Rca l37wd12 lcd tv

[gelatinous]

Hello,

I am working on an RCA LCD TV and have a question.

I've replaced some swollen caps on the cold side of the PSU but the tv exhibits the same trouble. The buttons are not responsive at all, if I leave the unit plugged in and then unplug and plug it back in it will sometimes turn on.

When the TV does turn on the smps does show 12V, 24V, and 3.3V, though I believe the 3.3V is coming from the main board as a power on signal.

I measure 5V for the standby voltage on the 1W board and at the main board, however the voltage on the power button only shows ~1V. To me it would seem that there is a problem with the main board given that the power button is not showing 5V or even 3.3V.

Does anyone have any idea what the problem might be?

Thanks,

Chris

PS: I'm off to add to the intro thread now.

[tom66]

Re: Rca l37wd12 lcd tv

Can you post some pictures?

Where did you buy the replacement capacitors and what manufacturer and series did you use?

[gelatinous]

Re: Rca l37wd12 lcd tv









I'm not sure what series the three smaller caps are, I can't see the brand. I do know that they are 1000uF 25V and 105C

The larger caps are 1000uf 50V 105C Nichon PJ(M)

however I do not believe that they are the problem as when the system does turn on the voltages are appropriate. I believe that the problem lies on the mainboard and has to do with the standby voltage coming from the 1W board.

The standby voltage is the only voltage on the mainboard until the power button is pressed to trigger the relay on the 1W board that supplies the 120V mains to the SMPS.

[tom66]

Re: Rca l37wd12 lcd tv

The Nichicon PJ series is suitable for the power supply and Nichicon make very high quality capacitors. It's mostly about longevity. General purpose capacitors may work but you could end up replacing them in 3-6 months time.

What voltage does the 1W standby measure? A user on this forum had a similar problem with a similar TV (I think 42") which has 3.6V on the 5V output. Does the voltage stay the same when the scaler board is plugged in?

Can you get a 10 ohm, 3W or higher resistor to test the 1W board?

[gelatinous]

Re: Rca l37wd12 lcd tv

I just measured the standby voltage with the board connected and not connected and what I found was that with the board connected the standby voltage was 4.4V but was also very very slowly increasing, like a capacitor was charging very slowly.

With the board not connected I measured 5.4V

I will have to see if my work has any resistors of that power tomorrow.

[tom66]

Re: Rca l37wd12 lcd tv

I just measured the standby voltage with the board connected and not connected and what I found was that with the board connected the standby voltage was 4.4V but was also very very slowly increasing, like a capacitor was charging very slowly.

With the board not connected I measured 5.4V

I will have to see if my work has any resistors of that power tomorrow.

You could go down to 0.5W, but it would get hotter. And see if you can get 15 or 22 ohms too because that would provide less load to the board.

It could be a problem with the standby board, but it could also be the main board drawing too much current due to a fault of some kind.

[gelatinous]

Re: Rca l37wd12 lcd tv

I will have a look at my work tomorrow and report back.

I just find it odd that if I leave the tv plugged in for say 10 minutes and then immediately unplug it and plug it back in it will turn on and work.

[tom66]

Re: Rca l37wd12 lcd tv

I will have a look at my work tomorrow and report back.

I just find it odd that if I leave the tv plugged in for say 10 minutes and then immediately unplug it and plug it back in it will turn on and work.

A bad cap can be thermally sensitive. As it warms up, its ESR can fall, allowing the TV to work eventually.

[gelatinous]

Re: Rca l37wd12 lcd tv

You could go down to 0.5W, but it would get hotter. And see if you can get 15 or 22 ohms too because that would provide less load to the board.

It could be a problem with the standby board, but it could also be the main board drawing too much current due to a fault of some kind.

Alright, I'm at work now so I can't do any tests, but I have in my hand a 10 ohm 100W power resistor, so once I am home I should be able to do some tests.

Am I going to be using this to jumper the standby voltage or am I going to be removing the smps from the circuit? Sorry but I've never done this kind of trouble shooting before.

[kc8adu]

Re: Rca l37wd12 lcd tv

am i seeing some sort of damage to the right of the flash?

[gelatinous]

Re: Rca l37wd12 lcd tv

am i seeing some sort of damage to the right of the flash?

I will try and get a better picture when I get home, but to me it looks like flux. I touched up the solder on a couple of resistors in that area. I'm also going to grab some Isopropyl so I can clean it.

[tom66]

Re: Rca l37wd12 lcd tv

Put it across the output and see if the voltage is more or less constant under load (it may vary a couple of percent, that's fine... but if it drops to say 4V, there's a problem.)

