Dell e172FPt

[trwilson70]

Have had good success recently repairing various e172/173Fp BenQ LCD's - but this one is different. Read in one of the threads here that this is made by Lite-On but I'm not sure. I replaced 7 of the 9 caps - the two in the corner by the logic board connector are nichicon he's - figured they were still good. What I ended up with is the infamous '2 sec to black' blinking. Have 160v on large cap, 5v and 13v on logic connector. When connected power I hear a squeal and ping noise - seems to come from transformer in middle of board. I don't have another set of lights with this connector to test if lamps are bad.
I will be posting my repair successes on HP, Dell, Viewsonic LCD's as soon as I can get time.
Any guidance appreciated!

[PlainBill]

Re: Dell e172FPt

Have had good success recently repairing various e172/173Fp BenQ LCD's - but this one is different. Read in one of the threads here that this is made by Lite-On but I'm not sure. I replaced 7 of the 9 caps - the two in the corner by the logic board connector are nichicon he's - figured they were still good. What I ended up with is the infamous '2 sec to black' blinking. Have 160v on large cap, 5v and 13v on logic connector. When connected power I hear a squeal and ping noise - seems to come from transformer in middle of board. I don't have another set of lights with this connector to test if lamps are bad.
I will be posting my repair successes on HP, Dell, Viewsonic LCD's as soon as I can get time.
Any guidance appreciated!

'Two seconds to black' is the inverter controller detecting a fault condition. Possible fault conditions are output voltage too high or too low, CCFL current too low or too high. Not all of these are necessarily checked. Possible causes include broken CCFL (or bad solder joint on wire at CCFL), open or shorted transformer, arcing from CCFL lead to chassis, and a failure in the monitoring circuitry.

The best approach is to hook up a known good panel. Another approach involves measuring the voltages produced by the monitoring circuitry. Since you have indicated you don't have the ability to test by substitution, I've marked up some test points.

Return 1 and 2 are the return lines from the CCFLs. Measure the AC voltage on them while the backlights are on. I Sense 1 and 2 are the filtered voltage from the return lines. Measure the DC voltage on them while the CCFLs are on. V sense 1 and 2 are the voltage sense lines for the transformer outputs; the voltage is reduced by the pair of capacitors. Again, measure the AC voltage while the backlights are on.

PlainBill

[trwilson70]

Re: Dell e172FPt

Thanks Bill,started to measure voltages and wasn't getting anything - rechecked voltage to logic board - nothing! I have been trying to keep good notes as I go along - but may have been confused with another monitor - have 5 different ones on my bench. Still getting 160v at large cap - but no power light. Just checked voltages across other caps, four are 24vdc, but the two near the logic connector = 0.

[trwilson70]

Re: Dell e172FPt

Ok, have let this project set off to one side too long -need to revisit and see if can't make it work before I forget where all the screws are. No voltage to A/D board - large filter cap is 164v, 24v across the others except the two by the A/D board connector - they read 0. D820 is YG902C2 hi-speed rectifier has 24v on middle leg, 0 on other two. D813 is YG802C04 Schottky Diode - 0 across all legs. Can these be properly tested in circuit or is it best to remove? Checked with ohmmeter - seem to allow current in only one direction from center leg.

[alexanna]

Re: Dell e172FPt

Well my insomina got the better of me,For some reasion I have that exact same monitor at this time,And it has been apart for a long time also.Seeing your post I got looking at mine.I cannot rember the exact problem I was having but most likley good power light w/no back lamp activity.This morning the monitor was working as it should. I was checking voltages on the back of board to see If I could be of some help to you. probing around transistor
I803 My voltages started to realy go crazy w/ flickering back lamp and all kind
of strange things.I am going to reflow the solder on my board in that area to see if it helps,I think there is a good chance it will. If you need any good pin voltages on the board let me know and will try to get them for you
Al.

[PlainBill]

Re: Dell e172FPt

Ok, have let this project set off to one side too long -need to revisit and see if can't make it work before I forget where all the screws are. No voltage to A/D board - large filter cap is 164v, 24v across the others except the two by the A/D board connector - they read 0. D820 is YG902C2 hi-speed rectifier has 24v on middle leg, 0 on other two. D813 is YG802C04 Schottky Diode - 0 across all legs. Can these be properly tested in circuit or is it best to remove? Checked with ohmmeter - seem to allow current in only one direction from center leg.

