Dell E171FP No power

[Welchs101]

Hi,

I am working on a Dell E171FP. Prior to this post i had posted a few earlier comments on this repair at this bad-caps forum link.

https://www.badcaps.net/forum/showthread.php?t=8951

I received the replacement parts yesterday and i put them in. I put in the new bridge rectifier, two new fuses, and a replacement thermistor SCK 103.

Note: I had to "drill" the holes for the bridge rectifier slightly bigger for the leads to fit in.

I put all the parts in with the board not connected to lamps or screen.....just the board on the table. I powered it up............did not hear any popping of the fuses so i thought this was a good sign.

I measured across the "Big" filter cap and i get about 170V DC.

Not sure what to measure next? Anyone have any ideas?

[PlainBill]

Re: Dell E171FP No power

Hi,

I am working on a Dell E171FP. Prior to this post i had posted a few earlier comments on this repair at this bad-caps forum link.

https://www.badcaps.net/forum/showthread.php?t=8951

I received the replacement parts yesterday and i put them in. I put in the new bridge rectifier, two new fuses, and a replacement thermistor SCK 103.

Note: I had to "drill" the holes for the bridge rectifier slightly bigger for the leads to fit in.

I put all the parts in with the board not connected to lamps or screen.....just the board on the table. I powered it up............did not hear any popping of the fuses so i thought this was a good sign.

I measured across the "Big" filter cap and i get about 170V DC.

Not sure what to measure next? Anyone have any ideas?

Measure the outputs of the supply (probably 5 VDC and 12VDC). If there is no output check all the diodes to the left of the transformer and near the heat sink. If those are good, replace C6, C8, and the cap to the right of the lower heat sink.

PlainBill

[Welchs101]

Re: Dell E171FP No power

Ok....I am enclosing a pic ture of the main board with one of the parts surrounded by a red box. Is this the part you want me to measure outputs on?

I looked up this part (i also took a side view picture of part). The markings are "DP502T". I could not find this part but i did find a web address that mentioned this part.

http://www.electronicspoint.com/sync...es-t64642.html

In this web address they say this is a fairchild part similar to this "KA5Q12656RTYDTU ". I found this part on mouser and the datasheet is here


From the data sheet:
Pin1=Drain
Pin2=Gnd
Pin3=Vcc
Pin4=FB (not sure what these are)
Pin5=Sync (not sure what these are)


I measured the following:
Pin1=166.8V DC
Pin2=did not measure....used this as a gnd to measure other voltages
Pin3=11.46V DC
Pin4=0.63V DC
Pin5=4.96V DC


What is this device? Are the voltages i measured ........what they should be? Hard to tell from datasheet.

[Krankshaft]

Re: Dell E171FP No power

That is not just a FET but it is also the PWM IC.

It is a switching FET, PWM, and protection on one chip that's why it has 5 leads. So it seems that Topswitch isn't the only manufacturer making these anymore.

Manufacturers use these compact switcher ICs to reduce the part count. All the better because it simplifies the troubleshooting process.

Measure the secondary voltages as Plainbill said if you get nothing.

If the bridge rectifier died then there's a chance the PWM section of the chip got zapped through the VCC pin. Sometimes fuses just aren't fast enough to stop semiconductor damage.

[Welchs101]

Re: Dell E171FP No power

What exactly are the secondary voltages? Just want to make sure i measure what it is your asking for.

Can i assume the DP502T is operating correctly?

Thanks again for your help.

[Krankshaft]

Re: Dell E171FP No power

When the PSU is plugged in measure for DC on the filter caps off the rectifiers the two packages on that heatsink.

Not sure how many rails this PSU has maybe one 12V and one 5V.

But in the past I've seen cheap Chinamart PSUs parallel output rectifiers to get more output current. So without a look at the board it could be just one 12V rail.

You can't assume anything about the PWM/FET until you check the outputs.

If the output voltage is low it's a feedback problem if the voltage is non existent it could be the IC/FET or an issue in the startup circuit like a startup cap or resistor.

