Dell Monitor E151FP Backlight Power Supplly

[WA3SCM]

Hi,
I need help with Benq switching supply 48.L2903.A23 which has failed killing the two backlights. Have been looking for schematic but nothing found yet.

Have removed and reinstalled the transformers finding the coil resistances reasonable and the solder joints fine.

Have checked all diodes and transistors found sat and see no failed caps.

Notice the high voltage inverters on the left side of the board seem isolated from the rest of the board and receives it's power from the right side of the board thru fuse F751 a 3 amp 125 vdc fuse. Found just left and down from the big fat cap at the top C605.

The measured voltage supplied to the fuse is 12 vdc which seemed reasonable until I noted that the zener diode ZD752 is rated 24vdc so was setting at 0.65 vdc since not enough voltage is being delivered to the left side of the board

1. What voltage would you expect the right side to be sending to the left side high voltage inverter?

Investigation of the right side of the board is limited because I found all the voltages at the right side connector correct as listed at the top middle of the board (5v-fix, 5v-fix, 3.3v)

I used a Fluke 87 III true rms dvm to take measurements around the big transistor in the middle of the board Q601. I find it is pulsing at 12 vdc about 60 HZ as measured by the Fluke. My current guess is the feedback signal controlling Q601 is failed so it is not oscillating at kHZ as it should.

2. What frequency would you expect Q601 to be oscillating?

I have been reading switching supply info from all over the web and beginning to understand a little but don't have much of a grasp about how the frequency is controlled to vary the average DC voltage.

Certainly I have spent too much time on this but I really want to understand how this stuff works and fix it. Newly retired I remain very busy doing what I like. Fixing stuff.

Thanks for your help!

later, slater

[selldoor]

Re: Dell Monitor E151FP Backlight Power Supplly

What do you main "Failed and killing 2 back lights" They dont come on?
They dont work when tested on something else?
They flash on then go off?
When you say the coil resistances are reasonable what does that mean
can you post figures.
What do you main "Failed and killing 2 back lights" They dont come on?
They dont work when tested on something else?
They flash on then go off?
When you say the coil resistances are reasonable what does that mean
can you post figures.

Have you tested Q759 , Q760 , Q753 and Q754
Soldering around Q201 looks a bit odd can you check it out and
the main transformer looks like it has been re touched but was all the old solder removed
and replaced?

Did you have this monitor and it stopped working? have you seen it working?

[WA3SCM]

Re: Dell Monitor E151FP Backlight Power Supplly

Hi Selldoor,

Failed and killing 2 backlignts = No high voltage from board to either of the two lamps.

Have not tested the lamps on something else.

They are off. I did not try to off/on the monitor just replaced it so my wife could continue working then tore it apart.

I did not record the coil resistances because they were so low they were similar to a short which I expected. None were even in the tens of ohms.

Q759, 760, 753, 754 are tested ok.

I assume you mean D701 instead of Q201. The solder was that way as found. I tested the diode pair in circuit thinking it was a transistor and removed it thinking it was failed. When I realized it was a diode pair and ok I reinstalled it returning the solder to as found conditions. It seemed to me they might have wanted the solder thick near the diode pair to make up for the narrow trace in that area compared to the rest of the trace.

Perhaps you meant Q601 instead of Q201. I just removed the solder from Q601 and resoldered with Kester 44 rosin solder. After doing so I powered up the board while monitoring the DC voltage at fuse F751. The voltage is still about 12.4 vdc however I note that the voltage is pulsed a couple times to almost 14 vdc when the AC power is first applied. It seems the voltage comes up to 12.4 then after a second jumps to 14 for a second then drops back to 12.4 for a second then back to 14 for a second then drops to 12.4 where it stays.

The main transformer (all transformers) were removed from the board, the holes cleaned, and resoldered with Kester solder.

This monitor has been working for years and suddenly the backlight failed.

