LG model RU-23LZ21 blowing inverter fuse

[dossman]

Hello, I have a small 23" LG TV that keeps blowing the 4 amp fuse located on the inverter board. I have replaced the fuse a couple times but as soon as I turn on the TV it blows again. Here is a photo of its location. I do not want to buy an inverter board if it is something else. The sound and tuner are still working it is just that the screen is black.

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

Hello, I have a small 23" LG TV that keeps blowing the 4 amp fuse located on the inverter board. I have replaced the fuse a couple times but as soon as I turn on the TV it blows again. Here is a photo of its location. I do not want to buy an inverter board if it is something else. The sound and tuner are still working it is just that the screen is black.

Instead of blowing fuses, why not try to find the component that is causing the fuse to blow? Use your DMM to check transistors; maybe you will find a shorted one.

PlainBill

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

Upon further review, it would have been better if I had answered your question.

If you can see an image in the screen even with the backlights off, the problem is confined to the backlight system - the CCFLs and inverter. I cannot imagine any way the CCFLS could be causing the problem; it is almost certainly due to one or more shorted transistors on the inverter.

If a replacement inverter is available at reasonable cost, or you feel your troubleshooting or soldering skills are not up to the task, replace the inverter. If you would like to try to repair the existing inverter (there is an excellent chance this can be done successfully), post pictures showing the entire top and bottom of the inverter.

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

I did have a look at the screen to see if there is any image but the fuse blows as soon as the unit is turned on. A fellow on ebay was selling the power supply board and I thought it could be sending over-voltage to the inverter because the fuse is located right next to the connection ribbon coming from the supply board. Upon inspecting the inverter board when I first got the TV it looked like all the capacitors were replaced on it. I held a flashlight to the screen to see if there was any activity within the LCD panel and I could not see any. It was also mentioned that a bad bulb could have caused the inverter to fail, would that be possible?

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

I did have a look at the screen to see if there is any image but the fuse blows as soon as the unit is turned on. A fellow on ebay was selling the power supply board and I thought it could be sending over-voltage to the inverter because the fuse is located right next to the connection ribbon coming from the supply board. Upon inspecting the inverter board when I first got the TV it looked like all the capacitors were replaced on it. I held a flashlight to the screen to see if there was any activity within the LCD panel and I could not see any. It was also mentioned that a bad bulb could have caused the inverter to fail, would that be possible?

I would say the problem is the inverter itself. The design of these inverters included over-current protection of the outputs. That would eliminate any chance that the CCFLs were responsible. You could verify this by unplugging all CCFLs and trying again. (solder a #1157 taillight across the fuse location). Also, the chance that over voltage from the power supply is causing the fuse to blow is quite remote. You SHOULD be able to check the output voltage of the power supply; most supplies have the output voltages labeled.

In my opinion, the two most likely causes of the fuse blowing are a shorted driver IC or a reversed cap (assuming the caps had been changed).

The closest analogy I can come up with is this: You have a small electronic device - a cd player. One day you discover it isn't working, the circuit breaker has tripped. You reset the breaker and it trips immediately. You unplug the cd player, and try resetting the breaker. It stays on - until you plug in the CD player.

Do you 1. Assume the line voltage must be too high? 2. Wonder if the CD in the player is defective? or 3. Assume the CD player is defective? 4. Check the CD player by plugging it into an outlet on a different breaker?

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

I would assume the CD is defective in your analogy. I'm posting a photo from the shopjimmy site until I can take more photos. I would really appreciate it if you could look at my other post on the 42" Visio that I got recently, I hope I did not mess anything up and cause more problems with it. Thanks

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

what do I check after soldering an 1157 across the fuse contacts bill?

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

what do I check after soldering an 1157 across the fuse contacts bill?

Actually, you can test the driver ICs using nothing more than a DMM. Set it on the 200 ohm range. If you look at the first picture you posted, there are a number of 8 pin 'ICs' with the number 4532M. One corner of each IC has a dot, that indicates pin 1. Pins are numbered counterclockwise. For extra credit, explain why. Measure the resistance from pin 1 to pin 2, from pin 2 to pin 8, and from pin 8 to pin 1. Also measure from pin 3 to pin 4, from pin 4 to pin 5, and from pin 5 to pin 3. A 'good' reading would be over 200 ohms, a definite 'bad' would be under 5 ohms. Do this for all of the drivers. You will probably find one that tests bad.

The IC is actually a SI4532. The original is obsolete, the 'new, improved' version is available for $.80 each at Digikey. They only have 56,040 available.

When you have replaced all shorted drivers, try powering the board up with the 1157 lamp in place instead of the fuse. When the backlights come on you can pat yourself on the back, then replace the lamp with a fuse.

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

Thanks for the info, I'll get to work on it ASAP.

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

The pins on a DIP (dual inline package) are numbered counterclockwise so that automated chip-insertion machine assemblers can locate pin number 1 and insert it with the correct orientation into the circuit into which is being built.