Whenever you have a blown fuse

[Welchs101]

Re: Dell E171FP No power

I already measured DC voltage across the big filter cap......~ 170V DC.

As far as the "two packages on the heat sink" are you talking about the parts i have drawn a box around (see enclosed pic). ARe these the devices you wanted me to measure?

Also, i did measure the DC voltages on the PWM
I measured the following:
Pin1=166.8V DC
Pin2=did not measure....used this as a gnd to measure other voltages
Pin3=11.46V DC
Pin4=0.63V DC
Pin5=4.96V DC

[PlainBill]

Re: Dell E171FP No power

!!!! There are two ways to solve this.

1. Measure the voltages across the caps on the right side of the board - the six caps in a group. You should get two sets of voltages - one about 5 volts, the other probably about 12 volts.

2. Measure the voltages on the cable linking the power supply to the logic card.

The power LED is controlled by processors on the logic card. I spent too many hours assisting people who thought they had a power supply problem when it was actually a reset controller problem.

PlainBill

[Welchs101]

Re: Dell E171FP No power

(See enclosed pic as a reference)

Voltage across caps:
DC Volts across 1000uF/25V caps: Voltage same across all 3. With red probe on + and black probe on -. I measured 7.71V DC

DC Volts across 2200uF/10V and 470uF/25Vcaps: Voltage same across all 3. With red probe on + and black probe on -. I measured 4.411V DC


I also measured the voltages at the connector between powersupply board and logic board (see pic for reference):
At 1: 4.11V DC
At 2: 3.18V DC
At 3: 0V DC
At 4: 0V DC
At 5: 0V DC
At 6: 0V DC
At 7(Blue): 7.71V DC

[Welchs101]

Re: Dell E171FP No power

I think the 7.71 is kinda low.

Any ideas?

[PlainBill]

Re: Dell E171FP No power

(See enclosed pic as a reference)

Voltage across caps:
DC Volts across 1000uF/25V caps: Voltage same across all 3. With red probe on + and black probe on -. I measured 7.71V DC

DC Volts across 2200uF/10V and 470uF/25Vcaps: Voltage same across all 3. With red probe on + and black probe on -. I measured 4.411V DC


I also measured the voltages at the connector between powersupply board and logic board (see pic for reference):
At 1: 4.11V DC
At 2: 3.18V DC
At 3: 0V DC
At 4: 0V DC
At 5: 0V DC
At 6: 0V DC
At 7(Blue): 7.71V DC

You're right, 7.71 is very low. Also, 4.11 is slightly low. That points to an excessive load somewhere. Does the inverter connect to the logic card?

PlainBill

[Welchs101]

Re: Dell E171FP No power

Yes, it does. How did you know that? I have included a picture for reference.

[PlainBill]

Re: Dell E171FP No power

Yes, it does. How did you know that?

I'm smarter than I look.

I have included a picture for reference.

If you took those voltage readings with the inverter hooked up, try disconnecting it and see if they improve.

PlainBill

[Welchs101]

Re: Dell E171FP No power

I disconnected the main board from the power supply. This means the power supply is not connected to either the inverter board or the logic board.

Voltage across caps:
DC Volts across 1000uF/25V caps: Voltage same across all 3. With red probe on + and black probe on -. I measured 12.2V DC (was 7.71)

DC Volts across 2200uF/10V and 470uF/25Vcaps: Voltage same across all 3. With red probe on + and black probe on -. I measured 4.98V DC (was 4.11)


I also measured the voltages at the connector between powersupply board and logic board (see pic for reference):
At 1: 4.98V DC (was 4.11)
At 2: 0.034V DC (was 3.18)
At 3: 0V DC (was 0)
At 4: 0V DC (was 0)
At 5: 0V DC (was 0)
At 6: 12.2V DC (was 0)
At 7(Blue): 12.2V DC (was 7.71)


What does this mean? Does this mean the power supply board is ok and there is something wrong on either the main or inverter board?

