hanns g hh251 not recognizing input

[younat]

Hello!

I recently acquired an LCD panel that the lab next door was throwing away. The only historical information I had was a post-it note with the word "Broken" on it. I took it home and powered it up. The green light comes on, a message saying "No input detected" appears on the panel for a few seconds, and it goes straight to standby with the power light turning amber. I tested the vga input with a few computers. The display is recognized by the computers, so I figured the logic board is ok.

I proceeded to take it apart and test the main capacitor (161 V) and the DC outputs (5.2 V as spec and 12 V as spec when the power light is green). The diodes, 12 V regulator, and fuses all appear to be in good shape as well from multimeter measurements. There are no bulging caps, but I don't have an ESR meter so I wanted to have a good reason before I desoldered all the caps.

I feel like I've given the power supply a good testing based on what I've read on this forum. Are my symptoms indicative of a bad component on the logic board? Despite my searching I haven't found a post with the same symptoms.

Any help is greatly appreciated. Thanks!

[angel-boy]

Re: hanns g hh251 not recognizing input

This model has been discussed, but different problems

https://www.badcaps.net/forum/showthread.php?t=25295
https://www.badcaps.net/forum/showthread.php?t=34009

I hope I was helpful

[selldoor]

Re: hanns g hh251 not recognizing input

You checked the fuse/s on the back of the power supply.?
Next things to check would be inverter transformers, bulbs, 2 mosfets on the back that drive the inverters.
You should post pictures of both sides of your boards then we would know what we are dealing with

[younat]

Re: hanns g hh251 not recognizing input

Thanks for the help! I did take a look at those other posts, angel-boy, but didn't seem to have the same issues as described. Here are the pictures of the PSU and logic board. I did some more probing and found a diode on the back of the PSU that appears to be shorted according to my multimeter (there is a red arrow pointing at it). Could that really be the issue that sends the LCD to standby? All the fuses I could find on both boards seem fine.

[Rtech]

Re: hanns g hh251 not recognizing input

At the moment, it is still not certain which PCB is faulty, it looks like either the Logic OR the Power Supply.I would determine that,by removing the logic board, plug the lamps into the PSU,fit a 1KΩ resistor between the 5v pin and the on/off pin on the connector,this simulates the voltage you would normally get from the Logic when all is working well.Then apply the AC mains as normal.If the lamps light and stay on then the fault would point to the Logic PCB.If the lamps do not work, then the PSU needs fault finding.

[younat]

Re: hanns g hh251 not recognizing input

That is a great idea! I'll give that a try tonight and let you know what I find.

[younat]

Re: hanns g hh251 not recognizing input

Ok, so I tried what Rtech suggested. I put a 1k resistor between the 5.2V and O/F pins (I also tried putting PSON high, but that didn't do anything that I could see). I heard a slight, regular clicking and the backlight flashes with the clicking sound. When I removed the resistor, the clicking sound remained, but the backlight didn't flash. The 5.2V stays constant for all of this, but the voltage on the input of the 12V regular fluctuates between 5-7V when the resistor is connected and 9-11.5V when the resistor is not connected. Somehow the backlight circuit is pulling this down.

When I plug the logic board in, the backlight behaves as before. 2 seconds on, then straight to standby. There is no clicking at all when the logic board is connected. So this means that the PSU is likely the problem, right? Any ideas? I'm going to replace the burned out diode that I labeled in the uploaded pictures and try again.

Thanks for the suggestions! I feel like I'm learning a lot.

[younat]

Re: hanns g hh251 not recognizing input

I replaced the diode that I thought was shorted, but the good one appears shorted once it is soldered in the circuit as well... I can't think of any useful circuit where a diode would be shorted in both directions, but I'm not too experienced in electronics.

I may have to amend my previous post. The clicking and flashing backlight now only occurs when the resistor is holding the backlight control high. The clicking goes away when the resistor holds PSON high. The 12V is stable and reading what it should when the PSON is high. So it appears that I've narrowed it down to something with the backlight circuitry. Does that make sense to you guys?

[budm]

Re: hanns g hh251 not recognizing input

"The green light comes on, a message saying "No input detected" appears on the panel for a few seconds, and it goes straight to standby with the power light turning amber. I tested the vga input with a few computers. The display is recognized by the computers, so I figured the logic board is ok." This means the main board of the monitor did not detect the Video signals from the computer (but the computer does see the signal on the ID pin of the VGA connector) so it goes into stand mode (amber LED). Unless I am wrong, the LED should stay green when it detects the Video, did it do that before it goes into standby? Otherwise you may have power supply or inverter circuit problem.
You need to check VGA connector pins for the H sync, V sync pin, and also the R, G, and B pins to see if any one of them shows very low resistance (due to bad protection diode on each one of those pins), most monitor detect the G signal pin to see if it is present or not. If you look real close around the VGA connector, you will see lots of Diodes in that area. The VGA port could have been zapped with static discharge.
BTW, yes, they do put diode in parallel with the resistor in certain circuits, that is why you always test the component off the board when you suspect the component is bad.
See attached PDF and pictures of typical circuits setup.

"The 5.2V stays constant for all of this, but the voltage on the input of the 12V regular fluctuates between 5-7V when the resistor is connected and 9-11.5V when the resistor is not connected. Somehow the backlight circuit is pulling this down." This pulling down of the Voltage by the inverter circuit or the 12V power supply section is weak (bad filter caps). I would check those two 8-pin power MOSFET S and D pin to see if it shows low resistance. The current draw that caused the 12V to drop may have cause the 5V to drop just fast enough to reset the circuit on the main board but your meter is not fast enough to detect.
You can remove the fuse F260 (next to jumper JP205) which feed the Voltage to the inverter circuits and see if the monitor still goes into standby mode, I would do this first to help narrow down the problem.

[Rtech]

Re: hanns g hh251 not recognizing input

Looks like you are getting somewhere now.BUDM has suggested some tests,lets see what happens ??

[younat]

Re: hanns g hh251 not recognizing input

Well, I removed the fuse. The monitor still went to sleep with and without a computer plugged in. I checked the MOSFETs and the resistance between the source and drain is in the megaohms for all four. I tried testing the VGA port and saw that the R and G ports are open circuit to their respective R and G grounds. The B port looks ok (it reads 74.7 Ohms to B ground which seems to match the schematic budm posted). So it is probably a case of ESD. All the diodes seem to be ok though. I don't think I want to go to the effort of troubleshooting the logic board anymore. I'm going to order an HDMI cable and with any luck one of the ports will work. As it stands now, the wife wants the dining room table back, so it looks like this project is closed.

Thanks for all the help! Although I didn't solve the problem, I learned a lot in the process which is well worth it.