Thanks for taking the time to review and comment on my problem.

later, slater

[budm]

Re: Dell Monitor E151FP Backlight Power Supplly

What resistance do you read on that fuse F751? <1 Ohm?
If you are getting 12V, the power supply is OK.
Does the power LED stay green when you turn on the power switch?
The variable B+ (the higher the B+ the more brightness) is fed by the D751 (next to L752, the Cathode side is the output side) to both Royer Oscillator circuits for the High voltage transformer through L751 and L753.
So check the DC voltage at the Cathode of D751.
By the way, what is the revision (REV XX) of this E151FP?
You can also make test lamps for testing the inverter circuits.
https://www.badcaps.net/forum/showth...t=19987&page=2

[selldoor]

Re: Dell Monitor E151FP Backlight Power Supplly

OK perhaps not much point proceeding if the inverter transformers have gone (unless they are cheap to replace)
I have marked what I believe are the secondary wiring joints - Meter on ohms 2000
test S1 to S2 and S3 to S4

[WA3SCM]

Re: Dell Monitor E151FP Backlight Power Supplly

Hi budm and selldoor,

Thank you so much for your reply. This project got interrupted by a chain saw piston seizure, but is now back on track.

Selldoor the data attachment shows your requested ohm meter readings of S1-S2 = 238.0 ohm and S3-S4 = 237.8 ohm with the transformers installed. I was wrong when I said all the readings were showing near zero ohms. The transformers are ok.

Budm the data attachment shows all pin to pin resistance measurements taken with the transformers removed from the pc board. My Fluke was in the "relative" mode to eliminate test lead resistance while taking these readings. Looks to me the transformers are ok.

To answer your questions:
Fuse F751 resistance = 0.1 ohm.
Thank you for info that 12.4 vdc is ok.
The green power LED never comes on at all. The orange light illuminates while the power switch (toggle switch not pushbutton) is in "standby" and goes dark when the power switch is moved to "on or 1" position.
The variable B+ goes from 0 vdc to 9.5 vdc when the power switch is moved to "on" and stays at 9.5 vdc for 1 second then goes back to 0 vdc.
I can find no Rev. number for this monitor but it must be a very early vintage E151FP since it has a toggle power switch. It was purchased from Dell as part of a Dimension 2350 system February 20, 2003.

I am still curious about the 24 vdc Zener diode ZD752 and will remove and test the Zen voltage later this evening.

Thanks again for your interest and questions.

later, slater

[budm]

Re: Dell Monitor E151FP Backlight Power Supplly

"The variable B+ goes from 0 vdc to 9.5 vdc when the power switch is moved to "on" and stays at 9.5 vdc for 1 second then goes back to 0 vdc" Is that the voltage reading at the Cathode of the Diode I mention? If that is the case, the lamps should flash in that second, you may have to look at them in the darkened room. The voltage drops down after second can be due to the error detection when the lamps are being fired up and shut down the circuits.
At this point you need test lamps for the inverter function verification.

[WA3SCM]

Re: Dell Monitor E151FP Backlight Power Supplly

Hi budm,

Well I should have done some testing before I disassembled it. With the video cable disconnected the display works indicating "NO SIGNAL" for the 1 second then goes dark.

I have an Acer laptop setup for a second monitor which is 1024x600. When I unplug the Acer second monitor and plug in the Dell monitor then turn it on the Dell monitor remains lit as per the attached image "ErrorMsg" indicating "OUT OF RANGE"

Readjusting the Acer second monitor display setting to 800x600 then attaching the Dell monitor showed the monitor is now working as per the attached image "800x600Success, and the LED is Green.

Sorry I did not start off cycling power and verifying the problem before I tore it apart. I did see the monitor operating in failure mode with no backlight and jumped to the conclusion it was busted. Pretty tough to find out what to fix on something that is not broken.

Thanks for your help budm and I apologize for wasting your time.

later, slater

[budm]

Re: Dell Monitor E151FP Backlight Power Supplly

Well, glad to hear that it is not the monitor problem.

[WA3SCM]

Re: Dell Monitor E151FP Backlight Power Supplly

I removed the zener and placed it in a test circuit finding the zen voltage correct at 24vdc. Perhaps it is some kind of a protection device.

As I was assembling the case I noticed another sticker showing another model number and revision:
CN-03K648-46633-315-95JT REVISION A01

Thanks again for your help!