Dell E173fpf - No led coming on

[quattro alex]

Model: Dell E173fpf

Fault: No power led comes on and nothing is displayed on the screen at all.

Observations: constant ticking noise heard from T850 and I get a constant 131v from the primary capacitor (using mains earthing point)

I have changed cap C506 (4.7uf 16v) as the ESR was slightly high - 2.1
I have also changed D854 (SRF 10-10) as I had seen on another post that this part was faulty and before I replaced it the solder points looked slightly different compared to the rest, maybe due to it running hot?.

I have checked the board and cannot see any burn mark at all. I have checked the larger resisters and these are fine. D850 which I think is known as a bridge rectifier tested fine.

Questions: If I put my negative lead only from the DMM on the primary negative I get some fluctuations in voltage normally between 0.5 to 6v's so what could I do to get an accurate reading from the pins on U850?

Could someone please confirm what Q850 does and how I might test this component?


Many thanks

Ps: I am aware that there is some excess flux which I didn't wipe off prior to taking the photo's.

[retiredcaps]

Originally Posted by quattro alex

Observations: constant ticking noise heard from T850 and I get a constant 131v from the primary capacitor (using mains earthing point)

1) Is the 131V that you measured AC or DC? Either way, that is super low. Try measuring DC voltage across the legs of C854. Careful, this will be high DC voltage. If it is working properly, it should be mains x 1.414 DC volts.

2) Ticking suggests

a) bad startup cap (likely C857 - 47uF 50V cap?)
b) bad SMPS IC U850
c) shorted component like a diode (check D851, D852, and D853)

Could someone please confirm what Q850 does and how I might test this component?

3) Look for the Q850 datasheet. This is your SMPS IC. Once you find it, look for the "typical" VCC startup voltage. It might be something like 16V. C857 feeds the Q850 VCC pin. Measure the VCC pin on Q850 with respect to Q850's GND pin.

4) See if Q850 VCC is shorted to GND.

5) Measure the DC voltage across C857. It should be steady.

6) Those are excellent clear focused photos. Easily in the top 5 best photos this year.

[retiredcaps]

Originally Posted by retiredcaps

3) Look for the Q850 datasheet. This is your SMPS IC. Once you find it, look for the "typical" VCC startup voltage. It might be something like 16V. C857 feeds the Q850 VCC pin. Measure the VCC pin on Q850 with respect to Q850's GND pin.

4) See if Q850 VCC is shorted to GND.

Sorry for #3 and #4, I meant U850 (the 8 pin IC) is likley your SMPS IC.

Q850 is your power mosfet (3 pin IC mounted on heatsink). You can see if the power mosfet is shorted by measuring the resistance between D-S, D-G, G-S.

[quattro alex]

Retiredcaps - that to do list has really helped, here's what I've got:

1) The 131v was DC on the primary cap. Measuring from the legs (solder side) I get 340v dc steady.

2a) C857 is a 10uf 50v - tested at 0.87esr 9.72uf, live test has dc voltage fluctuating up and down between 13-19v

b) diode / continuity test of U850 pins 1 to 7 gave a reading of 477 on the DMM - I am assuming this is ok?

c) diode / continuity test : D851 - 494
D852 - 472
D853 - 498

3) U850 has these part no's (used a DSLR sensor loupe for reading - very handy!!) on it SG6841S & KA126690501E I have located the datasheet and included some diagrams.

datasheet specs:
Min Typical Max
U850 - startup threshold voltage 15 16 17
- minimum operating voltage 9 10 11

I tested the resistance on Q850. I get 94K between G&S but testing S-D or D-G the resistance keeps changing normally increasing past the 200k mark.

What do you recommend next?

[retiredcaps]

Originally Posted by quattro alex

Retiredcaps - that to do list has really helped, here's what I've got:

That is why I write enumerated paragraphs.

1) The 131v was DC on the primary cap. Measuring from the legs (solder side) I get 340v dc steady.

Assuming there is no PFC, 340V DC implies 240V AC mains.

2a) C857 is a 10uf 50v - tested at 0.87esr 9.72uf, live test has dc voltage fluctuating up and down between 13-19v

The ESR looks good when compared to a Panasonic FC (1.3 Ohm).

b) diode / continuity test of U850 pins 1 to 7 gave a reading of 477 on the DMM - I am assuming this is ok?

Since I don't know what the 477 unit is, I would prefer to see a traditional resistance measurement versus a diode/continuity. Retest and report a resistance measurement.

c) diode / continuity test : D851 - 494
D852 - 472
D853 - 498

What is the measurement the other way? If it is out of range, then the diodes are good.

I tested the resistance on Q850. I get 94K between G&S but testing S-D or D-G the resistance keeps changing normally increasing past the 200k mark.

Q850 is not shorted.

[quattro alex]

Assuming there is no PFC, 340V DC implies 240V AC mains.

