HP 1905 No power. No led. No output voltage.

[newbie1]

hello guys.
i need some help on how to proceed repairing this monitor. I am not the orginal owner of this monitor.

LCD model : HP 1905

Problem : No power when plugged. No LED light up when on-off switch is pressed.

Whats been done :
1) Opened up the monitor and saw 1 bulging 1000uf 25V cap on the power board.
2) Replaced all the electrolytic caps.
-replaced all of them with Panasonic FC & FM, except 2 with Nichicon (PS on the 10uF 50V (C917) and VZ on the 1000uf 25V near the audio jack(C916) because that is what i have at hand for now.
3) Measured across the big 150uF 400V caps (C904) = 327V DC.
4) Measured the fuse (F901) for open but it is reading 0.03 ohms
5) Measured output voltage to the main/scaler board but NO voltage detected. No 5v, etc.
6) Measured for short at the 4-pin bridge rectifier (BD901) but NO shorted between them (but between Pin 2 & 3 there is a resistance of 982k ohms both ways).

Attached are the top & bottom of the power board & schematic. Sorry if they are not up to standard. I have tried following the flow diagram of the troubleshooting section BUT i got confused. For example it stated in the flow diagram to check Voltage at C905 but there is no C905 capacitor attached to it (i.e empty).

I hope my desciption of the problem is clear or is there any more that i should add?.
Hoping that somebody can point out what my mistakes are & how to proceed further..
thank you guys..

p/s : just to add, when i unplug the monitor and waited for a few seconds (about 20-30 sec). i still measure across the big 150uF 400V caps (C904) = 307V DC. i do wonder how long does it need to discharge? i do not want to add another 'burnt-imprint-marks-of-capacitor-legs-due-to-stupidity' on my fingers again. LOL..

[Wrog]

Re: HP 1905 No power. No led. No output voltage.

Did you replace both of those 5x11 caps in the upper center (I'm assuming one of those is C917)?

[jetadm123]

Re: HP 1905 No power. No led. No output voltage.

Actually, C917 and C918 are part of the startup circuit and will prevent the power supply from starting up if they have failed. I would change those first (make sure large cap is discharged before you start), and if the supply still does not start, check the voltage on pin 7 (VDD) of IC901. Then, with power off, monitor unplugged and large cap discharged, check for shorts on diodes D905,D906 and mosfet Q901.

[PlainBill]

Re: HP 1905 No power. No led. No output voltage.

The pictures are so-so. I've tried to deal with a lot worse. Two points I would like to make. Most boards have an orientation. For example, this board's 'normal' orientation is with the AC input connector to the bottom of the picture. In that orientation none of the labels, etc are upside down. When flipping the board to take pictures of the back, rotate the board around it's vertical axis. That way components near the top in the 'top' picture are near the top in the 'bottom' picture. It's a lot easier to translate let to right than it is to rotate 90° clockwise.

Those are minor issues because you supplied the service manual, complete with schematic. Even the poorest schematic is better than the best pictures. And you have done a good job of troubleshooting (so far), and gave us the history of the monitor.

The prime suspect is IC901 - the SMPS controller. It is a prone to failure. With the monitor plugged in points on the power supply are at over 300 volts, so be careful. Plug in the monitor and using pin 1 of IC901 as ground measure the voltage at pins 3 and 7. IC901 will not start until the voltage at pin 7 is about 16 volts.

If pin 3 is above 5 volts, below 16 volts, and steady, replace IC901. If it reads 0 volts check R912, R916, and R918. If it is pulsing, replace C917 and C918.

PlainBill

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Thanks for all the replies. .

Did you replace both of those 5x11 caps in the upper center (I'm assuming one of those is C917)?

Yes. Both of them were replaced
C917 - replaced with Nichicon PS 10uF 50V
C918 - replaced with Panasonic FM 22uF 50V

Actually, C917 and C918 are part of the startup circuit and will prevent the power supply from starting up if they have failed. I would change those first (make sure large cap is discharged before you start), and if the supply still does not start, check the voltage on pin 7 (VDD) of IC901. Then, with power off, monitor unplugged and large cap discharged, check for shorts on diodes D905,D906 and mosfet Q901.

With DMM set at 200ohms, i could not detect short at D905 & D906. Both are open on one side & on the reverse probe, a flash of reading (too fast for me to note the reading) before it goes open. With DMM set to diode mode, Q905 reads 535 and Q906 reads 525. As for Q901, again setting DMM at 200ohms, i could not detect short on the D-S or D-G or S-G.

