Blown series inductor in TFT monitor - Value? - Page 1
Forum section: electronics / repair

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 10:49:01 am
content: Hi all,An old 17inch computer TFT monitor has recently devolved a fault and when I opened it up I found what appears to be an SMD inductor blown.The monitor has an external power module supplying the monitor with 19V DC. This inductor was connected in series to at the DC input power jack on the negative rail. Choke?It has completely disintegrated, and the only way I can tell that there used to be an inductor there is because its written on the PCB L1!The power supply is rated 19V/3.16A so I guess this inductor must be able to withstand this much current but I have no idea what its inductance should be. Sizewise it resembles a 1206 SMD resistor.Anybody has any ideas / recommendations?Hmm ...What if I just short it out of the circuit?

author: zvonex66x
author_rank: Contributor
timestamp: January 21, 2018, 10:58:00 am
content: Put the 470 mH

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 11:07:15 am
content: Use some ferrite bead like thishttps://eu.mouser.com/ProductDetail/Murata-Electronics/BLM31KN121SZ1L?qs=sGAEpiMZZMtdyQheitOmRU8JDs8ytUOVNtyMRQJqspm98jXK10gR4A%3d%3d. But try just shorting it with a solder blob and check if monitor works because it might not. Not that this part is required for monitor operation at all.

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 11:25:56 am
content: Quote from: wraper on January 21, 2018, 11:07:15 amI shorted it to check if monitor works... and it doesnt!Then I spoted a 3A SMD fused further also blown. Shorted the blown fuse out, replaced the supply with a bench PSU with controlled current protection and I found that there is a short circuit somewhere...Now i am stuck! Some other component is shorting everything... but all descrte diodes and 3pin smds seem ok.There are also 4 soic -8 chips, 2 of which are marked with 5234 and the other two 5134. The seem to be placed in pairs .Could be regulators... but not sure how to check if they are shorted...

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 11:34:02 am
content: Quote from: Watcher on January 21, 2018, 11:25:56 amThose most likely are mosfets or dual mosfets, post a picture.

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 11:42:43 am
content: Quote from: wraper on January 21, 2018, 11:34:02 amSee attached.The yellow wire - jumper shown is the one I put in place of the blown inductor

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 11:59:50 am
content: Those are certainly mosfets, although I cannot find what model they are. As this should be quiet old monitor, it's probably possible to find a schematic.

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 12:02:06 pm
content: Quote from: wraper on January 21, 2018, 11:59:50 amIts an Acer 17 inch monitor and yes its old and probably not worth fixing..Where do you find schematics anyway for devices?

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 12:15:38 pm
content: google, some Russian forums, elektrotanya.com

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 12:36:09 pm
content: In any case they can be replaced with many other mosfets like Si4825DDY for P-channel and Si4884BDY for N channel.http://monitor.espec.ws/section27/topic222222.html

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 12:56:30 pm
content: Thanks for the info.Now i am thinking that i could order some mosfets and then find out that there are also other things blown.Like those video chips...It looks like the monitor suffered some kinda overvoltage surge. ..

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 01:04:47 pm
content: Quote from: Watcher on January 21, 2018, 12:56:30 pmNo, in such case PSU would blow up. Most likely one of the MOSFETs failed, which cased the short and other parts going out. You don't need to replace all mosfets, just check which are shorted.

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 01:35:16 pm
content: Will have to desolder them to check though

author: Watcher
author_rank: Contributor
timestamp: January 21, 2018, 06:52:34 pm
content: Yeap.. Two of them are smoked. One p-channel and an adjacent N-channel.Removed them both from PCB and now the short is gone.Powered the board.. 3.3V rails are OKBUT: I ve measured the full voltage of 18V DC across an Atmel EEPROM near the MPU!!MPU itself is powered down (monitor on standby) and the Vdd measures less than 1VSo something else is also smoked too since 18V have reached the EEPROM chip...which means more parts are dead.Maybe is not worth looking into it further...

author: wraper
author_rank: Unknown
timestamp: January 21, 2018, 07:03:19 pm
content: Guess something wrong with 5V buck converter. EEPROM should have 5V on it It's programmed with E-EDID data. You'd need to program it with at least some supported resolution data for monitor to be recognized by PC.QuoteYeah, does not sound good. I'd guess most of the stuff on 5V is dead.