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Power Supply board no output - HP LP2475W 24" Widescreen LCD Monitor

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bellarmine
bellarmine picture

Hello all,

I'm hoping one of you all here could give me some advice on my dead LCD monitor.

First off, let me say, great site! Without it, I wouldn't have been able to even begin diagnosing my monitor, I found the service manual for it here. Hope to be coming back many times in the future!

Now the details: The LCD I have is an HP LP2475W 24" LCD computer monitor 1920x1200. I was using it a while back, and then it just turned off. No picture, no power light, even the built-in USB hub had shut off. It's as if someone pulled the plug out, no smoke, noise or anything. I've left it unplugged for a while, and plugged it back in, still nothing. It's almost as if a fuse is blown, though if I listen very carefully I can hear the power supply 'ticking' very very softly.

I just bought the monitor factory refurbished by HP in 2009, only has about 6500 hours on it; I expected it to last much longer than this. :weep:

Before I spend several days troubleshooting it, I'm hoping someone could give me some guidance as to what usually tends to be the defective component(s). :smiley:

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I opened it up and checked it out, and the problem seems to be the power supply board is dead; no output to the main, or 'scaler' board, whether or not it's hooked up to the power supply board.

Would this be typical of some faulty electrolytic capacitors, or is it likely to be a more serious problem? I don't see anything obvious, no bulging caps, no burnt parts, nothing.

The circuit seems to be a standard switching supply, which sends the 120v power first through a charge pump, which raises the voltage to around 400v. It then goes into a switchmode transformer to create 5v, 12v and 24v to run the scaler board and inverter.

Upon checking further, the main 300-400v bus seems to be at only 170v, indicating the charge pump is not functioning or starting. The 120v in and charge pump diode check ok. The problem seems to be in the charge pump control IC (FAN7529 PFC Controller) or circuitry. The FAN7529 chip seems to be getting only about 0.5v on it's power lead, could this be due to it's own supply, or more likely the chip itself?

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On the other hand, it may be more economical to replace the power board,
HP gives me the following part numbers:

ADPC72411AA5 ----- POWER BOARD ADPC G2956-I-X-X-1-080520 (power board assembly number)
715T2956 1 ----------- POWER BOARD PCB (pcb number, sometimes used as assembly number)

When I do a Google search, the only places I find that maybe have something like this is China or Poland. Would any of you know where can I could obtain another one of these power boards?

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Here are the links to the Service Manual and Disassembly Guide here on ElektroTanya. Shows the Power Supply schematic and color pictures respectively:
http://elektrotanya.com/hp_lp2475w_lcd_monitor_sm.pdf/download.html
http://elektrotanya.com/hp_lp2475w_lcd_monitor_disassembly_guide.pdf/dow...

Also a new version of the schematic that my Dad and I cleaned up, much easier to follow logically:
http://elektrotanya.com/hp_lp2475w_lcd_power_supply_schematic.zip/downlo...

I've also included some detailed pictures of my Power Supply board, hope they help out.

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Any thoughts you might have I'd appreciate very much :super:

Cheers! :D

Robert
Ontario, Canada

Comments
balage
balage picture

I'm glad that you could finally fix it !

Regards
balage

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bellarmine
bellarmine picture

So am I Balage, so am I!.......... :taps2: :taps:

Thanks again, and all the best.
Cheers! :beer:

Bellarmine

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bellarmine
bellarmine picture

Hello Balage and Francisco Sanchez,

Well, it's been way too long since I posted here, been really busy as of late dealing with the worst winter here in over a decade, brrrrr.

Here's an update: I finally fixed it!

You're suspicions were right, it was the double schottkey diode D951 after all, turned out it's shorted. I substituted it with a normal diode for a few moments, and the power supply started working again, so I ordered a replacement part on eBay, should be here in a few weeks.

An interesting thing, the part on the board is a V40100PG, rated at 40amps, whereas the one specified in the schematic is a YG868C06RSC, rated at 30 amps, which is not as good as what is actually in the board. However, the resister capacitor network components (R952,953,955; C953,954) that would protect the diode are missing from the board. I suspect these changes might be cost-cutting and possibly a decrease in reliability, causing the diode to blow up. But just speculation, my opinion.