The pictures are so-so. I've tried to deal with a lot worse. Two points I would like to make. Most boards have an orientation. For example, this board's 'normal' orientation is with the AC input connector to the bottom of the picture. In that orientation none of the labels, etc are upside down. When flipping the board to take pictures of the back, rotate the board around it's vertical axis. That way components near the top in the 'top' picture are near the top in the 'bottom' picture. It's a lot easier to translate let to right than it is to rotate 90° clockwise.

Those are minor issues because you supplied the service manual, complete with schematic. Even the poorest schematic is better than the best pictures. And you have done a good job of troubleshooting (so far), and gave us the history of the monitor.

The prime suspect is IC901 - the SMPS controller. It is a prone to failure. With the monitor plugged in points on the power supply are at over 300 volts, so be careful. Plug in the monitor and using pin 1 of IC901 as ground measure the voltage at pins 3 and 7. IC901 will not start until the voltage at pin 7 is about 16 volts.

If pin 3 is above 5 volts, below 16 volts, and steady, replace IC901. If it reads 0 volts check R912, R916, and R918. If it is pulsing, replace C917 and C918.

PlainBill

Noted on the orientation of the boards. .In future, I will try not to forget to rotate them before posting. Well, i am still learning the ropes on repair & troubleshoot with, of course, the AWESOME help & guidance from YOU GUYS.. .After my first successful Samsung 740N recapped, through this Fantastic Forum, i kinda get hook doing them. I even spending $ on buying faulty LCDs without questioning whether the panel is crack/having dead pixels. The learning experience is priceless. Of course, it also doesn't hurt earning some extra bucks that comes along with it, isn't it?. LOL.
Now, back to the power board.

With monitor plugged and DMM set at 20V DC,
- the reading of Pin 3 with respect to Pin 1 of IC901 is 0.04V (stable)
- the reading of Pin 7 with respect to Pin 1 of IC901 is 0.07V (stable)
Then i continued measuring (with DMM set at 2000k ohms) R912, R916, and R918 as per your instruction, but the reading, at least to me, is surprising.
- R912 measured 367k ohms (& rising) but reversing the probe it reads 521k ohms (& rising),
- R916 measured 361k ohms (& rising) but reversing the probe it reads 532k ohms (& rising),
- R918 measured 365k ohms (& rising) but reversing the probe it reads 532k ohms (& rising),
From the schematic, they are supposed to be 665k ohms for each of R912, R916 & R918. So i conclude, correct me if i am not, that they need to be replaced.

So here are my other questions:
1) As i understand, & have tried it in front of my own eyes, resistor will give the same ohm readings even when you reverse the probe. So, how do you explain the measurement of R912, R916, and R918 that i did above?. Or did i make a mistake?
2) Can i replaced them with a single Axial resistor (linking the start point of R912 to the end of R918) instead of soldering 3 SMT resistors?. This is because i have never replaced an SMT resistor before, however, will dare myself to do it if there is no other choice.

[PlainBill]

Re: HP 1905 No power. No led. No output voltage.

It is most likely IC901 is shorted. Because of the design of this circuit you won't read the correct resistance on R912, T916, and R918 if IC901 is shorted.

PlainBill

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Ok. Roger that.
I will try to source the replacement for IC901. Appreciate your help.

[jetadm123]

Re: HP 1905 No power. No led. No output voltage.

Great job on troubleshooting so far. Just out of curiosity, can you take a resistance measurement between pins 1 and 3, pins 1 and 7 of IC901? Just wondering if VIN or VDD are shorted to ground. Thanks!

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Great job on troubleshooting so far. Just out of curiosity, can you take a resistance measurement between pins 1 and 3, pins 1 and 7 of IC901? Just wondering if VIN or VDD are shorted to ground. Thanks!

jetadm123, with DMM set at 2000 ohms
the reading of Pin 7 (VDD) with respect to Pin 1 shows no reading, BUT
the reading of Pin 3 (VIN) with respect to Pin 1 shows 229 ohms!!!

So PlainBill is SPOT-ON, as always, again.
IC901 (VIN - Gnd) seems to be shorted. I hope that is the only component that i need to replace. BTW, Pin 6 (Sense) is also shorted to Gnd having the same reading. And looking at the datasheet of the IC, it looks like neither Pins 1 nor 6 should be shorted to Gnd.

