Sanyo LCD-40R50F lcd tv shorted mainboard - need help

[jetadm123]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Now test the resistance from the pads of pin2 (1084) and pin3(1117) to ground. Is there still a short at each of those pads?

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Here are the reading from the pads to ground:

1117
pin1 - 55ohms
pin2 - 163 ohms
pin3 - 0 ohms

1084
pin1 - 76ohms
pin2 - 0 ohms
pin3 - OL

[jetadm123]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

With both regulators removed and the pads still showing shorted, that tells us that something other than the regulators are shorted on the 5V STBY line. I suggest you try tracing and measuring the resistance across the other components on that line for shorts.

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Any ideas how to trace the 5V STBY line without a schematics? I am looking at it visually and I don't know where to begin.

[CapLeaker]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Either with a very sensitive (multiple digit behind the dot) Ohm meter, a LeakSeeker, or cutting traces and unsoldering parts of that trace.
Not having any high res pics from the whole top and bottom of the board doesn't help either.

[vinceroger69]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

can he use a psu to apply power to the shorted line? like people do on laptops? (ive never tried this myself so not sure of voltage or amps to apply).

[CapLeaker]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

with a lab power supply maybe, where you can set CV and CC. Most don't have that.

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

I don't have a lab ps where I can control the voltage or amperage.

I have taken some of the caps off to look at the traces.....here are some high res pictures. Do these help?

[CapLeaker]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

have you taken another measurement with the caps off?

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

yes, and the measurements are still the same.

[budm]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

There are many SMD MLCC (Multi Layer Ceramic Cap) that are connected in parallel with the electrolytics caps you removed, for example C318 is in parallel with C329.
The MLCC usually failed as shorted circuit.

[jetadm123]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

In post #28, top of first photo, what is the part number of the 8-legged surface mount ic?

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

I guess I'll take off some of the MLCC to check whether they are shorted.

As for the 8 legged surface mount ic, the p/n is MP1482DS - how can I check this ic?

thanks for all the help so far!

[zomi]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

try touching them
If the caps are shorted the should be heating up
The ceramic ones..

Or freeze spray..

Something like the thermal imaging cameras would be ideal for this lol
But the too pricey..

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

I think the mlcc caps are too small to sense the temperature change. I took C331 out and tested the capacitance and it was 200nF... does that sound right, since it seems to be parallel to C325, which is 100uF. I'm confused.

Also, when I touch the pads of several of these capacitors that I took out (C325, C313, C323) to ground, both sides of the pads are shorted....any ideas?

thanks

[CapLeaker]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

you are getting closer. Now if you can find the item that actually is making the short, You Are Da Man!

Maybe note down the resistance values as you go along. As long as the meter shows little by little less resistance from the shorted trace to GND, you are getting closer. Your meter may or may not be that sensitive. As the resistance rises, you are going away from the short.

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Does anyone know what this ic (MP1482DS) does and how can I check whether it is shorted? I tested pins 3 and 4 and they are both grounded...could this be the problem? According to the data sheet, pin3 is the switch and pin4 is ground.

http://www.dz863.com/datasheet-83912...own-Converter/

Also, can I assume that the positive pads of the caps should not be shorted to ground?

[jetadm123]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Suggest you lift the leg of pin 3 off the pad, then measure resistance between pad and ground. If short is gone, then you've found the problem.

Assuming you're measuring both ends of the cap with respect to ground and seeing a short, then try measuring resistance across the cap. If it reads shorted, then remove that cap and test it again out of circuit.

[jackal68]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

Unfortunately, once i took out the ic (mp1428ds), the short was still happening on the pad of pin3 and ground....so I guess that was not the problem.

As for the caps...I am measuring the pads on the circuit board to ground (the caps have all been removed), and the positive side of these (C325, C313, C323) pads are shorted to ground.

[CapLeaker]

Re: Sanyo LCD-40R50F lcd tv shorted mainboard - need help

so there is still something else that goes from that line to ground that is shorted. It would take me less than 5 minutes to figure it out with my LeakSeeker, but unfortunately you don't have it, so you have to take everything off and on, until you find it. You may want to cut the trace and section it off that way.