Dell 1707FPt No Power

[maybenever]

Re: Dell 1707FPt No Power

What brand was the 10uf 50v cap (yellow)?

Chemi-con KY

[retiredcaps]

Re: Dell 1707FPt No Power

Chemi-con KY

The only time I have heard KY fail is in the infamous Hanns-G 28 inch monitor. That monitor runs temperature hot and bakes all the caps on the power and main board.

My "downstairs" computer has an used Corsair PSU with KY caps inside.

[maybenever]

Re: Dell 1707FPt No Power

Connected boards together, AC power connected, my 5.25vdc fell like a rock to zero. Took them back apart, powered up, got the 5.25v, then it started falling to zero like in the beginning. What next?

[retiredcaps]

Re: Dell 1707FPt No Power

Well, either something is dragging down the 5V or something is failing, but not outright failed.

You could try replacing the caps, but I don't know if that will fix it.

I spent 20 months tracking down a shorted 5V rail. See

https://www.badcaps.net/forum/showthread.php?p=114994

[budm]

Re: Dell 1707FPt No Power

Please test Q107, 108.

[maybenever]

Re: Dell 1707FPt No Power

budm Re:
Please test Q107, 108.

It will be tomorrow before I can do the test.

What does D802 do? Markings are U4K8 80R K468. When my voltage is steady at output pin, I can track 123vac from power cord connector to this rectifer (I think that is what it is), beyond it I get 185v (I assume it's ac). When the 5.25v output starts droping, this voltage also drops, to like 75v, also the voltage across the large filter cap drops to 48-50 volts. Am I on to anything here.

[retiredcaps]

Re: Dell 1707FPt No Power

What does D802 do?

It is a bridge rectifier.

http://en.wikipedia.org/wiki/Diode_bridge

Markings are U4K8 80R K468. When my voltage is steady at output pin, I can track 123vac from power cord connector to this rectifer (I think that is what it is), beyond it I get 185v (I assume it's ac). When the 5.25v output starts droping, this voltage also drops, to like 75v, also the voltage across the large filter cap drops to 48-50 volts. Am I on to anything here.

We almost never need to measure AC. I have yet to see a problem attributed to household mains AC being bad.

All your measurements at the bridge rectifier should be DC volts. When you are on the hot side of the power supply, the negative leg of the large filter capacitor is ground.

Bridge rectifiers, when not working, are usually shorted and will blow the fuse right away.

[maybenever]

Re: Dell 1707FPt No Power

Budm

Test on Q107, lower pins left to right, pins 1 thru 3 = open, pin 4 = 1615ohm
Test on Q108,""""""""""""""""""""""""""""""", pins 1 thru 4, all = 1610ohm

[budm]

Re: Dell 1707FPt No Power

"When the 5.25v output starts droping, this voltage also drops, to like 75v, also the voltage across the large filter cap drops to 48-50 volts. Am I on to anything here."
Is the voltage on the large cap being measured right at the two legs of the cap? Fot it to drop that much, the load has to be tremendous which should have blown the main fuse, that large cap should be able to hold staedy since the inverter is not even running. The voltage on that cap should be around 165~170vdc.

[maybenever]

Re: Dell 1707FPt No Power

"When the 5.25v output starts droping, this voltage also drops, to like 75v, also the voltage across the large filter cap drops to 48-50 volts. Am I on to anything here."
Is the voltage on the large cap being measured right at the two legs of the cap? Fot it to drop that much, the load has to be tremendous which should have blown the main fuse, that large cap should be able to hold staedy since the inverter is not even running. The voltage on that cap should be around 165~170vdc.

When 5.25 is steady, measuring at the legs of the large cap, 168vdc. Sometimes the voltage will flip flop between 5.3v & 5.1v, when it does that, the large cap is 148 to 154vdc. Then if the voltage starts droping, it goes down to 48 to 50vdc. When I was holding the board in front of the heater, I thought it was the heat, but I think it was the way I holding the board, by the AC input socket, because as I was flipping it from front to back on the bench, I create the same voltage drop. So I reflowed, resolded all the legs on that end of the board, including the large cap. Since then, the voltage hasn't dropped, but it will flicker between 5.17 & 5.3v.
Is my mosfett bad Q107?