Another question i have is, anybody knows whether is there any other subtitute for this SG6841S IC which can fit & works? Maybe from other manufactures like TI, Fairchild, On semicon, STM, as such. I am staying where ordering from Digikey is not cost effective. Usually i order my Caps from RS components and i could not find SG6841S on their website. The worst case scenario is i would have to order through ebay which would, hopefully, take around 10-15 days to arrive.

[jetadm123]

Re: HP 1905 No power. No led. No output voltage.

Hello! Unfortunately, most of the PWM controller chips used have no substitute(s). However, if you figure out a replacement, please share! Have you tried Ebay? Since your location is in asia, there seem to be several sellers in Hong Kong that sell the SG6841.

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Hello! Unfortunately, most of the PWM controller chips used have no substitute(s). However, if you figure out a replacement, please share! Have you tried Ebay? Since your location is in asia, there seem to be several sellers in Hong Kong that sell the SG6841.

jetadm123, sorry for not replying sooner..
work, kids and especially the 'Queen' are needing more attention nowadays..
noted on using the same PWM controller IC. i have heed ur advise and ordered through Ebay from a seller in Hong Kong.. Once i am able to replace it, i will update again, which i guess will take 1-2 weeks (at best) to arrive, hopefully.

[NeiltheDruid]

Re: HP 1905 No power. No led. No output voltage.

Hi newbie1. I'm very interested in seeing the results of you replacing your SMPS controller, since like you say, seem to have very similar issues with our monitors. I am going to be testing my controller tonight to see if I get similar results to your tests - as unfortunately a full recap of my board was not the issue. Keep us posted.

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Hi newbie1. I'm very interested in seeing the results of you replacing your SMPS controller, since like you say, seem to have very similar issues with our monitors. I am going to be testing my controller tonight to see if I get similar results to your tests - as unfortunately a full recap of my board was not the issue. Keep us posted.

after about 5-6 hours of trying to fix this, i am EXHAUSTED!...

coincidentally, the SG6841S came from Hong Kong this afternoon.. removed the old IC.. measured 665k ohms for each of R912, R916 & R918.. was hopeful that all will go well.. replaced the faulty one (with some difficulty, but managed not to break any traces) with the new IC,. measured for any shorting in between pins but confirmed none, so i am convinced i did not short them during soldering the IC. tested before closing it.. Blue LED light-up and backlight stay on with the 'no signal' box flying around.

continue to assemble it & connect it to a PC, & then things start to go slowly from bad to worse..

first up, LED light-up, but no backllight. ok fine, i can faintly see the 'flying box' still even without backlight.. then i disconnect & disassembled to test it. Power LED on but no backlight still but when i slightly nudge the panel around the backlight stay on a while (about 20sec).. decide to partially assemble (w/out base) & test it.. this time Blue power LED turns to pulsing orange LED!!.. this is new to me.. disassembled again & test.. this time Blue LED light-up, backlight on.. but when i pressed off the on/off switch, the backlight stays on!!.. hmm...ok this is getting on my nerves.. now i unplugged it from the main power.. measure resistance on the pins of the IC but none was shorted.. feel that the processor on the logic card + 2 voltage regulators are HOT, but visually did not detect any burning on them,.. ok now for the final try.. plugged it back, this time NO power LED light-up.. so i am back to square 1.. so now, voltage on the SG6841S is Pin 3 is 0.18V.. same goes with Pin 7.. but no shorting in between them to Gnd..

sorry if the long story above if confusing to you guys,, just feeling tired, agitated + frustated with this ECCENTRIC monitor.. .. in addition to being sleepy right now.. its almost 3AM here.. & i need to wake up in a few hours time to send my kids to school..

even still, will not give up trying to figure out what went wrong.. until you guys say that it is the processor or something that really is beyond my capability of repairing..

hope you guys can assist.. *YAWN!!*

[seanc]

Re: HP 1905 No power. No led. No output voltage.

In my opinion you have a shorted transistor somewhere, test the all.

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

In my opinion you have a shorted transistor somewhere, test the all.

After a good night sleep, & in between some fantastic & bizarre EPL games, i managed to check all the transistor for short but was unable to detect any. So i decided to replace the new SG6841SZ IC with another (yes, i bought 2 pieces) because the voltage on Pin 3 is <5V & steady. But unfortunately destroyed pin 6 pad/trace on the board so i had to hardwired Pin 6 to the connecting smd cap.

