ViewSonic N2751W dead power board

[budm]

Re: ViewSonic N2751W dead power board

I can clearly see that one end of R303 is connected to pin 3 VCC, pin 7 is also VCC and connected to the startup running cap?.
I can see d304 Cathode connected to pin 7 VCC? And the anode is connected to???? gnd pin 1??. If it is then you need to check the resistance of that D304 also.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

It's M Ohms, and I have to correct my last post.......

I mistakenly lifted pin 7 which also happens to be VCC, I just re checked it with pin 3 lifted(the one that connects to the 3 series resistors and the filter cap).

When putting the leads to pin 1(GND) and pin 3 lifted (the correct VCC), I get open, so I'm guessing that means the IC is definitely bad.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

You are correct, it is connected to the + leg of the 10 uf 50V cap.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

so it looks like I need to order the LD7552BS?

https://www.utsource.net/ic-datashee...S-1702334.html

[budm]

Re: ViewSonic N2751W dead power board

I updated post 41, so please read it again.

[budm]

Re: ViewSonic N2751W dead power board

It's M Ohms, and I have to correct my last post.......

I mistakenly lifted pin 7 which also happens to be VCC, I just re checked it with pin 3 lifted(the one that connects to the 3 series resistors and the filter cap).

When putting the leads to pin 1(GND) and pin 3 lifted (the correct VCC), I get open, so I'm guessing that means the IC is definitely bad.

Yes, I think you found the problem.
D304 ZENER is good then (REF TO POST 41).

[budm]

Re: ViewSonic N2751W dead power board

so it looks like I need to order the LD7552BS?

https://www.utsource.net/ic-datashee...S-1702334.html

Yes, I would order more than one since that IC is used alot in other power supply too. I order lots of part from UTSOURCE before without any problems so far.

[budm]

Re: ViewSonic N2751W dead power board

BTW, it is much easier to see how the IC is connected on the board because you can see traces running under the IC when the IC is not installed.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board




Hope that helps, D304 shows 0.00 to OL in one direction, and 1.200 M Ohms falling to ~0.850 M Ohms in the other direction.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Last question for the evening, I've used UTSOURCE many times before and sometimes it can take a while for the pieces to arrive(for someone with no patience). I tried to match the LD7552BS to some ICs from mouser, is that too much of a long shot?

http://www.mouser.com/Semiconductors...roller&FS=True


It must be time for bed, I was way off with that link, this one is closer but there's no SOP-8 packages.......

http://www.mouser.com/Semiconductors...roller&FS=True

[budm]

Re: ViewSonic N2751W dead power board

Those are for PWM Motor drive circuits.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

I believe what little brain capacity I have is now depleted, thank you so much for your help, I will order that IC from UTSOURCE and update you when it's on the board!

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

Bud, thanks to your incredible diagnostic skills, I've replaced the STDBY SMPS IC and am now getting 5V at the standby points! Do I have the green light to start buttoning the monitor back up or is there anything else you want me to verify?

[budm]

Re: ViewSonic N2751W dead power board

Thanks for the follow-up on the repair, I would hook everything back up and see if the TV will work, if it does then I would recap the secondary side next.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

No problem, what exactly do you mean by secondary side?

[budm]

Re: ViewSonic N2751W dead power board

There are 7 lytics caps in the secondary (cold) side of the board.

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

ahhh I see, I'll definitely do that, I'lll test the monitor when I get home from work and let you know how it went!

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

The plot thickens....


I connected everything and turned the unit on, and it's working! About 20 seconds later, I see a nice trail of smoke coming from power supply board followed by the classic smell of burning PCB.

I immediately unplugged it, removed it from the monitor and found the culprit. I can take a picture later if you'd like but there's really no need, I found a shorted diode near the 12V/24V power supply but I can't seem to identify it.

It looks like a normal rectifier diode, with the markings S1 AR 62 on it....I wonder if it's a variation of the 1N4001?

[budm]

Re: ViewSonic N2751W dead power board

What is the designator as printed on the board, D????

[WHaThEFLuX]

Re: ViewSonic N2751W dead power board

D402