Your correct I am on 240v ac

Since I don't know what the 477 unit is, I would prefer to see a traditional resistance measurement versus a diode/continuity. Retest and report a resistance measurement.

The resistance goes from 30k increasing through to about 330k then it starts decreasing. This is for pins 1-7 (positive on pin 1)

What is the measurement the other way? If it is out of range, then the diodes are good.

They are out of range when tested the other way.

Q850 is not shorted.

Thanks for confirming this.

[retiredcaps]

Originally Posted by quattro alex

The resistance goes from 30k increasing through to about 330k then it starts decreasing. This is for pins 1-7 (positive on pin 1)

1) Okay, I have one more test that I can think right now.

Measure the resistance between pin 1 (GND) and other pins. That is, 1-2, 1-3, 1-4, 1-5, 1-6 and 1-8. Report all readings.

2) If none of them are shorted, my best *guess* is that SMPS IC is bad even though it is not shorted. If you search this forum for SG6841, you will see similiar cases where this IC is bad.

If you have to order a new SMPS IC, I suggest replacing the startup cap with a Panasonic FC even though it tested fine with your ESR meter.

SG6841 is available on ebay for $2.50 + $1.00 shipping.

[retiredcaps]

Originally Posted by quattro alex

Your correct I am on 240v ac

I could be wrong, but I don't think this monitor (due to its age?) uses PFC otherwise the DC voltage should be in the 390V DC range.

[quattro alex]

Measure the resistance between pin 1 (GND) and other pins. That is, 1-2, 1-3, 1-4, 1-5, 1-6 and 1-8. Report all readings.

1-2 = 78.2K
1-3 = 30K-->130K+
1-4 = 26K
1-5 = open
1-6 = 41ohm
1-7 = 90ohm --> open
1-8 = 172ohm -->open

I could be wrong, but I don't think this monitor (due to its age?) uses PFC otherwise the DC voltage should be in the 390V DC range.

I think it was made in 2005 and even though the mains is probably closer to 230v I take this is outside the correct spec?

[budm]

You may want to download one of these service manuals:
http://elektrotanya.com/?q=showresul...oria=&kat2=all

[budm]

Please see the notes in the pictures, see what happen when you lift one end of the resistor and let us know what happen.
What I am trying to do is try to remove one load at a time to find out if the loads are causing the power supply to go into shutdown, or the power supply itself is having the problem.

[retiredcaps]

Originally Posted by quattro alex

1-6 = 41ohm

I think this is shorted, but I can't be sure since I don't have this SMPS IC.

[quattro alex]

Please see the notes in the pictures, see what happen when you lift one end of the resistor and let us know what happen.
What I am trying to do is try to remove one load at a time to find out if the loads are causing the power supply to go into shutdown, or the power supply itself is having the problem.

Ok I lifted one end of the resistor at R870 ( and may have pulled part of the pad unfortunately at the same time) and the ticking still persists. I get no voltage at all when testing to the ground point or between the legs of L853.

[budm]

OK, the shorted circuit is not in the logic circuits side, you will have to remove the round inductor and see if the problem goes away next, you have to use good 40watts or more with large tip (2.5~3mm wide tip) soldering tool to work on this double sided circuit board, otherwise you will be damaging the traces on the board.

[quattro alex]

OK, the shorted circuit is not in the logic circuits side, you will have to remove the round inductor and see if the problem goes away next, you have to use good 40watts or more with large tip (2.5~3mm wide tip) soldering tool to work on this double sided circuit board, otherwise you will be damaging the traces on the board.

I have removed the inductor but the ticking is still there. Also primary cap DC voltage is fluctuating between 280 and 330volts.

[budm]

Originally Posted by retiredcaps

I think this is shorted, but I can't be sure since I don't have this SMPS IC.

That pin 6 of the IC should be connected to the resistor (R859)Source leg of the MOSFET through R861 (value as printed on the body?) Can you tell us the value of R861 that is connected to Pin6?
OK, I see that the R861 is 470 Ohms, so you are reading 41 Ohms, it must have an internal short inside the IC as retiredcaps has suspected. You can try lifting pin six and measure the resistance between PIN 6 and Pin 1 again.

[quattro alex]

Can you tell us the value of R861?

470 is the value and when I test the resistance with the DMM on the 200 scale I get 47.0

[budm]

47 ohms is correct, I thought I saw 471 as printed but not sure if I was seeing it right.
I do not belive you will be reading 41 Ohm between pin6 and Pin 1 if the resistor is 47 Ohms.

[quattro alex]

I do not belive you will be reading 41 Ohm between pin6 and Pin 1 if the resistor is 47 Ohms.

Your correct - I've just measured again and the reading was 48.2 (-0.07 for leads resistance) = 47.5ohm

[budm]

OK pin 6 is good.