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

[jackal68]

I'm troubleshooting my Sanyo 40" LCD-40R50F where the standby red light will turn on for a second and then turn off. Once off, nothing happens to the tv when power is pressed. I need to unplug the a/c cord, wait a few minutes and the same thing happens again once I plug the cord back in.

Originally, I thought it was a bad power board, but after checking all the caps and diodes on the board, I was able to detect the standby 5v from the power supply. I also bypassed the main board connector so I was able to detect 24v from the power supply and for the backlight (and the backlight was turning on when I bypassed the main board), so I know the power supply is working fine.

When I disconnect the main board from the power supply, the 5v standby is constant. As soon as I plug the connector to the main board, the standby drops down to less than 1v. I believe the main board is faulty.

I am looking to troubleshoot the main board; having checked all the caps and diodes on the main board, I don't know what else to check. The main board is a 569MA0101A.

Does anyone have the schematics or know what else I can check?

thanks

[CapLeaker]

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

Either you find a schematic, or you trace the 5V rail on the main board. Without pics not much joy. If you disconnect the main board from PSU and measure the resistance on the 5V STBY pin on the main board to GND pin? What do you get?

[jackal68]

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

I have tried looking everything on the net for a schematic but I cannot find it. I have tested the resistance and it seems quite high (all in kilo-ohms).

BON - 12.7 Kohms
STB 5V - 5.2 Kohms
5V - 12.2 Kohms
Pwr on - 6.3 Kohms
12V - 10.1 Kohms
24V - 8.2 Kohms

I am attaching some pictures…hopefully someone can help me out more.

thanks

[CapLeaker]

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

good and bad news. Good news, the rail isn't shorted to GND. bad news... Maybe some IC that's fried or some voltage regulator that is using that 5v rail. I think I see a regulator on top left corner. Can you post high res pics? What happened to it? a lightning strike special?

[budm]

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

I would check the output pin of the Voltage regulator ICs to see if any of them show low resistance.

[jackal68]

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

I don't know what happened to the tv as I picked it up as is.

As for the ic regulator, here are the markings:

AZ1084S
ADJEI
037J12

As for checking the voltage, I'm not sure if I am doing it correctly: I reconnected the main board to the back of the tv and reconnected the cable from the power supply. When I plug in the a/c cord, and try to read the pins (left to right, 1,2,3), I get 0v on all three pins.

I'm not sure if I am doing this right.

let me know as I am attaching a high resolution of the regulator

[budm]

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

You check the resistance of the Regulator pin against GND, no power apply to the board. There is no other regulators on the bottom side of the board?

[jackal68]

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

The following are the reading I got:

Pin 1 - .08 Kohms
Pin 2 - 0 Kohms
Pin 3 - 5.21 Kohms

There are no other regulators on the board.

[budm]

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

Well,
Pin 1 is ADJ (for Adjustable regulator) or GND (for fixed regulator).
Pin 2 is the output pin
Pin 3 is the input pin
So the output pin is connected to a shorted circuit (load) or the internal output pin of the IC is shorted, shorted MLCC (Multi Layer Ceramic Capacitor), etc.
You will have to isolate what ever are connected to the output pin, you can try lifting the output pin first and see if the shorts will go away.
BTW, without the diagram to see what are the loads that are connected to the output pin will make it very difficult to troubleshoot.

[CapLeaker]

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

The following are the reading I got:

Pin 1 - .08 Kohms
Pin 2 - 0 Kohms
Pin 3 - 5.21 Kohms

There are no other regulators on the board.

sounds like you had the negative probe on the tab of the voltage regulator, which is connected to the the output pin 2 in this case. Anyway... it's fubar. So you need one of those. No wonder it is dragging the 5v rail down. Take it off or lift the output pin and redo measurement.

Are you sure there isn't anything on the backside of the board?
Right next to the big regulator there is another smaller something looking like having 3 legs and a tab. What is it?

Aahh crap... just read budm's post all the way through... oh well now you have it double!

[jackal68]

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

Thanks for all the help so far!

I had my negative probe on ground (screw hole metal) all the time.

Yes, I noticed another smaller regulator (thanks CapLeaker!). I measured the resistance on that and here are the markings and results:

01c (line 1)
1117 (line 2)

Pin1 - 55 ohms
Pin2 - 162 ohms
Pin3 - 5 ohms

[jetadm123]

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

1117 pin 3 is the input pin. Try lifting pin 3 off the board and take a resistance measurement from the pin 3 pad of the circuit board to ground. If the short goes away, then remove the 1117 and check for shorts between the pins.

[jackal68]

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

Hi

I lifted pin3 and took a resistance reading between it and ground - the short went away. So, I removed 1117 and checked resistance between all the pins and there is NO SHORT!! (1-2, 1-3, 2-3). I also checked between the tab and the pins and, as expected, it is open for pins 1,3 and shorted for pin2.

What else can I check?

[CapLeaker]

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

Well... Since you know now what is bad, replace it and fire the board up again. Cross your fingers that all is well.

[jetadm123]

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

Actually, with pin 3 lifted I wanted you to check the pad it was soldered to ground. Since you have removed the 1117, go ahead and check the resistance of each solder pad to ground.

[jackal68]

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

Okay. Here are the resistance reading from the pads to ground:

Pin1 pad - 55 ohms
Pin2 pad - 164 ohms
Pin3 pad - 0 ohms

CapLeaker, I don't follow you, as I don't think the regulator is bad since there are no shorts between the three pins on the regulator....should there be?

[budm]

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

I wonder if the output pin2 of the large regulator (1084) that shows as short is fed to the input pin 3 of the 1117. So if you remove both regulators and check the PAD to see if the shorts is still present or not.
There is not direct connection to any of the 3 pins of the ADJUSTABLE regulator to ground (the adj pin has resistor to ground but not 0 Ohms resistor) so if there are shorted circuit inside the IC, it will not show as O Ohm to GND. Fixed regulator has pin 1 connected to GND.

[jackal68]

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

I wonder if the output pin2 of the large regulator (1084) that shows as short is fed to the input pin 3 of the 1117. So if you remove both regulators and check the PAD to see if the shorts is still present or not.
There is not direct connection to any of the 3 pins of the ADJUSTABLE regulator to ground (the adj pin has resistor to ground but not 0 Ohms resistor) so if there are shorted circuit inside the IC, it will not show as O Ohm to GND. Fixed regulator has pin 1 connected to GND.

I removed the large regulator (1084) and tested the resistance between the pads of pin2 (1084) and pin3(1117) and it is shorted (still present). I am not sure if this helps, but I also tested the resistance between all the three pins of the large regulator and there are all open (not shorted).

I don't know what this means. Help?

[CapLeaker]

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

The output pad #2 of the 1084 should feed the input pad#3 of the 1117. Hope that helps

[jackal68]

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

in that case, the pads are good. As such, I don't know what else to check.....is this board toast? I can't find a replacement board.