ViewSonic N2751W dead power board

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

That was with the caps cold, resistance between VCC and GND = ~2.5K

P.S. checked the IC with a loupe, that's definitely the P/N on the chip

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Just heated the caps, no change.

When checking for +5VSTBY, the leads should go on +5VSTBY and the GND pins on the connector correct?

Also, how do I know that PS-ON is present?

[budm]

Re: ViewSonic N2751W dead power board

Just heated the caps, no change.

When checking for +5VSTBY, the leads should go on +5VSTBY and the GND pins on the connector correct?

Also, how do I know that PS-ON is present?

Either at the +5VSTBY pins and the GND pins of the connector, or at those points I marked in the picture.

You will have to have the board connected to the main board and activate the power switch, there is pin on the power supply board that receives the PS-ON signal, I do not know which exact pin that is yet but not going to worry about it until you have the +5STBY first.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Ok gotcha, will I need to hold the power switch while testing? If I just depress the power button, nothing happens.

[budm]

Re: ViewSonic N2751W dead power board

Ok gotcha, will I need to hold the power switch while testing? If I just depress the power button, nothing happens.

At this point nothing is going to happen since you do not have the +5VSTBY to run the main board that is why you need to concentrate on that STBY power supply section first, no STBY power then nothing else will matter.
Please read my basic troubleshooting guide.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Ok, I will do that, so at this point I should be focusing on finding bad components like the rectifiers and caps, I will let you know what I find.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Bud, I've checked all the circled components on the board(except for the ICs), they all seem ok unfortunately.

Is there a way to test the actual Standby Transformer itself?

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

bump

[budm]

Re: ViewSonic N2751W dead power board

I doubt that if it is bad transformer, right now you are not getting the startup Voltage at the VCC pin to kick start the IC.
BTW, so what resistance do you get between the VCC pin and the GND pin.
I will also replace those two small lytics caps by the STBY SMPS IC unless you have ESR meter to verify them, I always replace the startup/running caps when I do the repair, especially the TV that old.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Good point....

I get 2.30 between VCC and GND, I will replace those caps tomorrow, no ESR meter for me just yet.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

The caps are 47 uf and 10 uf, both 50V, could I replace them both with higher uf 50V caps that I might have lying around?

[budm]

Re: ViewSonic N2751W dead power board

How about the resistance reading in both direction of the meter probes between VCC pin and the GND pin?
2.5K in both direction of the probes? That is kind of low resistance.
I wonder if the internal 32V Zener diode on the VCC pin may be bad.

[budm]

Re: ViewSonic N2751W dead power board

The caps are 47 uf and 10 uf, both 50V, could I replace them both with higher uf 50V caps that I might have lying around?

That will be fine for testing.

[budm]

Re: ViewSonic N2751W dead power board

The VCC has to reach the startup Voltage which is 16VDC, that startup Voltage comes from the 3 series connected resistors (R301, 302, 303), one end is connected to the + leg of the main filter cap, another end of the string is connected to VCC pin 7.
I cannot read the value of those resistors, you should test them out also.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

With the upgraded caps the values are still the same, VCC to GND in one direction shows 2.5K and in the other direction 2.2K, that doesn't seem like good news......

The 3 series connected resistors are all 3903s (390K). When tested in circuit they all start at around 250K and keep climbing, I assume they're being influenced by the main filter cap, should I remove them and test them?

I really appreciate you taking the time to help me with this BTW, I'm determined to get it sorted out, I've even found the LD7552BS so if you feel that could be the culprit I'll order one right away!

[budm]

Re: ViewSonic N2751W dead power board

Supply Voltage of 165VDC feeding the Voltage divider circuit that has three 390K Ohms (1.117M Ohms) resistors in series feeding 2.5K resistor equivalent to GND, that is not going to give you 16V.
At this point it looks like you may have bad IC. You cn some how lift VCC pin off the board and then measure the resistance between the VCC pad and GND to see if it still show 2.5K or not, then measure the lifted VCC pin against the GND pin to see if it will show 2.5K or not .

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

I can do that right now, give me a min.....

[budm]

Re: ViewSonic N2751W dead power board

BTW, one end of R303 is connected to the VCC pin of the IC, right?
And if you look at the application circuit of the IC spec sheet, you can see how the IC is used, it is really close to what you have on your board.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Wow you're right, they are extremely similar.

You are correct, R303 is connected to the VCC pin of the IC.

So GND to the VCC pad showed 1.200 and rising, I'm assuming because it's connected to the main filter cap?

GND to the VCC pin showed 3.10, not much of a difference. When I removed the IC to lift the VCC pin I thought there was a burnt spot but it turned out to be some sort of glue:

[budm]

Re: ViewSonic N2751W dead power board

What is that 1.200 reading for the VCC pad (VCC pin not connected) and gnd? Ohms, K Ohms?
IC pin vcc and gnd pin shows 3.1K Ohms, I would say that it is bad.