[budm]

Re: Dell 1707FPt No Power

I expect that large cap top be bad, if it is is bad MOSFET that draws too much curent, it will be either shorted out or not switching, but it is able to drive the transformer to produce 5v out put, it means it is swithcing but the 168V power supply is not steady to supply the constant voltage.
Is your meter True R.M.S? If it is, put it in AC mode and measure AC ripple voltage on the two legs of that large cap.

[maybenever]

Re: Dell 1707FPt No Power

I expect that large cap top be bad, if it is is bad MOSFET that draws too much curent, it will be either shorted out or not switching, but it is able to drive the transformer to produce 5v out put, it means it is swithcing but the 168V power supply is not steady to supply the constant voltage.
Is your meter True R.M.S? If it is, put it in AC mode and measure AC ripple voltage on the two legs of that large cap.

Don't know about my meter, it's the best you can get at WalMart for $20. GB Instruments I think. Didn't see your post until after I put this project up for the night, I didn't make that AC measurment. But get this, I order all new replacement caps, they came in today, but before I replace them, I'm checking one more time. Remember I resoldered all those joints on the AC socket end of the board, and after that, I couldn't get the voltage to drop like before. I powered up the main, voltage flickering beteen 5.1 & 5.3v, large cap = 167 & 168v, just like before. Reconnected both boards,& on/off buttons, checked voltage, steady 5.25v on pins 1 thru 4, no voltage on pin 9 thru 11. Large cap 168.7v steady. Hit the power button, and the led burned blue (WTF). Pin out voltage now, pin 1 thru 4 = 5.24v, pin 9 = 3.26v, pin 10 = 3.24v and pin 11 = 2.56v. Large cap = 167.8v, all steady as a rock. Hooked up two spare ccfl sets, they didn't light (they are known good, but for a 19" monitor). Sorry for the long post, tomorrow I replace caps and see what happens. I wish I had one of those mosfett, markings are P609 B06S4E.

[budm]

Re: Dell 1707FPt No Power

OK, so now the 5V is steady and the 168v is also steady, then it is OK, you had indicated that it was dropping like a rock. Now we need to find out if you do have 12~18vdc feeding the inverter section or not. Check the voltage on jumper J117 (or on C811 LEGS) and ground to see what you have.

[maybenever]

Re: Dell 1707FPt No Power

OK, so now the 5V is steady and the 168v is also steady, then it is OK, you had indicated that it was dropping like a rock. Now we need to find out if you do have 12~18vdc feeding the inverter section or not. Check the voltage on jumper J117 (or on C811 LEGS) and ground to see what you have.

It was dropping, until did all that reflowing. Thanks for the help, I'm back on it first thing in the morning. Should I go ahead and replace the rest of the caps, or do some more testing?

[budm]

Re: Dell 1707FPt No Power

I need to know if we do have B+ for the inverter circuit first.

[maybenever]

Re: Dell 1707FPt No Power

Now we need to find out if you do have 12~18vdc feeding the inverter section or not. Check the voltage on jumper J117 (or on C811 LEGS) and ground to see what you have.

23.5 vdc at J117 23.6vdc at C811 But this time, power button would not work, no led, no voltage at pin 10, which I think is the BL/ON. no voltage at pin 9 or 11, but other voltage are steady.

[retiredcaps]

Re: Dell 1707FPt No Power

It was dropping, until did all that reflowing.

I fixed a Dell 19 inch once by simplying reflowing the solder joints at the AC plug.

Do more testing before changing out the caps. As I said before United Chemi-con KY and Rubycon are generally good caps.

[maybenever]

Re: Dell 1707FPt No Power

Test on Q107, lower pins left to right, pins 1 thru 3 = open, pin 4 = 1615ohm
Test on Q108,""""""""""""""""""""""""""""""", pins 1 thru 4, all = 1610ohm

I made a mistake on this report, those reading were 1715 ohm & 1710 ohm. Q107 is not a good reading, right.

[budm]

Re: Dell 1707FPt No Power

At least now we know we have steady but voltage for the inverter seems high for 17" monitor. But at this point we need to get your main board working again. I would retrace your works to find out what happen now that you have no control, no LED.

[maybenever]

Re: Dell 1707FPt No Power

I haven't done anything to it for a few hours. When I tried to turn it off earlier, it wounld't power off, I had to ulplug it. Now it won't turn back on. May be a button think as all voltages are steady with boards connected.