TV powersupply 100microFarad filter capacitor and fuse keep blowing up - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: September 26, 2025, 11:59:40 am
content: Hi,I have a tv powersupply and the big yellow 100microFarad filter capacitor and fuse keep blowing up. There is no short, i tested it with continuity, but i think something is drawing a lot of current. I need some advice from an experienced tv repair technician who can give me some hints where and what to look for. I attached a picture of the powersupply. Many thanks in advance

author: Psi
author_rank: Super Contributor
timestamp: September 26, 2025, 12:37:04 pm
content: I assume you have already tried to replace the blown up cap at least once and it blew up again and blew the fuse?I would be checking that the 4 diodes in the black bridge rectifier module are all ok.It wont be happy if AC is getting through and onto the cap.Might as well check the other 2 diodes while you're at it.Mains voltage electrolytic cap explosions are usually either reverse polarity or overvoltage.It won't be overvoltage because it's a 450V cap and you have less than that coming in.So my money is on AC getting onto the cap, causing it to expand, short out inside and then blow the fuse.

author: robsims
author_rank: Frequent Contributor
timestamp: September 26, 2025, 12:56:08 pm
content: Yes Psi i replaced the cap and the fuse and they both blew up. The cap expanded till it blew up. I already checked the bridge rectifier and the two diodes with my multimeter in diode mode. They are all fine but i will replace the bridge rectifier to see if it solves the problem

author: robsims
author_rank: Frequent Contributor
timestamp: September 26, 2025, 01:48:10 pm
content: I replaced the filtercap on the powersupply again and the bridge rectifier. I disconnected the cable from pwersupply to the mainboard and also disconnected the backlight cable on the powersupply. After that i connected the powersupply to mains and i measured the voltage on the fltercap . The voltage was 179 volts which is good. After disconnecting the powersupply i measured the voltage on the filtercap again. The voltage decreased very rapidly which is not a good sign. So i suspect something is not shorted but is drawing a lot of current. What can it be? How can i find out what is drawing too much current

author: jmelson
author_rank: Super Contributor
timestamp: September 26, 2025, 04:15:34 pm
content: This is a switching power supply. There will be a self-start circuit that sends power from the main cap to start the switching controller chip. I would expect it to drain the main cap within a few seconds when power is removed.Jon

author: robsims
author_rank: Frequent Contributor
timestamp: September 26, 2025, 10:56:50 pm
content: With power cable connected and plugged in and backlight cable and cable to the mainboard disconnected, the filter cap doesn't get warm. As soon as i connect the cable to the mainboard (white connector upper right- see picture) the big yellow filtercap gets very very hot in a very short time. I hope someone can give me some hints what to look for, which component is failing

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 07:05:59 am
content: If that psu-board belongs to an LG flat, esp LG-42LNxxx, the fat C* isn't the smoothing capacitor. Its part of the boost converters charge pump aka PFC. Probably it will be overcharged under load conditions, so one may check the PFC mosfet, the diode on heatsink & the driver IC and corresponding fb circuit*C610

author: robsims
author_rank: Frequent Contributor
timestamp: September 27, 2025, 10:09:39 am
content: Yes Uunocticum that board belongs to a LG flat panel Tv. The fat Cap is the smoothing capacitor because it is connected to the plus and minus of the bridge rectifier. On the heatsink on the right side of the board there is a mosfet (upper side) and a diode (lower side). Tested them, they are good, in fact i tested almost all components on the board and they are good. Things i'm not sure of are the transformers and the IC on the right side in the middle of the board. Ive attached a picture of the underside of the board. I flipped the board to the left. On the board you can see the filter Cap pins, pins of the switching mosfet and the pin of the transformer which is switched on/off. There is also a cut i made. If i connect the two sides of the cut, the filter Cap will heat up very very quickly. Hope this clears thing a bit better. Does anyone has other suggestions, please let me know

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 02:59:11 pm
content: I've drawn my understanding of that circuit simplified to the right and don't see C610 as a smoothing capacitor behind the bridge rectifier, it is the charge-up capacitor from PFC.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 04:35:22 pm
content: My oracle says that diode D601 is highly suspect. It is possible that it is conducting partially in reverse or has a short circuit. Dosn't affect idle mode, but under load conditions, when the driver IC pushes the dutycycle towards full speed we got an backwards discharging path thru the mosfet for the capacitor. In other words, that results in an exessive current thru C610 - like it's connected to AC

author: Sorama
author_rank: Frequent Contributor
timestamp: September 27, 2025, 04:50:22 pm
content: Quote from: Uunoctium on September 27, 2025, 04:35:22 pmYou mean D603 instead of 601 I suppose?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 04:51:26 pm
content: yes, of course

author: robsims
author_rank: Frequent Contributor
timestamp: September 27, 2025, 04:55:17 pm
content: No it can't be D603. It was shorted the last time the cap blew up. I replaced it with a good one so D603 can't be the problemI've tested all the components in you're drawing Uunoctium and they are all fine. The only thing i can't test is the IC601. So what can i look for next? Can it be that the IC501 is bad, because that IC's drain pin is connected to pin EL40 of the transformer. That IC maybe needs to regulate the current coming out of the transformer. Maybe that IC is bad

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 27, 2025, 05:15:57 pm
content: IC501 drives the low voltage 3V3 and should be active in idle modeBut i see in SM, there are two values given for C610, depending on model / screen size.68µ / 450V and 100µ / 500V!

author: robsims
author_rank: Frequent Contributor
timestamp: September 27, 2025, 05:41:47 pm
content: the filtercap that was originally on the powerssupply is a 450V 82 microfarad capacitor. The yellow 450V 100 microfarad Cap in the picture also blew up. I've installed new 82 microfarad 450V cap

author: robsims
author_rank: Frequent Contributor
timestamp: September 27, 2025, 07:47:23 pm
content: I replaced IC501 and the tv worked good for 3 minutes. Standby light, backlight came on, no blown C610 capacitor, the capacitor stayed cool and after 3 minutes the tv was dead again. I dont get no voltages on the upper right white connector which connects the powersupply to the mainboard. So IC501 was bad but something else is bad now. Do you have a clue what it might be Uunoctium?

author: robsims
author_rank: Frequent Contributor
timestamp: September 27, 2025, 11:08:22 pm
content: TV is fine again now. It was IC501 which was bad. Many thanks guys for you're input. I will order a few of these IC's and keep them in stock

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 28, 2025, 07:02:45 pm
content: Quote from: robsims on September 27, 2025, 07:47:23 pmIf the replacement IC501 died within 3minutes, it makes me question whether the replacement chip is genuineorthat another fault (ex. over-voltage) is still present in the circuit. Where did you purchase the chips?