[gelatinous]

Re: Rca l37wd12 lcd tv

Put it across the output and see if the voltage is more or less constant under load (it may vary a couple of percent, that's fine... but if it drops to say 4V, there's a problem.)

ok, I connected the 10 ohm resistor across the 5V standby voltage with the mainboard disconnected and found that it was pulled down to ~2V but would very very slowly climb. I didn't leave it connected for too long but it was slowly climbing.

I know there is a schematic for the 1W board in another thread so I'm going to print it off and have a look at it.

Thanks for all your help!

EDIT: I just looked in the other thread... I will check these possibilities as soon as I have time, hopefully tonight:
https://www.badcaps.net/forum/showpo...48&postcount=9

[tom66]

Re: Rca l37wd12 lcd tv

ok, I connected the 10 ohm resistor across the 5V standby voltage with the mainboard disconnected and found that it was pulled down to ~2V but would very very slowly climb. I didn't leave it connected for too long but it was slowly climbing.

I know there is a schematic for the 1W board in another thread so I'm going to print it off and have a look at it.

Thanks for all your help!

EDIT: I just looked in the other thread... I will check these possibilities as soon as I have time, hopefully tonight:
https://www.badcaps.net/forum/showpo...48&postcount=9

This clearly indicates a fault with the 1W board, instead of the main board. Good - easier to fix.

The first step would be to replace all the capacitors on the board, including the smaller ones. Can you post a picture of your board?

[gelatinous]

Re: Rca l37wd12 lcd tv

This clearly indicates a fault with the 1W board, instead of the main board. Good - easier to fix.

The first step would be to replace all the capacitors on the board, including the smaller ones. Can you post a picture of your board?

In the pictures I posted above it's the third one down, with the large blue relay. If better pictures are required I will take some more when I get home from work.

Seriously, thank you for all the help. Troubleshooting PSU's is not something that I do often....

I will have a look at the caps on the board and see if I have any similar ones, or if my work does. I will also check the diodes.

[tom66]

Re: Rca l37wd12 lcd tv

In the pictures I posted above it's the third one down, with the large blue relay. If better pictures are required I will take some more when I get home from work.

Seriously, thank you for all the help. Troubleshooting PSU's is not something that I do often....

I will have a look at the caps on the board and see if I have any similar ones, or if my work does. I will also check the diodes.

Can you take a picture as a bird's eye view? It might be necessary to remove the board to do this.

Remember to only use low impedance (ESR) capacitors. In general I tend to recommend (as do others on this forum) Panasonic FR, FM or FC, with FR being the best.

[gelatinous]

Re: Rca l37wd12 lcd tv

Can you take a picture as a bird's eye view? It might be necessary to remove the board to do this.

Remember to only use low impedance (ESR) capacitors. In general I tend to recommend (as do others on this forum) Panasonic FR, FM or FC, with FR being the best.

I will take a new picture as soon as I'm home, top and bottom of the board. I will have to order parts as I don't believe I have the values necessary for the repair. That's not a big deal though as Digikey is next day and I have to put an order together for my RGB to YUV converter for my PS1

[gelatinous]

Re: Rca l37wd12 lcd tv

Alright, I spoke with a coworker that used to work in a television repair shop and he thinks that the 10 ohm resistor was too big of a load for the standby supply, he suggested measuring the standby supply and pressing the buttons on the TV to see if there is any appreciable drop in voltage on the standby line.

He also suggested there there might be a problem around where the main board looks fluxed, possible memory controller issue?

I'm going to be bringing the main board in to my work and examining it under our microscope for issues in that area, after I clean it tonight.

He also had a suggestion of unplugging the TV and using a meter probe to discharge any accumulated charge on the processor pins.

[budm]

Re: Rca l37wd12 lcd tv

At 5v output and with 10 1Ohms load, you will be only drawing 0.5A which is only 2.5Watts, most Standby power supply can handle 2A, if you do not feel safe, you can use 20 Ohms instead. But the power supply should be able to supply 0.5A and still be able to maintain steady 5V output.
So from your test result, you do have power supply problem, it can be the feedback circuits are not working properly or the main DC feeding the primary circuits are bad.
You may look here for the diagram of the power supply:
https://www.badcaps.net/forum/showth...648#post235648

[gelatinous]

Re: Rca l37wd12 lcd tv

My coworker did suggest that the feedback circuit could be a problem, specifically he was suspicious of the 3 lead Zener on the optocoupler. He said that a similar feedback method was used on the old RCA tube TV's that he's worked on. I will hopefully be able to inspect the boards at work tomorrow.

Thanks for the help!