A further development, the reason I was actually able to fix it, is about a month ago, I finally found some auctions on AliExpress selling bunches of 715T2956 used power supplies for my monitor, at only $25 a pop free shipping! I was going to link it here, but the auctions have disappeared unfortunately, lucky I got it when I did!

So anyway, at that price, I couldn't resist getting one and potentially saving myself days of diagnosing. So when it arrived, I installed it and it worked perfectly. I then powered up the bad supply and stepped through it checking voltages using the new supply as a comparision, and sure enough quickly found the problem at the Schottkey diode as you all suspected. It's too bad I didn't pursue it just a bit further, but it's just as well. I own three of these monitors, so now I have an extra supply for the future.

One wierd thing is measuring the secondary of T902 in both supplies show 0 volts (?) strange. However on the good supply there was 5 volts present on the far side of the schottkey, whereas on the bad one there was zero, so that was pretty revealing. I cut the trace to the middle leg and measured it and it indeed was shorted, so there it was.

See picture of it working again, and my favorite wallpaper (Mariachi).

Balage, Sanchez, I would like to express my heart-felt thanks for all your trouble! You were right all along, I just didn't have any time to pursue it further at the time, but all's well that ends well. I really appreciate it, and wish you all the best!

This is such a great place, hope to have more to do on here in the future. Best wishes for an early Spring this year......:^)

Bellarmine

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Francisco Sanchez
Francisco Sanchez picture

Just a test.

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bellarmine
bellarmine picture

Hello all,

I FINALLY got back to checking out this HP 24" LCD, and as promised I'm letting you know what I found.

I've replaced the IC902 as you recommended, took me a while to learn how to solder such a tiny chip. It was interesting, and soldered it in without a hitch. Unfortunately, there's no change, the power supply still ticks softly.

Balage, you said before to try replacing the IC950 opto, can I desolder it and check it?

I'd like to apologize for the loooong delay getting back to this, been busy all winter helping my Dad with his work, and staying warm, been a pretty long winter. If you have any other suggestions, that'd be great.

Many thanks for all your help so far Francisco & Balage, it is MUCH appreciated!

All the best,

Bellarmine

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balage
balage picture

Hi Bellarmine!
The winter was long here in Hungary too.And it seems to have not ends.
Try to replace the power FET (Q902).The PSU ticks
when the FET have G-S defect, S/C or low resistance.

Regards

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Francisco Sanchez
Francisco Sanchez picture

Hi, Bellarmine, nice to hear from you again.
Did you check/replace schottky diodes?
It is not easy to check the IC950 opto, but there are some tricks you can do: short IC 953; by doing this, the current in the diode of the opto will reach its maximun, and if the opto is ok, the power supply ticks should change, and ptobably the PSU will start to work at a reduced duty cicle . (I hope so)
Regards.

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Francisco Sanchez
Francisco Sanchez picture

Hi, I have found the datasheet for the YG868C06R, (Low IR Schottky barrier diode) and I think it could be of interest to repair the power supply of your HP LP2475W lcd monitor.

File csatolás: 

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bellarmine
bellarmine picture

Hi Francisco,

Thanks for this, it should give me some insight into the Schottky's specs.

I'm still waiting for those ICs I ordered, so in the meantime I'm going to look into the power supply with my Dad's oscilloscope and such. But first I've got to finish some chores outside before the snow flies (groan)......so it'll be a few days before I can get back to this. When I do I'll be sure to post back with my findings.

Thanks again Francisco, stay warm!...... :king:

Bellarmine

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bellarmine
bellarmine picture

Hi Balage & Francisco,

Thanks again for these tips. I decided to go with your repeated recommendations, and buy some replacement IC902 (LD7522PS) chips on eBay. $6 USD for a pack of 10, free shipping, how can I lose....... ;)

No, I haven't extracted the D951 schottky diode yet. I checked the other schottky, D953, and it's also apparently shorted (unlikely? :hmmm: ), which leads me to believe, as you say, that I have to remove the Schottkys to check them. It'll take a few weeks for the ICs to get here to Canada, so I'll spend the time checking out the circuit and those Schottkys with my Dad's oscilloscope. I'll let you know how it goes Balage & Francisco.

