Rca 42la45rq

[CapLeaker]

Re: Rca 42la45rq

YouTube "how to test bridge rectifier".
You should should see 4 diodes. If one or more is shorted, it is bad. A good bridge rectifier has a voltage drop of 0.5 to 0.7 on the diode check function. A good diode has a voltage drop one way and an open circuit the other way. Most DMM's with a diode check fuction will beep a continuous tone when there is a shorted diode. If you find a bad one in circuit, lift a leg or take it out and remeasure it again. Different diodes have different voltage drops, so you have to do research and check what that diode supposed to have for a voltage drop.

[bleed4me]

Re: Rca 42la45rq

Thanks again CapLeaker, so I took it out of circuit and according to the video the rectifier is good. The video however did not mention voltage drops as you did. Safe to assume that would get done in circuit with power on? Also if the rectifier is good is the next step to test the 7 mosfets on this board? Just looking for confirmation that I am at least going in the right direction to solving this power board. If not what else would make a fuse instantly blow?

[Electronic Handyman]

Re: Rca 42la45rq

Check all the voltages and pins their suppose to be bec those two boards you mention sure look the same.Same amount of heatsinks, same parts I bet connected to them and same plug in connectors, I checked most of the two boards and compared part layouts etc and so far the parts are the same too.Note they might not list this board as a sub bec they've a lot of the originals for sale and just want to get rid of them.Again compare the plug in connectors and voltages mentioned on your board to the original board to be sure but again it sure looks like that board would work.By the way I doubt that many ppl would buy a board etc for those sets RCA's are known for several different faults one being poor quality capacitors, poor soldering connections, those panels fail a lot especially the panel ribbon cables.

Oh one last note on your power supply that big capacitor sure doesn't look good to me but that's just my opinion from your pic.

[budm]

Re: Rca 42la45rq

One board (*2002) has blk/red/yel jumper wires between the two connectors, the other board (*2302 for this model TV) does not have it.

[bleed4me]

Re: Rca 42la45rq

So I have spent some time reading and watching videos about bridge rectifiers and mosfets, enough to make a brain hurt. Its a struggle to understand all the differences and how to test them. I am noticing the more I watch the more these videos contradict each other. One video will test one way then another will test in a completely different way and here I stand wondering which one is right for what I am doing. I watched one video and thought that the rectifier was good then when I watched another and did their test I now think I believe it is bad. I will post the results and let you be the judge. These are the values after taking it out of circuit in diode mode .

1-2--.467
1-3--.468
1-4--.855

2-1--ol
2-3--ol
2-4--.479

3-1--ol
3-2--ol
3-4--.484

4-1--ol
4-2--ol
4-3--ol


Transistors get me really tripped up. Bipolar junction, FET, NPN or PNP and how to test them all I just cant connect the dots. I dont know if I am just testing them wrong and not understanding but all of mine MOSFETS are OL with the exception of a few out of circuit. Its probably me or the fact that I dont have a fancy multimeter UNI-T UT136B ( just got it and still getting use to it honestly). With all this said I am sorry but I am new to all this and am still learning and thank you for any and all insight. On a better note I am understanding the drain gate source concept and am wondering is there like a master list to look these up or do you just have to google it and search?

[bleed4me]

Re: Rca 42la45rq

I am not going to lie but I have been dreading this particular thread and wishing it would just disappear. This ones hasnt exactly been my brightest moment. I have spent count less hours reading and watching videos on mosfets and I just got overwhelmed. The readings they got were completely different from the ones I got no matter how I tried it. Granted they were using far superior equipment than I was. I just dont think diode mode on mine is up to par with accurate readings. With that being said I wanted to follow up on this and let everyone know how it turned out. I took a shot with my readings and replaced PQ101, crossed my fingers and powered it on.......and success. Powered on and the picture looks great. Like always thanks to all that were involved your knowledge is much appreciated. I will promise this though... this wont happen again...

[bleed4me]

Re: Rca 42la45rq

I retract my previous statement, powered on the TV again and the fused blew and PQ101 went bad again. So it is not fixed but it is progress! I have the right mosfet but there is something else in the equation that is unknown. So back to square one...

[CapLeaker]

Re: Rca 42la45rq

Your bridge rectifier is good. Since the Mosfet blows, you have to look at the gate drive circuit and the PWM.

[bleed4me]

Re: Rca 42la45rq

I will be honest with you what you just said I have no clue as to what that means...sorry. On a brighter note I am much more comfortable now with bride rectifiers and mosfets and how to read them. If you wouldnt mind explaining or point me to a good reading source it would be much appreciated.

[CapLeaker]

Re: Rca 42la45rq

There is nothing much to it. The mosfet by itself doesn't do anything. It has to be told to switch on and off all the time with a device called "PWM". The Pulse Width Modulator IC tells the Mosfet's to turn on and of at it's Gate. Between the gate pin of the mosfet and the PWM may be resistors, diodes etc. called gate drive circuit. So when the mosfet blew and got shorted to the gate pin, It put a whole lot of juice into the gate drive, in which parts are damaged. Resistors go high in value, diodes short etc. The PWM might be damaged to and just "holds" the Mosfet latched on all the time in which in turn will blow it up, because it isn't switching.

[bleed4me]

Re: Rca 42la45rq

So what you are saying CapLeaker is that there was a chain reaction to this situation. Really appreciate your insight on this. So U701 is the PWM IC you are referring to am I correct? From what I am reading I dont have the correct gear to know if that is bad or not. As far as the gate drive circuit goes would it be scattered throughout the board or would it be more isolated around the problem mosfet like D101,L102,D103,RS101 and so on? Also what do the prefixes L and Z stand for?

[KK4PGM]

Re: Rca 42la45rq

I know this is somewhat of an old post, but I too am having some of the same issues with this particular powerboard. Fuse pops every time I plug it into an outlet. Seems next to PQ101 there may be some damage to a diode & resistor. Curious to see if this is a somewhat commonly occurring problem among these boards due to design flaws & curious to hear of any update to this thread. Appreciate any help folks.