a suitable replacement.