Dell E173FPf with Su'scon caps

[yotefn]

Hi, monitor has no power at all, no LED at power button.
Attached are pictures of the board, the video and power are on the same board.
Probing with a meter, I get 169VDC at the big filter cap, and at the primary connections at T850. I do not measure any other voltages anywhere else on the board, including any direction on the secondary of the transformer. My guess at this point would be the transformer is shot. It's marked "SPW-039 TDK X 0434". I have no idea what the output power or current would be.

2 Questions:
1)Does anyone have any ideas what might be wrong or what else I should measure?
2)Without a datasheet posted online for the transformer, anyone know how to replace it?

Thanks

[selldoor]

Check the resistances of the secondary windings on the transformer

[jetadm123]

Try reading the DC voltage of the vcc pin of U850. Sorry , I can't read the part # of U850, so you'll have to look up the spec sheet yourself.

[selldoor]

I have been reading other posts
http://www.badcaps.net/forum/showthread.php?t=9297

http://www.badcaps.net/forum/showthr...hlight=E173FPf


They have both had problems around R852 q850 and one person came in and said they fixed one by replacing these ,so perhaps you should check yours.
Spec sheet may be in one of these threads

[retiredcaps]

Originally Posted by jetadm123

Try reading the DC voltage of the vcc pin of U850.

And the DC voltage across C857. Note if voltages are stable or fluctuating.

[yotefn]

Woa! After seeing this page and billy66's post:

http://www.badcaps.net/forum/showthr...ht=u850&page=2

Monitor wont power on. No LED, no picture, nothing. The transformer T850 clicks once or twice (sounds like it is charging up). Voltages are as follows:

- Filter cap: 156V
- U850 Pin 1- 0
2- 2.6V
3- 24.8v
4- 1.29v
5- 4.5v
6- 0.75v
7- 24.7v
8- 0.25v

I measured my U850, which I assume is the PWM chip. Other than pin 1, I think every pin measured -68.7VDC (negative), except for one pin which measured positive same voltage.

The center pin on Q850 was also -68.7VDC

Now I'm no electronics geek, but after rectification why would I be measuring negative voltage? If the whole thing is shorted out I think I'll just call it a day, the monitor only sells for $50 on ebay.

[retiredcaps]

Originally Posted by yotefn

Other than pin 1, I think every pin measured -68.7VDC (negative), except for one pin which measured positive same voltage.

The center pin on Q850 was also -68.7VDC

Now I'm no electronics geek, but after rectification why would I be measuring negative voltage?.

Negative voltage is a result of you using the wrong ground for your black probe. There is a hot ground and cold ground. Usually the cold ground are the ground screws and chassis. I normally use the negative leg of the large filter capacitor for hot ground.

[yotefn]

OK, that's good to know.

Here's a list of measurements:

Across primary and secondary windings of transformer is non-infinate resistance so I guess thats good for now considering all the other bad measurements.

Using filter cap as ground:

to cold ground: 67VDC

I850 Pins 1 and 2, U801, U802 U851: 67VDC (all pins of these same as cold ground)

I850 Pins 3,4: 0V

U850
Pin 1,2,4,6: 0VDC
Pin 3: 4.4
Pin 5: 0.1 (consistently bumped meter slightly)
Pin 7,8: 1.7

Center of Q850: 170VDC

Across c857: 1.7VDC

U850 part number SG6841S

http://pdf1.alldatasheet.com/datashe...C/SG6841S.html

Any ideas?

[retiredcaps]

Originally Posted by yotefn

U850
Pin 1,2,4,6: 0VDC
Pin 3: 4.4
Pin 5: 0.1 (consistently bumped meter slightly)
Pin 7,8: 1.7

Across c857: 1.7VDC

In order for the SMPS IC to start, it requires 16V DC (pin 7).

Either C857 is bad or the SMPC IC is bad. Start by replacing C857 with something like a Panasonic FC series cap and retest.

[yotefn]

Recapped C857, no change.

Analyzing the datasheet and looking at the voltages on U850 at pins 7 and 8, I checked continuity with the IC on the board of those two pins and they were shorted. So I removed the IC and checked again and they're shorted. Ordered a new IC on ebay from Hong Kong (no one reputable carries the chip anymore), will replace and report.

Thanks for your help.

[retiredcaps]

Originally Posted by yotefn

Analyzing the datasheet and looking at the voltages on U850 at pins 7 and 8, I checked continuity with the IC on the board of those two pins and they were shorted.

Good job finding the short. Yes, if you can't find it at digikey or mouser, ebay is a good alternative.

[yotefn]

Alright, replaced U850, but now I have fluctuating voltage across C857 (equal to pins 1 and 7 on the SMPS) between 12.1V and 15.8V, with no power to other pins at all, other than pin 3 at 18V but fluctuating slightly. Replaced cap C857 again with no change.

[retiredcaps]

Follow along with this thread. There are a number of suggestions that you can try.

http://www.badcaps.net/forum/showthread.php?t=20469