LG47LC7DF Fuse Blows

[killian6pk]

Re: LG47LC7DF Fuse Blows

see below

[killian6pk]

Re: LG47LC7DF Fuse Blows

Well good news and bad. I installed the shorted mosfet and a couple of other suspect components and put the board back in and turned on the set. The main fuse did not blow and has 120v across it. The bad news is that there is another fuse near the two large caps and it does not have any voltage across it, but checks good. My guess is a bad bridge rectifier, does that sound likely? I tested the BR out of circuit first before doing anything else to the board and I thought it tested good, but I must have been wrong.

[budm]

Re: LG47LC7DF Fuse Blows

"The main fuse did not blow and has 120v across it." What do you mean by 'ACORSS IT'?
Fuse is a shorted device so there should have no Voltage across it unless it is open (blown).

[killian6pk]

Re: LG47LC7DF Fuse Blows

What I meant was that I put one probe on one end of the fuse and the other probe on the other end of the fuse. Was that not correct?

[budm]

Re: LG47LC7DF Fuse Blows

You have open fuse. Just think about it, fuse is a shorted device so should you get Voltage reading on shorted wire?
Just use Ohm meter.

[killian6pk]

Re: LG47LC7DF Fuse Blows

With the meter set on ohms and the board not plugged in the fuse reads OF. So the fuse did blow.

[killian6pk]

Re: LG47LC7DF Fuse Blows

While I was waiting I pulled the Bridge Rectifier off of the board again to test it. In diode mode on dmm red lead on ac pin black lead on + reading of .489, Switch lead same two pins reading of OF. Red on other ac pin black on neg. pin reading of OF. Switch leads same two pins reading of .491. Am I correct in thinking the BR is good?

[killian6pk]

Re: LG47LC7DF Fuse Blows

I have done some other component checking. On the heat sink opposite the Bridge Rectifier there are two Power Mosfets Q607 and Q606 and one Diode. The diode reads OF one direction .387 the other is that likely a good or bad diode? The two mosfets test bad there are no readings in any direction I test the pins in. Source to Drain or Source to Gate so I will replace them. Let me know if the Diode needs to be replaced.

[vinceroger69]

Re: LG47LC7DF Fuse Blows

mosfets can be awkward to test at times this video helped me
https://www.youtube.com/watch?v=RkWy...dex=38&list=WL
the diode should be ok

[killian6pk]

Re: LG47LC7DF Fuse Blows

Thanks for the link that did help.

[killian6pk]

Re: LG47LC7DF Fuse Blows

I have attached a picture of 1 of 2 resistors that are shorted. They both show a dmm measurement of 0.3 ohms that fluctuates from 0 to 0.3 ohms. Doing a continuity check they measure shorted. When I look on a resistor calculator (like the one on digikey) they don't have a black stripe as a choice on either band 1 or 5. I think this is a 1 watt resistor due to it's size compared to others I have. What I am trying to figure out is what is it's true resistance so that I can replace these 2. Also if I can't find that resistance what can I replace it with. Can I go up in resistance and or wattage etc.?

More info. If the 1st band is not black then band 1 and 2 both appear to be brown. Also the 3rd band looks silver on the resistor but the only choice on the calculators is grey.

I have been replacing bad diodes and mosfets (having to wait for orders etc.) I had all of them replaced and was checking voltages at the cable connectors to the main board I actually got a 6.5v and 12v reading but the 3.4v was low at about 2 volts. I was searching for what might be causing this low voltage when I shorted a large mosfet near the bridge rec. That shorted the bridge and the two resistors and the two fuses. So I had to order a new bridge and was trying to find the resistors when I encountered the problem described above. Should have come back to you guys for help with that instead of my trying to search blindly.

[vinceroger69]

Re: LG47LC7DF Fuse Blows

0.11 ohm?
https://www.hobby-hour.com/electroni...calculator.php

[killian6pk]

Re: LG47LC7DF Fuse Blows

Ok thanks, what about the going up or down in ohms and watts?

[R_J]

Re: LG47LC7DF Fuse Blows

The resistors are ok, they are very low resistance (.1 Ω) and will check like a short

[killian6pk]

Re: LG47LC7DF Fuse Blows

Ok thanks,
So could they be showing up in circuit as a shorted component? I was testing them out of circuit when they tested shorted. The reason I ask is that I saw a guy on youtube who was checking a tv power board for shorts by setting his dmm on diode mode and putting his leads on the ac power imput pins (no ac going into board) and he said if it had a reading there was a shorted component and if it read OL then there were no shorts. Does that make sense?

[happyade123]

Re: LG47LC7DF Fuse Blows

If you want to save on blowing up fuses Killian6PK remove blown fuse and replace with light bulb ( 60 to 100 watt) in series with fuse holder, if when you turn on power supply the light bulb lights and stays lit you have a short somewhere and need to check again. If the light bulb lights briefly then dims right out should be fine to put fuse back in and remove light bulb before applying power again.

[killian6pk]

Re: LG47LC7DF Fuse Blows

Questions: Is ESR important on Polypropylene capacitors as a means of deciding if they are good or not? What is considered a Low ESR for Polypropylene capacitors? The one Datasheet I could find that listed the ESR of a 1uf 400v Poly Cap said the ESR was 54 mohms and that these were low ESR caps. When dealing with Electrolytic caps this would be considered a high ESR.

When I test a poly cap pulled from the circuit board I am working on, it tests (with cap meter) at 6.3 ohms. That is a far cry from 54 mohms.