Re-assembled & Plugged the monitor but still No Power LED light up. So i continue to measure the voltages that i assume is relevant;
- across the big 150uF 400V caps (C904) = 327V DC.
- output voltage 12V to logic card = 12.79V DC
- output voltage 5V to logic card = 4.95V DC
- SG6841SZ Pin 3 to Gnd (Pin 1/-ve of big cap) = 16.9V DC
- SG6841SZ Pin 7 to Gnd (Pin 1/-ve of big cap) = 14.2V DC
- IC701 (OZ9RRB) Pin 4 & 5 (DRV) to Gnd = 145-146V DC (unstable)
- IC701 (OZ9RRB) Pin 6 (VDDA) to Gnd = 149-150V DC (unstable)
- IC701 (OZ9RRB) Pin 8 (ISEN) to Gnd = 145-146V DC (unstable)

However, i am stuck what to look for next.. any input is greatly appreciated..
thanks y'all..

[alexanna]

Re: HP 1905 No power. No led. No output voltage.

your seeing 12.79v and 4.95v at the video board connectors,To me that suggests the SMPS is working. Please someone smack me if i am wrong.
Can you post a pitcure of the video board?
Al.

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

your seeing 12.79v and 4.95v at the video board connectors,To me that suggests the SMPS is working. Please someone smack me if i am wrong.
Can you post a pitcure of the video board?
Al.

Hi Al,
hhhmmm.. not too sure about that.. what i understand, from PlainBill's explanation was that IC901 (SG6841SZ) will start when voltage at pin 7 is about 16 volts.

The prime suspect is IC901 - the SMPS controller. It is a prone to failure. With the monitor plugged in points on the power supply are at over 300 volts, so be careful. Plug in the monitor and using pin 1 of IC901 as ground measure the voltage at pins 3 and 7. IC901 will not start until the voltage at pin 7 is about 16 volts.

If pin 3 is above 5 volts, below 16 volts, and steady, replace IC901. If it reads 0 volts check R912, R916, and R918. If it is pulsing, replace C917 and C918.

PlainBill

But mine was measured at

Re-assembled & Plugged the monitor but still No Power LED light up. So i continue to measure the voltages that i assume is relevant;
- SG6841SZ Pin 3 to Gnd (Pin 1/-ve of big cap) = 16.9V DC
- SG6841SZ Pin 7 to Gnd (Pin 1/-ve of big cap) = 14.2V DC

will try to take the images of the video board, & upload them in the next few days..

[jetadm123]

Re: HP 1905 No power. No led. No output voltage.

Okay, let's do a rehash of what's going on:

1) You measured 12.79V and 4.95V outputs at the output of the power supply. GOOD! The 12.79V is used to drive the inverter section and the 4.95V the logic card and LCD panel. So, it looks like the SG6841 and power supply are doing their job.

2) Now you have no picture and no power light. This can be caused by several problems:

a) With the power on, try shining a flashlight on the screen to see if you can see an image.

b) Possible burned out fuse(s). I didn't see one on the schematic, but it doesn't mean one doesn't exit. Check for fuses: glass, surface mount, bare wire or resistor types on both power and logic boards marked as FX (where "X" is a 1-3digit number). Look on the top and bottom of the board around the output of the power board and the input of the logic board.

c) Possible shorted CCFL drivers (Q701 and Q705). These are duo mosfets and should be checked for shorts.

d) Possible bad voltage regulator(s) on the the logic card or bad logic card. Will need photos of the logic card.

e) Possible bad CCFL tube(s). Do you notice a quick flash on the screen when you first power on the monitor?

f) Possibly a bad CCFL controller (IC701).


3)I think you measured the voltage of IC701 incorrectly. If you are truly seeing over 145V, then you would have seen the "magic smoke". Pins 4 and 5 are outputting a square wave voltage.


PlainBill: did I leave anything else out?

[PlainBill]

Re: HP 1905 No power. No led. No output voltage.

Okay, let's do a rehash of what's going on:

1) You measured 12.79V and 4.95V outputs at the output of the power supply. GOOD! The 12.79V is used to drive the inverter section and the 4.95V the logic card and LCD panel. So, it looks like the SG6841 and power supply are doing their job.

