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**momaka** · 12-06-2015, 05:36 PM

Re: Acer LCD monitor on-off repeatedly

Attach some pictures of your boards so that we can see what you are seeing.

Sometimes, the monitor's video/logic board may also have caps that go bad, and the symptoms will be similar. A good example of this is the Hanns-G HG281D monitor. What's worse is that the caps on the video/logic board are often small ones, so in most cases they won't show any signs of problems.

But before you replace anything further, again I suggest you post some pictures so we can see what your are dealing with.

**paulfxh** · 12-08-2015, 03:07 AM

Re: Acer LCD monitor on-off repeatedly

Hi Momaka
Thank you for your reply.
Here's the power supply board:

And here's the main board

Attached Files

**momaka** · 12-08-2015, 07:01 AM

Re: Acer LCD monitor on-off repeatedly

Excellent pictures, paulfxh! You really make it easy on us.

The power supply board appears to be properly recapped with what looks like Panasonic FC/FR/FM caps. So really good job there!
The only thing I see slightly off is resistor R506 - it looks slightly overheated. Its located on the lower-right corner of the power supply board in the picture you posted. The 4-color code suggests it should be 1 MOhm (1000000 Ohms), just like R525. If you have a multimeter, can you verify this?

Also, the main board does have some small caps like I mentioned. But before replacing them, it may also be worthwhile to check the voltage output of the two linear regulators, both when the monitor is cold and works and when the monitor starts acting up. The two linear regulators are those small rectangular boxes with numbers on them "MT1117". The one on the top-left side of the picture you provided is a 3.3V regulator (as the "3.3" on the second line of its part number suggests), while the one of the bottom-left side is a 1.8V regulator.

For these regulators, the rightmost pin is the input voltage (pin 3 on their datasheet, just look up any "1117" datasheet) and the center pin / big tab (pin 2) is the output voltage (i.e. 3.3V or 1.8V). These voltages should be pretty close to their value. Let me know what you get, though. Check voltages with respect to ground (monitor chassis).

There is a slight chance that on or more of those caps may be bad and causing the regulators to oscillate. So consider replacement if you have the caps.

Finally, if you could also post a picture of the underside of the power supply, that would be ideal. Just to see if everything else there is in order too.

**budm** · 12-08-2015, 10:35 AM

Re: Acer LCD monitor on-off repeatedly

What DCV do you see on that 8-pin white connector on the power supply board that goes to the main board, are they varying up and down?

**paulfxh** · 12-08-2015, 01:10 PM

Re: Acer LCD monitor on-off repeatedly

Originally posted by momaka View Post

Excellent pictures, paulfxh! You really make it easy on us.

The power supply board appears to be properly recapped with what looks like Panasonic FC/FR/FM caps. So really good job there!
The only thing I see slightly off is resistor R506 - it looks slightly overheated. Its located on the lower-right corner of the power supply board in the picture you posted. The 4-color code suggests it should be 1 MOhm (1000000 Ohms), just like R525. If you have a multimeter, can you verify this?

Also, the main board does have some small caps like I mentioned. But before replacing them, it may also be worthwhile to check the voltage output of the two linear regulators, both when the monitor is cold and works and when the monitor starts acting up. The two linear regulators are those small rectangular boxes with numbers on them "MT1117". The one on the top-left side of the picture you provided is a 3.3V regulator (as the "3.3" on the second line of its part number suggests), while the one of the bottom-left side is a 1.8V regulator.

For these regulators, the rightmost pin is the input voltage (pin 3 on their datasheet, just look up any "1117" datasheet) and the center pin / big tab (pin 2) is the output voltage (i.e. 3.3V or 1.8V). These voltages should be pretty close to their value. Let me know what you get, though. Check voltages with respect to ground (monitor chassis).

There is a slight chance that on or more of those caps may be bad and causing the regulators to oscillate. So consider replacement if you have the caps.

Finally, if you could also post a picture of the underside of the power supply, that would be ideal. Just to see if everything else there is in order too.

