Vizio VL370M power supply troubleshooting

[aaronwt6]

Ok everyone. I've got two of these identical power supplies here with the same issue. Tired of replacing boards on most of these sets and ready to try more component repairs. Both of these boards have 5V-STBY but will do nothing else. I can apply power to the PS on and inverter on and get no 12V or Backlight. Where should I start on it?

I replaced one on another set months ago and have the other set here waiting on this repair.

Oh and the filter caps show 170V in either state. Standby or with power applied to one or both pins.

Thanks as always,

Aaron

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Oh, and I'll add that this is an LG power supply.

[budm]

Re: Vizio VL370M power supply troubleshooting

I would start looking at the PFC section since your voltage on the cap did not go up to 380VDC or so when PS_ON is applied.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Thanks budm. Where am I looking at on the board? I'm trying to familiarize myself with the terms here.

Thanks,

Aaron

[budm]

Re: Vizio VL370M power supply troubleshooting

There is a 8-pin SMD FAN7530 PFC IC at the bottom of the board, check the VCC pin and ground pin to see if it has any voltage on it when PS_ON is applied. Since this board has built-in Inverter, do not apply any BL_ON signal otherwise you will get high voltage arc at the lamp connectors if you run the board without the lamps attached.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Thanks! I wish I was at home right now to check but unfortunately its work until around 6:00 this evening. I will definitely test as soon as I get home.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Ok. So this seems odd but there is about -6V with reference to ground on pins 1, 3, 4, 5, 7, & 8. Does that make any sense?

Bad chip?

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

I'm confused. Rechecked and I'm getting -60V both on AC and DC on those.

[budm]

Re: Vizio VL370M power supply troubleshooting

What ground point are you using? When work on the Primary (HOT) side, you need to use the Negative leg of that the 450vdc (those two caps are connected in parallel) as the Ground ref point, do not use the chassis. You use the chassis as the ground ref point when you work in the secondary side (COLD) of the circuit.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Sorry. Amateur mistake. I have 1.2V on pin 1 and 0.8V on pin 3. Less than a tenth on the rest.

[budm]

Re: Vizio VL370M power supply troubleshooting

Pin 8 is the VCC pin, pin6 is ground. The board is marked to show the pins of the IC. Can you show me the picture of the bottom of board where you check that IC?
If all the readings against Ground are less than a volt, then it is not getting the power or it may be bad.
Next test will be the resistance testing between the Ground ref (negative leg of either C607 or C608) and all 8 pins.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Let me know if this is clear enough. Just taking these with my phone.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Lower left is pin 1. Impossible to see the mark in the photo.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

OK. Here's my readings:

Pin 1 -- 2 M Ohms
Pin 2 -- 51 k Ohms
Pin 3 -- Open
Pin 4 -- 1 k Ohm
Pin 5 -- 67.8 k Ohms
Pin 6 -- 0
Pin 7 -- 4.65 k Ohms
Pin 8 -- 3.9 Ohms

Thanks again for your help!

[budm]

Re: Vizio VL370M power supply troubleshooting

OK, it has shorted VCC pin, so it is either the IC or the components (caps, etc) that is connected to the VCC pin 8 is bad. Do you get the same reading on the other board?
You can try lifting pin 8 off the board and see if the resistance at the pad where pin 8 will still show low resistance, if not, then the IC has internal shorts.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Ok. I'll give that a shot this evening. The other board shows about the same result. 5 ohms on pin 8. Seems that they both have exactly the same issue.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Does anyone have a schematic for this board so I can trace the power coming to that pin if the feed is bad? Getting ahead of myself but thought I could be prepared this evening just in case. If the chip is bad I may be able to rob one off another PS.

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Ok. Removed the IC and now have 11.3V on that pad so I guess the chip is bad?

[aaronwt6]

Re: Vizio VL370M power supply troubleshooting

Figured I'd have another PS here with that same IC but no joy.

[budm]

Re: Vizio VL370M power supply troubleshooting

OK, it does have bad IC then, You can try measuring that pin 8 and the ground pin of that IC to see if it shows low Ohm. That pin 8 VCC hss internal Zener Diode that may have shorted out.
I have the board at home so it will awhile before I have time to draw out the diagram.