Wish me luck; cheers! :beer:

Bellarmine

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Francisco Sanchez
Francisco Sanchez picture

Hi Bellarmine, did you desoldered and extract from the pcb, the diode D951 dual shottky?, because it only should conduct one way. If you don’t have a replacement for it, you can replace it for a pair of normal diodes, (fast switching ones) only for checking that power supply starts to work, then replace for original parts or near the same. (equivalent)
Regards.

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balage
balage picture

The shottky diode can be measured to conduct in both directions, with a multimeter. It's OK If it's not S/C.

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Francisco Sanchez
Francisco Sanchez picture

Shottky diodes has a significant reverse leakage current, but most multimeters should see it as normal diodes. (not in forward voltage drop)
Regards

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bellarmine
bellarmine picture

Hi Balage & Francisco,

Thanks for these new tips. Yes, I realized that I should find out more about these SMPS. In fact, that's just what I've been doing yesterday. My Dad and I spent some time rearranging the power supply schematic so that it was easier to read, it drives him crazy when things are written sloppily. My Dad is an electrical engineer, but he hasn't had an awful lot of experience with SMPS. I've uploaded the schematic, see what you think of the changes...... :yes:

http://elektrotanya.com/hp_lp2475w_lcd_power_supply_schematic.zip/downlo...

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Learned a lot doing it, I now understand how the supply starts up. It feeds half- or full-wave rectified voltage to the xformers, and also directly to IC902, which starts the pulsing on the main B+ line,.

I also understand that DC won't go through a xformer. However, there is still 0v on the secondary, and since there is half- or full-wave rectified voltage going into the primary, there's someting wrong here. I re-checked the D951 dual shottky again, and it conducts both ways! I don't see how my meter can get around any other way in the circuit. Not to mention that R952, R953, C953, C954, R955 on the secondary don't exist, they are just solder pads!

My Dad does have an oscilloscope, so I guess we'll get it out and start trying to trace the waveform through. Balage, which opto coupler are you specifying, and where is TL431, I can't find that anywhere?

Francisco, yes there is voltage going to pin7 of IC902, so the resisters are ok. Maybe the chip is bad. Also, you did just fine, I appreciate your help (by the way, I'm new too!..... :izom:

Thanks again Balage & Francisco, I'll let you all know how it goes. :beer:

Bellarmine

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balage
balage picture

Hi!
The circuit starts up through the R951,952,953.When started , feed itself from the xformer's pin4 through the D907, D905. The opto: IC950,TL431:IC953(AZ431 only other brands) I think the prime suspect is the IC902.

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Francisco Sanchez
Francisco Sanchez picture

Hi, I think if there is not 5v. you should check R9251,R9252,R9253 and IC902. Probably one of those resistances are open.
I hope this could help.
Regards.

PS: I hope not to do anything wrong. (I am new, here)

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bellarmine
bellarmine picture

Hi Azza & Balage,

Thanks for your suggestions, I finally got around to checking 'em out, and here is what I found:

I tried substituting another 100uF 450v cap (C951), but there was no change - supply still ticks softly, once per second. I checked the new cap and the existing cap with my meter by measuring the time they take to charge up (I don't have a cap meter), and they both seem ok.

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As for the dual shottky (D951), it conducts both ways, but I checked it in circuit, so it could be going around another way, though I don't quite see where.

However, I don't see how that's coming into play yet, as there is 0v on the secondary side, nothing at all. On the T902 power xformer, both coils have between 70-90v on the primary side, and 0v on the secondary side. I assume this is why the charge pump is not working, because there is no +5v power coming from the secondary side.