[PlainBill]

Re: Dell E171FP No power

I disconnected the main board from the power supply. This means the power supply is not connected to either the inverter board or the logic board.

Voltage across caps:
DC Volts across 1000uF/25V caps: Voltage same across all 3. With red probe on + and black probe on -. I measured 12.2V DC (was 7.71)

DC Volts across 2200uF/10V and 470uF/25Vcaps: Voltage same across all 3. With red probe on + and black probe on -. I measured 4.98V DC (was 4.11)


I also measured the voltages at the connector between powersupply board and logic board (see pic for reference):
At 1: 4.98V DC (was 4.11)
At 2: 0.034V DC (was 3.18)
At 3: 0V DC (was 0)
At 4: 0V DC (was 0)
At 5: 0V DC (was 0)
At 6: 12.2V DC (was 0)
At 7(Blue): 12.2V DC (was 7.71)


What does this mean? Does this mean the power supply board is ok and there is something wrong on either the main or inverter board?

First of all, there are no absolutes with the limited test you just did. A more refined test would involve putting a 2 ohm power resistor across the 5V output and testing the power supply at full load. But the odds are that the power supply is fine, so let's go with that.

The next test would be to hook the logic card back up to the power supply, but leave the inverter disconnected. Then repeat measuring the voltage. There is no need to measure the voltage at the caps, measuring at the connector is fine. If the voltages are still 5V and 12V it improves the odds that the problem is in the inverter (Case A). If it drops again, then it is either the power supply or the logic card (Case B).

In Case A, hook the inverter back up and carefully feel for any point that is getting unusually warm. I'm guessing that the bad part is dissipating something in the order of 5 watts.

In Case B, it is not necessary to hook the inverter up, just check the logic card for any part that is getting very hot.

Do this fairly quickly; don't leave the power supply on for more than a minute when the 12V output is down around 7 volts.

PlainBill

[Welchs101]

Re: Dell E171FP No power

ok. With logic board connected and Inverter NOT connected i measure the following:Voltage across caps:

I measured the voltages at the connector between powersupply board and logic board (see pic for reference):
At 1: 4.11V DC
At 2: 3.18V DC
At 3: 0V DC
At 4: 0V DC
At 5: 0V DC
At 6: 0V DC
At 7(Blue): 7.71V DC

I will not try and find the part that is getting hot. Will post what i find in just a min.

[Welchs101]

Re: Dell E171FP No power

ok. With power supply and main board connected (no inverter connected) I touched around the logic board and i could not find anything "hot".

[PlainBill]

Re: Dell E171FP No power

ok. With power supply and main board connected (no inverter connected) I touched around the logic board and i could not find anything "hot".

Darn!! Maybe it is the power supply. One more thing to check: What is the voltage at pins 3 and 4 of the 'power switch' (the 5 pin device you measured in post 7) with the logic card disconnected and with it connected?

PlainBill

[Welchs101]

Re: Dell E171FP No power

Bill,

I measured the following (using the black probe set to the AC ground. Gnd coming from AC plug)

With logic board connected:
Pin1: -81.5V DC
Pin2: -81.4
Pin3: -70.6
Pin4: -81.6
Pin5: -76.5

With logic board DIS-connected:
Pin1: -81.0V DC
Pin2: -81.7
Pin3: -64
Pin4: -81.3
Pin5: -76.5


Did i use the right gnd.........or such i use pin2 (gnd) measure relative voltages. Recall that according to this



pin 2 should be gnd.

[Welchs101]

Re: Dell E171FP No power

I measured the voltages again this time using the gnd pin from the device to measure relative voltage levels.

Note: Inverter board is not connected for any of the measurements. Same as in previous post.

With logic board connected:
Pin1: 167.8V DC
Pin2:
Pin3: 11.45
Pin4: 0.061
Pin5: 4.96


With logic board NOT connected:
Pin1: 168V DC
Pin2:
Pin3: 18.24
Pin4: 0.331
Pin5: 4.96