This is a very standard power supply design. Since you have 24 volt output you know the primary side of the power supply is fine. A shorted diode would dump AC into the filter caps on the 5 volt output, generating a lot of heat or shutting down the SMPS controller. I'd check the connections from the transformer secondary to the diode, and from the diode to the caps for any bad solder joints.

PlainBill

[trwilson70]

Re: Dell e172FPt

Progress maybe - reflowed connections although nothing appeared bad. Still have the ticking noise and no 5v. Then I flipped the board over and didn't hook up the Logic board, just the CCFL's - I now have 5.22v! Appears I may have a direct short on logic board - about the same time I got a call for lunch so will focus on it after a bit. Will post photos of board. TR

[trwilson70]

Re: Dell e172FPt

So I started to look over the logic board - and I noticed the connector from the power supply and the connector to the control buttons were the same style connector. DUH - in flipping the board over - taking photos etc. I had connected the PS to the wrong connector! Swapped connectors - no more click - and light in the power button! Now I have 2 sec. to black - I think that's where I started this tread. One of the inverter transformers seems to be making noise, makes the 'no signal' box jutter for the few seconds it is on - will re-flow and check things after. Then go to PlainBills original response and check things.

[alexanna]

Re: Dell e172FPt

So I started to look over the logic board - and I noticed the connector from the power supply and the connector to the control buttons were the same style connector. DUH - in flipping the board over - taking photos etc. I had connected the PS to the wrong connector! Swapped connectors - no more click - and light in the power button! Now I have 2 sec. to black - I think that's where I started this tread. One of the inverter transformers seems to be making noise, makes the 'no signal' box jutter for the few seconds it is on - will re-flow and check things after. Then go to PlainBills original response and check things.

I am sure you have way more experince than I have at this.But I would like to share what I found on my monitor.I also was noticing a ticking noise around I803 after reflowing solder in that area the ticking noise went away,Runnig the monitor for a very short time I started noticing the jittery noise around the transformers but it was very difficult to tell the exact location even with the card board tube aproach.This monitor uses a three wire connector to the ccfls shairing the return line.I was lucky to have a known good lcd with the correct connectors and it has corrected the problem
Al

[trwilson70]

Re: Dell e172FPt

OK, finally got time to measure things. After reflowing transformers and several other components.

Return 1 = .233v ac
I sense 1 =1.38v dc
V sense 1 = .027-.03v ac

Return 2 = .285v ac
I sense 2 = 1.63v dc
V sense 2 = .03v dc

The V sense was difficult to measure - seemed they would start at higher voltage (.06v), then trickle lower till lights shut off.

I tried to use a stethoscope to listen where the noise was from - couldn't really pin point. Seemed to come from one inverter transformer. I did note that sometimes the lights stayed on 1/2 second, sometimes 10-15 seconds.

Not sure it is wise to run PS/inverter without the load - but I disconnected momentarily to see if noise would go away - it did not.

I don't have another CCFL with this connector to swap, but may try to rig something in order to test. Thanks, TR

[alexanna]

Re: Dell e172FPt

Here is what I found inside mine,There sharing the return with two lamps and each has its own hot.You could swap connectors and With six solder joints you could rule alot out quickly.
Al.

[trwilson70]

Re: Dell e172FPt

Thanks for the photo - gives me idea for an adapter to test things. Smelled ozone a bit while testing - supposed to be from poor connections. But still had some noise when disconnected the lights. Am real surprised that the stethoscope didn't pinpoint the sound.

[trwilson70]

Re: Dell e172FPt

OK - robbed the lights form another monitor and plugged them into the Dell - one set was good, one fair - but they stayed lit. Pulled the CCFL's from the Dell, plugged them in and one set was definitely bad with one tube pink before it shut down. Put the best two sets of the four into the Dell - re-assembled and everything worked as it should! Now I have at least one extra set for testing.

Also have the inverter from the salvage monitor for testing with this style of connector - just have to figure out the pin out. Thanks

[PlainBill]

Re: Dell e172FPt

OK - robbed the lights form another monitor and plugged them into the Dell - one set was good, one fair - but they stayed lit. Pulled the CCFL's from the Dell, plugged them in and one set was definitely bad with one tube pink before it shut down. Put the best two sets of the four into the Dell - re-assembled and everything worked as it should! Now I have at least one extra set for testing.

Also have the inverter from the salvage monitor for testing with this style of connector - just have to figure out the pin out. Thanks

Well done.

PlainBill