Dell 1707FP no power

[bob12345]

Hello,

I have a Dell 1707FP which when connected to mains power shows no sign of life - no power light nothing and no response when pressing the buttons. I have taken the monitor apart and there appears to be no blown or damaged capacitors. Does anyone have any ideas as to what could be wrong with it? I have tested the LCD panel itself with another monitor and that works fine.

Thanks

[pedro]

Re: Dell 1707FP no power

Hello,

I have a Dell 1707FP which when connected to mains power shows no sign of life - no power light nothing and no response when pressing the buttons. I have taken the monitor apart and there appears to be no blown or damaged capacitors. Does anyone have any ideas as to what could be wrong with it? I have tested the LCD panel itself with another monitor and that works fine.

Thanks

There's no rampant signs of cap failure to me in the pics or the symptoms.

First - power OFF - check the red fuse to the left of the AC inlet for continuity, then if that passes measure (carefully) - power ON - the voltage across the large 450V cap to the right

[bob12345]

Re: Dell 1707FP no power

I've measured the resistance across the red fuse (T 5A 250V) with power off, on the lowest resistance setting on the digital multi meter (200ohm setting) I get a reading of 0.4 which to me sounds like a good fuse? (low resistance but not open circuit). The highest setting on the DMM (2000Kohm) shows 0 probably as the fuse is low resistance (i assume - correct me if i'm wrong).

When testing the 450V cap, do I need to connect the rest of the parts, ie the logic board, inverters, LCD etc? or can I just plug in the AC to the board as pictured? I also assume that the DMM will need to be set to AC as this cap is on the AC side? Thanks

[pedro]

Re: Dell 1707FP no power

I've measured the resistance across the red fuse (T 5A 250V) with power off, on the lowest resistance setting on the digital multi meter (200ohm setting) I get a reading of 0.4 which to me sounds like a good fuse? (low resistance but not open circuit). The highest setting on the DMM (2000Kohm) shows 0 probably as the fuse is low resistance (i assume - correct me if i'm wrong).

Yes, continuity is there.

When testing the 450V cap, do I need to connect the rest of the parts, ie the logic board, inverters, LCD etc? or can I just plug in the AC to the board as pictured? I also assume that the DMM will need to be set to AC as this cap is on the AC side? Thanks

Just in case there is a glitch, reconnect to the proper configuration then test.

[bob12345]

Re: Dell 1707FP no power

Ok I hooked up all the components, testing the voltage across the 450V blue large cap, (DMM set to AC - 600V scale) I measured 735V AC and (DMM set to DC - 600V scale) 330V DC. I did notice the first time I took the measurements there was a ticking noise coming from somewhere on the board. This was not apparent when I unplugged - reconnected mains for the second retest. - I've on EU power so 220v-240v ac.

[bob12345]

Re: Dell 1707FP no power

Just to add, I'm assuming that this AC measurement is peak to peak so half waveform would be 367.5V AC or this capacitor is connected to the DC side behind a rectifier? or else this 450V rated cap would be up in flames!

[PlainBill]

Re: Dell 1707FP no power

Ok I hooked up all the components, testing the voltage across the 450V blue large cap, (DMM set to AC - 600V scale) I measured 735V AC and (DMM set to DC - 600V scale) 330V DC. I did notice the first time I took the measurements there was a ticking noise coming from somewhere on the board. This was not apparent when I unplugged - reconnected mains for the second retest. - I've on EU power so 220v-240v ac.

Ticking sound?? Replace C857.

PlainBill

[bob12345]

Re: Dell 1707FP no power

Its strange it doesn't seem to make the ticking noise any more - but I can hear a constant faint static noise and C857 looks from the outside OK, do you reckon it will solve the problem?

[PlainBill]

Re: Dell 1707FP no power

Its strange it doesn't seem to make the ticking noise any more - but I can hear a constant faint static noise and C857 looks from the outside OK, do you reckon it will solve the problem?

Sarcasm mode ON. No, I don't think it will fix the problem. I just want you to replace if so you will be unhappy and I will look like an idiot! Sarcasm mode OFF.

Perhaps I got ahead of myself. Have you checked the outputs of the power supply? Understand, the power LED is controlled by the processor on the 'logic card' (I've got to start using a better name for that). Just because the power LED is off doesn't mean the power supply is dead.

If you do not have any output voltage, yes, replace that cap. It provides the power for the SMPS controller during the power supply startup. If it fails (and it won't show any signs when it does), the result will be a ticking sound and a dead power supply.

PlainBill

[mathog]

Re: Dell 1707FP no power

If you do not have any output voltage, yes, replace that cap. It provides the power for the SMPS controller during the power supply startup. If it fails (and it won't show any signs when it does), the result will be a ticking sound and a dead power supply.

Since there do not seem to be speakers in this model I'm curious as to which part of the circuit makes the noise. Specifically, what component ticks, and why?

Thanks.

[PlainBill]

Re: Dell 1707FP no power

Since there do not seem to be speakers in this model I'm curious as to which part of the circuit makes the noise. Specifically, what component ticks, and why?

Thanks.

Usually the SMPS transformer. The capacitor has enough energy stored to drive the power FET at least once, and that drives a 166V pulse into the SMPS transformer. Since the filter caps are discharged, the secondaries are driving into a very low resistance; a lot of energy is transferred. In normal operation each subsequent pulse would charge the filter caps, so much less power would be transferred (and the pulses would be at ultrasonic frequencies). Because the start-up cap is bad, it discharges quickly, before the tertiary winding that supplies the SMPS with run power can charge the capacitor.

