Rca l37wd12 lcd tv

[gelatinous]

Re: Rca l37wd12 lcd tv

I have not yet replaced any caps on the 1W board. The 470uF cap was the only one that measured low at 300uF so I'll try replacing it first.

Unfortunately I don't have an ESR meter and my personal multimeter can't measure capacitance.

[budm]

Re: Rca l37wd12 lcd tv

2V per division on the scope setting using X10 probe, so you are getting almost 4v peak-to-peak. I would say yes, you do have bad power supply problem.
So did you replace any electrolytic caps on the board yet?

[gelatinous]

Re: Rca l37wd12 lcd tv

Ok, I fired up my scope and measured the output of the 5V standby with the main board connected. My results are below, along with my scope settings. I used a 10x probe.

I also measured the supply with the main board disconnected and saw an almost complete sine wave, there were small flat areas at the zero crossings but I didn't take a picture.

I also noticed that the waveform on the scope would change depending on the pitch of the tone I can hear coming from the 1W board, this made it difficult to trigger on.

I'm assuming that this is indicating a problem with the 1W board.

[tom66]

Re: Rca l37wd12 lcd tv

One interesting thing I noticed was how they drive one of the 1.8V regulators. 5V standby comes into the board and runs through 4 diodes in series to drop the input voltage to the regulator. I've never seen this done before, though my knowledge of power supplies is limited.

It's cheaper than a bigger voltage regulator or a buck converter. All in the name of cost savings, whether or not that is a good thing. Potentially, it shows the regulator is clearly going to be under some stress (having to dissipate quite a bit of heat) and adding the diodes was the fix that was needed, but it can't be guaranteed to last.

[gelatinous]

Re: Rca l37wd12 lcd tv

I will report back here with a screenshot of what I measure.

For the other voltages, it just threw me off because some of the 3.3V regulators come up, as well as a 2.5V (for memory I'm guessing).

I think this would be so much easier with a schematic of the main board, I haven't been able to find one anywhere.

One interesting thing I noticed was how they drive one of the 1.8V regulators. 5V standby comes into the board and runs through 4 diodes in series to drop the input voltage to the regulator. I've never seen this done before, though my knowledge of power supplies is limited.

[budm]

Re: Rca l37wd12 lcd tv

The other volatges you mentioned, 6.5v, 1.2, etc will be present when the relay is turn on and supply the 120Vac to the main power supply. The purpose of rhtis small 5v pwoer supply is to supply enough power for the contrller IC to run and process the signal fro the power switch on the TV or from the remote receiver to turn on the relay, that is why it has such small power. You can bypass the realy contact so the main power supply will be on and see if the TV will work, taht is how I found out the proble was in the main board, nit in my power supply board.

[tom66]

Re: Rca l37wd12 lcd tv

I do, it's a Tektronix T912, 10MHz analog scope. I will scope the standby voltage and try to get a decent picture of it.

Perfectly capable for the task. Set to 100mV/div, 5ms/div and AC coupled.

[gelatinous]

Re: Rca l37wd12 lcd tv

4.8V does seem a little low, especially as that's poor load regulation. Do you have a scope available?

I do, it's a Tektronix T912, 10MHz analog scope. I will scope the standby voltage and try to get a decent picture of it.

[tom66]

Re: Rca l37wd12 lcd tv

4.8V does seem a little low, especially as that's poor load regulation. Do you have a scope available?

[gelatinous]

Re: Rca l37wd12 lcd tv

This weekend I hope to be beeping out some traces and measuring the various voltage regulators on the main board. I will try and get some better pictures of the main board and note the designators and part numbers along with voltages.

[gelatinous]

Re: Rca l37wd12 lcd tv

Ok, I used 2x 68ohm in parallel and found the 5V to be ~4.8V that slowly charged to 5.

I have found that the 6.5V supply to the button board is not present, though I'm not sure that it is supposed to be until after power up as the 5V standby also goes to that board. I've found some of the regulators (3.3, 5) are not running, the 5V shows 1.52V and the 3.3V shows 0.19V, again, I'm not sure if they are supposed to be runningn or not.

The large diode on the main board would show 6.5V when the tv was working, but only ~0.5V when not working. Also, unplugging and plugging the tv in no longer causes it to turn on.

Just for the hell of it I decided to set my meter to ACV and found that my standby voltage is showing ~4VAC. I still have to hook my scope up to it to be sure.

I am unsure where to look next, it's either this standby supply, though the DC is stable... Or it's something on the main board.

[tom66]

Re: Rca l37wd12 lcd tv

measures 0.8 - 0.9 ohms if I account for the lead resistance of 0.1 - 0.2 ohms. meter read 0.9 to 1.0

Okay, that seems close enough. Lower is okay. Higher is not. Once you get the load resistor, check it is stable under load.

[gelatinous]

Re: Rca l37wd12 lcd tv

Does that 1.2 ohm resistor measure 1.2 ohms +/-5%? You might need to remove it to test it.

measures 0.8 - 0.9 ohms if I account for the lead resistance of 0.1 - 0.2 ohms. meter read 0.9 to 1.0

I don't think I have the parts here to replace it... most of the stuff at my work is SMD 0402, up to 1206. and 1/4W through hole. I will check my stash at home and see if I have any 1/2W parts.

[tom66]

Re: Rca l37wd12 lcd tv

Does that 1.2 ohm resistor measure 1.2 ohms +/-5%? You might need to remove it to test it.

[gelatinous]

Re: Rca l37wd12 lcd tv

How is the 22uf 200Vdc cap for the primary side checked out? Very good clear pictures! What do you use for lighting?

I haven't pulled that cap yet, but I will shortly and report back. Lighting is just the ambient lighting in the lab I'm in... just standard fluorescent lights and the camera is my Galaxy S Captivate lol.

is that a bad solder joint on bp610 or just the picture.

I think it's just the picture, but I'll remove and replace the solder to be sure.

EDIT: My bad, it's a 47uF and measures 44.8uF on my FLUKE 87

[vinceroger69]

Re: Rca l37wd12 lcd tv

is that a bad solder joint on bp610 or just the picture.

[budm]

Re: Rca l37wd12 lcd tv

How is the 22uf 200Vdc cap for the primary side checked out? Very good clear pictures! What do you use for lighting?

[gelatinous]

Re: Rca l37wd12 lcd tv

It is labelled "1W board". Maybe try 20 ohm as budm suggested or even 25-30 ohm... It could be current limiting. (I think I got a little confused when I worked out the power dissipation of the resistor, it's V^2/R.)

I will try a higher value resistor. I have the board with me at work and I measured the 470uF output cap, measured as 300uF.

Here are a couple pictures, there's no solder on the one cap because I had it out for measurement.

The 470uF 16V cap is a Lelon (?) 85°C H545 (M)
The other large cap is an Elite 22uF 200V 105°C ES(M)

[tom66]

Re: Rca l37wd12 lcd tv

It is labelled "1W board". Maybe try 20 ohm as budm suggested or even 25-30 ohm... It could be current limiting. (I think I got a little confused when I worked out the power dissipation of the resistor, it's V^2/R.)

[budm]

Re: Rca l37wd12 lcd tv

Diagram, should be close to what you have, the TL431 is the ref Diode and part of the feedback.