2) Now you have no picture and no power light. This can be caused by several problems:

a) With the power on, try shining a flashlight on the screen to see if you can see an image.

b) Possible burned out fuse(s). I didn't see one on the schematic, but it doesn't mean one doesn't exit. Check for fuses: glass, surface mount, bare wire or resistor types on both power and logic boards marked as FX (where "X" is a 1-3digit number). Look on the top and bottom of the board around the output of the power board and the input of the logic board.

c) Possible shorted CCFL drivers (Q701 and Q705). These are duo mosfets and should be checked for shorts.

d) Possible bad voltage regulator(s) on the the logic card or bad logic card. Will need photos of the logic card.

e) Possible bad CCFL tube(s). Do you notice a quick flash on the screen when you first power on the monitor?

f) Possibly a bad CCFL controller (IC701).


3)I think you measured the voltage of IC701 incorrectly. If you are truly seeing over 145V, then you would have seen the "magic smoke". Pins 4 and 5 are outputting a square wave voltage.


PlainBill: did I leave anything else out?

I'm multitasking (watching football and checking in here), but you have described the situation succinctly. The SMPS is working, now it's time to look at the logic card.

One point you may have missed: When the OP disassembled the monitor he probably disconnected the link between the logic card and the front panel.

PlainBill

[newbie1]

Re: HP 1905 No power. No led. No output voltage.

Okay, let's do a rehash of what's going on:

1) You measured 12.79V and 4.95V outputs at the output of the power supply. GOOD! The 12.79V is used to drive the inverter section and the 4.95V the logic card and LCD panel. So, it looks like the SG6841 and power supply are doing their job.

2) Now you have no picture and no power light. This can be caused by several problems:

a) With the power on, try shining a flashlight on the screen to see if you can see an image.

b) Possible burned out fuse(s). I didn't see one on the schematic, but it doesn't mean one doesn't exit. Check for fuses: glass, surface mount, bare wire or resistor types on both power and logic boards marked as FX (where "X" is a 1-3digit number). Look on the top and bottom of the board around the output of the power board and the input of the logic board.

c) Possible shorted CCFL drivers (Q701 and Q705). These are duo mosfets and should be checked for shorts.

d) Possible bad voltage regulator(s) on the the logic card or bad logic card. Will need photos of the logic card.

e) Possible bad CCFL tube(s). Do you notice a quick flash on the screen when you first power on the monitor?

f) Possibly a bad CCFL controller (IC701).


3)I think you measured the voltage of IC701 incorrectly. If you are truly seeing over 145V, then you would have seen the "magic smoke". Pins 4 and 5 are outputting a square wave voltage.


PlainBill: did I leave anything else out?

jetadm123, thanks for the inputs.. just a quick updates below,..
p/s- i may not have all the answers to all the questions above.. but i will try my best to answer them..

1) Glad to know & hope so it lasts, because that is my last IC.. i am not sure whether i busted the one i replaced earlier while soldering it.

2a) Currently i have not have the chance to assembled them back as i want to prioritize the taking of the logic card photo. But as far as i can recall, i can see no image.

2b) I tried scanning with my untrained eye but could see none also.

2c) Pins 5, 6, 7 & 8 of Q701 are all shorted (0.08ohms) to each other. Same goes with Q705. But i guess they should be shorted, because from schematic they are tied to the common Pins 1, 3 & 5 of PT701. Same goes with PT702. Right?

2d) Photos as attached. Note on the bottom side there is some stain which is flux.
Ok, i hope u can confirm this.
U301 (AIC1084-18cm)
Pin 1-2 = 81.3 ohms, the rest of the pins are not shorted (brief flash of reading then open)
U302 (AIC1084-33cm)
Pin 1-2 = 4.5 ohms, the rest of the pins are not shorted (brief flash of reading then open)

so are they shorted?. fyi, i continue measuring resistance across the electrolytic caps & all of them (except C401) are having the reading of either 81.3ohms or 4.5ohms. I tried removing one of them (C309) (that's where the flux came from) & measure on the pad. It shows the same with the caps off as well. Anyway i replace it since i have a spare. Not sure what's the logic or whether it is significant?

2e) same answer as 2a. AFAIK none.

3) geez!.. my bad. Probably i hook to the wrong Gnd, i guess.

I'm multitasking (watching football and checking in here), but you have described the situation succinctly. The SMPS is working, now it's time to look at the logic card.

One point you may have missed: When the OP disassembled the monitor he probably disconnected the link between the logic card and the front panel.

PlainBill

PlainBill, true. When i disassembled it i need to remove the panel switch/pcb board. But upon testing i need to fix back because the on-off switch is located over there.. I hope i am understanding what you mean & answering it correctly.. Talking about multitasking.. I am now watching a cooking program with the other half while typing this, lol.. ok, off for my showers now..