Finally, about the PS-ON signal, where exactly does this signal come from? I can't seem to find it's source. Though if it comes from the scaler board, I don't see how the PS_ON signal would come into play, as the scaler board electronics are not receiving any power at all to even send the signal, unless I'm wrong. :hmmm:

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Would you have any more thoughts? Again, thanks for your past input, it's a great help! :king:

Bellarmine

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balage
balage picture

Your basic problem that no 5V for the scaler b/d. If no 5V, no PS_on signal neither. Maybe s/c on the 5V line,or simply defective IC902.

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bellarmine
bellarmine picture

Hi Balage,

You're right, that would make sense. I'm testing the power board with the scaler b/d disconnected, as the shop manual suggests, but there is no change whether the scaler board is connected or not - power board just ticks.

If there were a short circuit in the scaler board, the power supply would work with it disconnected, right? Since there is no change, I guess any short circuit would be on the power board, or IC902 is bad, as you say.

If IC902 is bad, would that cause IC901 not to come on? Would you recommend buying a replacement IC902 chip and trying that?

Also, I don't understand how there can be voltage on the transformer primary side, but 0 on the secondary. Maybe because the PFC circuit is not working?

What puzzles me, is how does the PFC system in the primary turn on to power the secondary if there is 0v in the secondary to feed the PFC circuit in the primary (endless loop....) :hmmm:

Thanks for these new suggestions, I'll puzzle over the schematic some more :smiley:

Bellarmine

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balage
balage picture

You must learn more about how working of a smps, The DC voltage can't be transformed.The drive ic drives the FET with 40-100kHz and it switches the primary suupply voltage to the gnd through the Xformer. Generates the same frequency, lower AC the secondary side.A simple multimeter not enough to repair this circuit. You need an oscilloscope too. Try to replace the opto coupler and the TL431 in the control loop.

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balage
balage picture

Hi!
The PFC circuit is working only when the Vcc1 is present, and the PS_ON signal is on H-level. What about with the D951 dual shottky?

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bellarmine
bellarmine picture

Hi Balage,

Ok, I think D951 is on the secondary side of the supply? I haven't checked out those parts yet. I'll check into the PS_ON signal, and definitely let you know how it goes.

Thanks for your advice Balage, it's much appreciated! :taps2:

Bellarmine

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fega72
fega72 picture

Hi!

-Disconnect the cable connecting main board and PWPC board, check if the 12V and 5V voltage in
PWPC board is normal? (Attention: 1. Be sure the grounding is OK. 2. Cut off the power when soldering.)
Check C914 (+) = 300V -> if not good then Check F901, NR901, DB901, D901, Q901 - Have you checked these parts?
I found this in the SM - page 42.
Good luck!
Gab.

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bellarmine
bellarmine picture

Hi Gab,

Thank you very much for your suggestions. Sorry for the delay, it took me a while to check out those parts, plus been busy with the fall chores :king:

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To answer your queries:

  • I've disconnected the main board to PWPC board cable
  • Also disconnected inverter board to PWPC board cable as well.
  • There is 0v on the outputs, no 5v, no 12v, no 24v.
  • Grounding was good, I left the power board screwed down to the chassis.
  • C914 was about 170v

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All the following 'seem' to check out ok, thought I haven't checked them out of circuit as of yet:

  • F901 (fuse, ok)
  • NR901 (varistor, ok)
  • DB901 (main diode bridge, not open or shorted)
  • D901 (charge pump diode, not open or shorted)
  • Q901 (transistor, not shorted out, not sure if turning on though)

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Hope this doesn't exhaust your ideas ;)

Thanks again for your help Gab, it is much appreciated! :oke:

Cheers!

Robert

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AzzA
AzzA picture
*

Hi Bellarmine,

If ticking power supply, on the primary side of the replace the 100uF 400V capacitor for sure.
Trying to go power supply, transformer heard ticking.
Unfortunately, this is a typical switching power supply failure.

Best regards,

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bellarmine
bellarmine picture

Hi Azza,

Thanks for the tip, I'll be sure and check that out, sounds like that could fix the problem. I'll check into it and let you know. Here's hoping....

Thanks again Azza, I appreciate it. :super:

Cheers! :beer:

Bellarmine

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