Look at this thread for more information.

PlainBill

[bob12345]

Re: Dell 1707FP no power

Sarcasm mode ON. No, I don't think it will fix the problem. I just want you to replace if so you will be unhappy and I will look like an idiot! Sarcasm mode OFF.

Perhaps I got ahead of myself. Have you checked the outputs of the power supply? Understand, the power LED is controlled by the processor on the 'logic card' (I've got to start using a better name for that). Just because the power LED is off doesn't mean the power supply is dead.

If you do not have any output voltage, yes, replace that cap. It provides the power for the SMPS controller during the power supply startup. If it fails (and it won't show any signs when it does), the result will be a ticking sound and a dead power supply.

PlainBill

In terms of the output, i assume you mean the connection between the power board and the "logic board"? I see that there is one red wire and the other 9 black. I assume not all are power but some also signal cables?

[PlainBill]

Re: Dell 1707FP no power

In terms of the output, i assume you mean the connection between the power board and the "logic board"? I see that there is one red wire and the other 9 black. I assume not all are power but some also signal cables?

Yes. Sometimes there is a legend on the top or bottom of the board. Another approach is (with the power supply fastened in place) to measure the voltage from ground to each of the pins in the cable.

PlainBill

[EGuevarae]

Re: Dell 1707FP no power

...

Look at this thread for more information.

PlainBill

I agree. The little guy is sometimes responsible for either the no-startup tick or the 5v line bouncing like crazy.

[pedro]

Re: Dell 1707FP no power

Since there do not seem to be speakers in this model I'm curious as to which part of the circuit makes the noise. Specifically, what component ticks, and why?

Thanks.

Google magnetostriction

[bob12345]

Re: Dell 1707FP no power

Yes. Sometimes there is a legend on the top or bottom of the board. Another approach is (with the power supply fastened in place) to measure the voltage from ground to each of the pins in the cable.

PlainBill

I've measured the voltage from ground (or what I think is ground - please correct me if i'm mistaken) - the three pins on the right of the (pins 0 - 2) connector CN853 (PSU -> Logic card) which appear to be connected to the ground track of some nearby caps and that track also goes to the chassis mounting screw hole so I assume thats a suitable ground location (see attached underside pic). The voltages relative to these GND pins (right to left as shown on underside pic):

Pins 0-2: GND (0V)
Pin 3: 0V ---- (as expected as this appears to be connected to the 12V optional speaker attachement - not attached)
Pin 4: 5.18V
Pins 5-7: 5.21V
Pins 8-9: 0V ---- (appear to go to one of the inverter transformers T2?)

All parts where connected: LCD panel, inverters, logic board (but not the usb header board)

No ticking noise heard now, only the constant buzz/humm of something.

Thanks for all your help so far!

[PlainBill]

Re: Dell 1707FP no power

I've measured the voltage from ground (or what I think is ground - please correct me if i'm mistaken) - the three pins on the right of the (pins 0 - 2) connector CN853 (PSU -> Logic card) which appear to be connected to the ground track of some nearby caps and that track also goes to the chassis mounting screw hole so I assume thats a suitable ground location (see attached underside pic). The voltages relative to these GND pins (right to left as shown on underside pic):

Pins 0-2: GND (0V)
Pin 3: 0V ---- (as expected as this appears to be connected to the 12V optional speaker attachement - not attached)
Pin 4: 5.18V
Pins 5-7: 5.21V
Pins 8-9: 0V ---- (appear to go to one of the inverter transformers T2?)

All parts where connected: LCD panel, inverters, logic board (but not the usb header board)

No ticking noise heard now, only the constant buzz/humm of something.

Thanks for all your help so far!

One thing is apparent; another can be inferred.
1. The power supply is working.
2. You have very good ears.

As I've said repeatedly, the power LED is controlled by the processor on the logic card. It's time to post pictures of the logic card and see if we can find the problem.

PlainBill

[bob12345]

Re: Dell 1707FP no power

One thing is apparent; another can be inferred.
1. The power supply is working.
2. You have very good ears.

As I've said repeatedly, the power LED is controlled by the processor on the logic card. It's time to post pictures of the logic card and see if we can find the problem.

PlainBill

I'm dreading that the logic board is dead, probably hardest to source/repair i'm assuming! Here are two pictures of either side of the logic board (model is a 1707FPt). Nothing appears to be burnt out visually.
Sorry about the blur in the second pic, camera doesn't like to focus with no flash in macro mode.

[bob12345]

Re: Dell 1707FP no power

I'm dreading that the logic board is dead, probably hardest to source/repair i'm assuming! Here are two pictures of either side of the logic board (model is a 1707FPt). Nothing appears to be burnt out visually.
Sorry about the blur in the second pic, camera doesn't like to focus with no flash in macro mode.

My mistake the monitor is a 1707FPf not t.

[PlainBill]

Re: Dell 1707FP no power

I'm dreading that the logic board is dead, probably hardest to source/repair i'm assuming! Here are two pictures of either side of the logic board (model is a 1707FPt). Nothing appears to be burnt out visually.
Sorry about the blur in the second pic, camera doesn't like to focus with no flash in macro mode.

We may have a winner!!! What is the part number for U404 (three pin device to the left of the gm5621-LF).

PlainBill