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

pin 1 to pin 2 = 1.3
pin 2 to pin 8 = 2.2
pin 8 to pin 1 = 1.2
pin 3 to pin 4 = 48.3
pin 4 to pin 5 = 48.3
pin 5 to pin 3 = .4

2nd chip pin1 to pin2 = 5.8

all other pins and all other DIP chips showed nothing, there are 12 DIP chips

I tested them still connected to the board with the entire board removed from the set.

I guess its off to digikey

http://search.digikey.com/scripts/Dk...=SI4532DYTR-ND

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

pin 1 to pin 2 = 1.3
pin 2 to pin 8 = 2.2
pin 8 to pin 1 = 1.2
pin 3 to pin 4 = 48.3
pin 4 to pin 5 = 48.3
pin 5 to pin 3 = .4

2nd chip pin1 to pin2 = 5.8

all other pins and all other DIP chips showed nothing, there are 12 DIP chips

I tested them still connected to the board with the entire board removed from the set.

I guess its off to digikey

http://search.digikey.com/scripts/Dk...=SI4532DYTR-ND

Good work, that certainly looks like you found the problem.

If you don't already have it, get a Chip-Quik kit. It makes removing a SMD chip easy. Solder Wick makes clean-up easy.

PlainBill

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

The pins on a DIP (dual inline package) are numbered counterclockwise so that automated chip-insertion machine assemblers can locate pin number 1 and insert it with the correct orientation into the circuit into which is being built.

Nope. Way back in the dark ages, the pins on vacuum tubes were numbered clockwise when viewed from the pin end. That meant that when you viewed the tube from the top the pins were numbered counter-clockwise, but that wasn't a problem, troubleshooting was done from the bottom of the chassis.

When the first ICs were developed the TO-5 package was a popular package - plastic or ceramic dips had not yet been developed. Again, numbering was clockwise when viewed from the pin end. One common form for op-amps was an 8 pin to-5 package, but with the leads formed into two rows, .1" between pins, .3" between rows. And of course, they still kept the same numbering.

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

I might as well order all twelve divider chips, I would like to find out what type of fuse it takes but the service manual does not list it because it is on the inverter board. It is the SMD fuse under the pico fuse shown in my first photo.

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

I might as well order all twelve divider chips, I would like to find out what type of fuse it takes but the service manual does not list it because it is on the inverter board. It is the SMD fuse under the pico fuse shown in my first photo.

Driver, not divider. It's not necessary to replace all of them, but I would definitely replace the pair that are bad, and ordering a couple of spares makes sense. You wouldn't be the first person to have a part take a dive off the workbench and disappear.

It's possible to figure out a part number for the fuse. What numbers (or letter) are on the fuse, and what are it's dimensions? Is there any warning such as "Replace with 4 Amp 32V fuse only" in the area?

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

Hello Bill, The fuse has an Le 4A on it and it is white ceramic looking about 1/4" long and 1/8" high. I beleive it is made by Littlefuse but I do not know if it is fast or slowblow. there are no other surrounding markings on the inverter board. And that's what I get for not proofreading right? Thanks

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

There are more than one of these type SI4532 driver chips on digikey. I'm assuming the more expensive one is the best to get.http://search.digikey.com/scripts/Dk...eywords=SI4532

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

There are more than one of these type SI4532 driver chips on digikey. I'm assuming the more expensive one is the best to get.http://search.digikey.com/scripts/Dk...eywords=SI4532

Well, you can get a better price if you buy a roll of 2500, but that's probably more than you need. So order SI4532DYCT-ND

For the fuse, it appears 507-1070-1-ND is the proper choice.

PlainBill

[dossman]

Re: LG model RU-23LZ21 blowing inverter fuse

You know I had a funny feeling that the fuse was a slow blow with all the high voltages and switching going on. The local electronics supplier did not even know WHAT kind of fuses that had in stock for a replacement. That is probably why they sold me in-line fast-acting pico fuses in the first place. Is there any other things that you think I might check on this board before ordering from Digikey? I'm off to get my gas tank filled, hope the price is still at $3.45 Thanks for the help Bill

[PlainBill]

Re: LG model RU-23LZ21 blowing inverter fuse

You know I had a funny feeling that the fuse was a slow blow with all the high voltages and switching going on. The local electronics supplier did not even know WHAT kind of fuses that had in stock for a replacement. That is probably why they sold me in-line fast-acting pico fuses in the first place. Is there any other things that you think I might check on this board before ordering from Digikey? I'm off to get my gas tank filled, hope the price is still at $3.45 Thanks for the help Bill

I honestly don't know if that would be a fast blow or slow blow fuse. Looking at it rationally, if a slow blow is required and you put in a fast blow, it'll blow sooner or later, possibly on the first use. If you put in a slow blow instead of a fast blow, a shorted driver might pop it's top. If it had been an option, I would have use a normal blow.

If you don't have it, Chip-quik makes removing the drivers easy. And solder braid makes it easy to clean up the mess left by Chip-quik.

PlainBill