Need Circuit board troubleshooting procedures on a computer monitor

Thread Starter

Swofra

Joined May 19, 2020
11
Hello all and thanks- don’t want to waste your time so I’ll get right to it
Asus VK246H monitor does not power on. I have no electrical testing background but do have a multimeter and know how to use it (thanks YouTube!)
I have confirmed that the board is receiving 120V as read at the power receptacle solder points on the board.
The fuse shows 0 when tested in diode mode unplugged and 120V AC when powered so as I understand it, that means it’s working.
I’ve tested the bridge rectifier (assuming it’s the black standing rectangle next to the brown capacitor) in diode mode and it reads ~.5 at all three points, and zero if i reverse the probes.
However, when I test the BR two inner contacts for incoming AC volts, I only get readings in the 2 millivote range, and the dc test of the outer two contacts (black probe on the negative, red on the positive) reads about .001. To my noob brain, this seems to indicate that the BR isn’t getting any power. I also can’t detect any power at the power at the exit power that runs to the display board
I’ve attached the front and back of the PCB for reference. I searched for the service manual online but did not have any luck. Thanks again if you’ve read this far and I look forward to learning from you!
- Swofra
image.jpgimage.jpg
 
Last edited:

AlbertHall

Joined Jun 4, 2014
9,873
Measure the voltage across the large black capacitor - it should be around 170V. Be very careful not to short the capacitor with the meter probes. That will make a big bang.
If that voltage isn't there then the problem is between the bridge rectifier and the power input and that should be fixable.
 

BobTPH

Joined Jun 5, 2013
2,391
Are you saying you put the probes across the fuse and read 120V? If that is the case, the fuse is blown, it should read 0V. And why would you test a fuse on the diode check range? Try the Ohms range, again, it should read zero.

Bob
 

AlbertHall

Joined Jun 4, 2014
9,873
With no power applied, measure the resistance from each power input terminal to each AC leg of the bridge rectifier (four measurements).
 

Dodgydave

Joined Jun 22, 2012
9,162
Without any AC power, put the DVM on Ohms, and measure the resistance across the mains input socket, from each pin to the bridge rectifier ~ side , there should be a low ohms or Zero ohms reading. Check the fuse, also the Mov TR601.
 

Thread Starter

Swofra

Joined May 19, 2020
11
Ok, thanks. Here’s what I found:
*Fuse shows 0 ohms
*Power input to BR rectifier (without power applied): The left power input terminal has 0 ohms to either BR AC terminals. The right power input has .6 ohms to the BR AC leg nearest the positive DC terminal. It has .99 ohms to the BR AC leg nearest the negative DC terminal.
*TR601 5.8 ohms

Let me know what else I can do/test. Thx!
 
Ok, thanks. Here’s what I found:
*Fuse shows 0 ohms
*Power input to BR rectifier (without power applied): The left power input terminal has 0 ohms to either BR AC terminals. The right power input has .6 ohms to the BR AC leg nearest the positive DC terminal. It has .99 ohms to the BR AC leg nearest the negative DC terminal.
*TR601 5.8 ohms

Let me know what else I can do/test. Thx!
I have repaired a couple of these Asus VK246H monitors, both with the same problem 'would not power up', in each case the fault was the 3.3v regulator (G1084-33) had failed. Worth checking. Good luck.
 

Thread Starter

Swofra

Joined May 19, 2020
11
I have repaired a couple of these Asus VK246H monitors, both with the same problem 'would not power up', in each case the fault was the 3.3v regulator (G1084-33) had failed. Worth checking. Good luck.
I found that part on the logic (or whatever it’s called) board. How do I test it? Additionally, does the ohms readings I found on the BR indicate that is in working condition? Thanks!
 

Thread Starter

Swofra

Joined May 19, 2020
11
Tested the bridge rectifier per the video and it checks out ok. I’m not detecting any power to the voltage regulator on the logic board as it shows .000 Dc to the input regardless if it is the red or black probe. I also don’t get any dc readings at the power junction leaving the power board to the logic board. Any other ideas or suggestions?
 

AlbertHall

Joined Jun 4, 2014
9,873
Ok, thanks. Here’s what I found:
*Fuse shows 0 ohms
*Power input to BR rectifier (without power applied): The left power input terminal has 0 ohms to either BR AC terminals. The right power input has .6 ohms to the BR AC leg nearest the positive DC terminal. It has .99 ohms to the BR AC leg nearest the negative DC terminal.
*TR601 5.8 ohms

Let me know what else I can do/test. Thx!
These readings are not self-consistent.
"The left power input terminal has 0 ohms to either BR AC terminals" implies that the bridge AC terminals are shorted to each other. Is this so?
If not please re-check the resistances from mains input to the bridge terminals.
 

Thread Starter

Swofra

Joined May 19, 2020
11
Verified the previous resistance readings from ac terminal to br. Didn’t know exactly how to test for short in ac terminals so I ended up attaching a paper clip so that it touched both ac terminals and used the same method. I got the same reading from both ac terminals for the .990 reading, the br ac input nearest the + terminal was giving highly variable and unstable readings (4-23) from the left ac terminal. If I remove the paper clip and check the ac probes then I get 0
 

Dodgydave

Joined Jun 22, 2012
9,162
Put your meter on Ohms and measure across the AC terminals on the bridge rectifier. It should read around 100ohms rising .

Then the check from the mains socket from the bridge rectifier.
 

Thread Starter

Swofra

Joined May 19, 2020
11
Ok thx. I’m attaching another picture to make sure we are all on the same page. Here are the readings:
- Br ac terminals (1&2 on the pic): .990 M ohms (rose slowly from .7)
- R main Ac terminal (R on pic) to br ac (2): .6 ohms
- R main Ac terminal to br ac (1): .991 M ohms
- L main Ac terminal (L on pic) to (2): 0 M ohms
- L main Ac terminal to (1): 0 M ohms
- (1) to (2): 0 M ohms
- BR diode test:
*Black probe (on + br) & red probe on (2): .545 dcvolts. Reversed OL
*Black probe (on + br) & red probe on (1): .543 dcvolts. Reversed OL
*Red probe (on - br) & black probe on (2): .540 dcvolts. Reversed OL
*Red probe ( on - br) & black probe on (1): .550 dcvolts. Reversed OL
*Red probe (on - br) & black probe on (+ br): .551 dcvolts. Reversed OL
Hopefully that helps and makes sense
 

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Thread Starter

Swofra

Joined May 19, 2020
11
Thx. Referencing the previous pic, with ac power plugged in: I get: From L to 1 and 2 122V, from R to 1 & 2 about 5mV slowly dropping
Red probe on +br, black on -br I get 10.5Vdc and slowly dropping. I retested later and only got 1.7
If that indicates that br is working, then where do I go from here? I tried following the connections along the circuit board and found the same readings in the large brown capacitor. I’m probably doing it wrong or missing several steps but I’m not sure
 

Thread Starter

Swofra

Joined May 19, 2020
11
Last ask before I have to toss the monitor as unsaveable. I truly don’t know what steps would be next. Thanks
 

Thread Starter

Swofra

Joined May 19, 2020
11
Thanks for the input and continued help.
Between + and - ac br dc terminals I get .25 Vdc and rising the longer I leave it plugged in.
Between 1 & 2 on ~ I get declining mV down to less than 1
I get the feeling that the cause is a cheap component to replace and I’d hate the thought of getting rid of the monitor if that is the case. Plus my inquisitive mind wants to figure out what it is. Thanks again for the help!
 
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