Thank you, Momaka, for your most instructive reply.
Well, as I said, I had never taken a monitor apart before so I'm sailing in what are for me uncharted waters.
Nevertheless, I'm starting to see light at the end of the tunnel.
I hooked up the two boards together with the monitor keyboard and the four connectors from the screen to the power board.
Only the ribbon cable from the main board to the backlights on the screen remained unconnected.
Then with main power cord inserted I switched it on.
The 3.3v linear regulator gave a fairly steady 4.6v in and 3.6v out.
The 1.8v regulator gave 3.5v in and 2.0v out which was also fairly steady over about 20 minutes.

I also want to say thanks to budm who asked me to measure the voltage on the 8 pin connector between the two boards. Here I got 4.8v which was again very steady.

However, the big surprise here was that the blue power light on the monitor keyboard remained blue throughout the 20 minute test period.
No turning off, no orange light. Just blue.
Does this indicate that the problem is in the backlights?

On the other questions raised in Momaka's post, I had actually previously checked all nine resistors on the power supply board and found that both R506 and R525 both gave readings well below their rating.
Incidentally, both of these resistors seems to me to be 2.5 MOhm (mega-ohm) resistors rather than the 1.0 MOhm that you see.
The band colors I see are Red-Green-Green-Gold on both.
I've attached close-up photos of both.

However, in both cases I get only 1.5MOhms on the mutimeter. Bear in mind, however, that I did not remove either pin of the resistors from the PCB to make these measurements.
All the same, the seven other resistors on the power supply board all gave resistance readings within the allowed tolerance (5%).
And, yes, R506 does look just a little darker than R525.

I've also attached a photo of the solder-joint side of the power supply board as requested and I've included the solderjoint side of the main board as well.
Thanks to everyone for the help so far.
I'm learning a lot here.

Attached Files

**budm** · 12-08-2015, 01:48 PM

Re: Acer LCD monitor on-off repeatedly

'However, the big surprise here was that the blue power light on the monitor keyboard remained blue throughout the 20 minute test period.
No turning off, no orange light. Just blue.
Does this indicate that the problem is in the backlights?'
So the blue indicator light is on steady but no backlights?
How about the pin 5 ON/OFF (Backlights command pin)? What do DCV do you get on that pin? <1VDC = Backlights OFF, >2VDC = Backlights ON.
Can we get the pictures of the bottom side of the power supply/inverter board?

**paulfxh** · 12-08-2015, 04:12 PM

Re: Acer LCD monitor on-off repeatedly

Originally posted by budm View Post

'However, the big surprise here was that the blue power light on the monitor keyboard remained blue throughout the 20 minute test period.
No turning off, no orange light. Just blue.
Does this indicate that the problem is in the backlights?'
So the blue indicator light is on steady but no backlights?
How about the pin 5 ON/OFF (Backlights command pin)? What do DCV do you get on that pin? <1VDC = Backlights OFF, >2VDC = Backlights ON.
Can we get the pictures of the bottom side of the power supply/inverter board?

Thanks for the comments, budm
Well, here's another surprise. I have tried to repeat what I did and reported in my last post, but now I am getting different, but this time, the expected behaviour from the system as set up.
Also, it seems I was mistaken in assuming that the ribbon cable from the main board to the screen was the backlight cable. The manual refers to this as the LVDS cable which provides data to the screen.
The backlights are powered from the four connectors on the power supply board.

In any event, this time (and I repeated this about four times with the same results), the blue light came on but went off and on again with more or less the same frequency as I noted in my first post.
Additionally, the backlights came on and off with a similar, but not identical frequency.
[Why on my first attempt the backlight did not function is something I cannot explain. Also I cannot explain why the blue light stayed on for fully twenty minutes with no perceived voltage variation across the board. I had thought that perhaps one or more of the backlight connectors were incorrectly inserted but even when I took out all four of them this time, the blue power light continued to flash on and off as before although, of course, the screen stopped functioning].
So, it doesn't after all look like a CCFL light problem.
I measured the voltages again, under these "new" conditions, on the 3.3v Linear Regulator, the 1.8V L. Regulator and the 8-Pin connector between the Power Supply Board and the Main Board.
All three voltages varied significantly as the blue power light flashed on and off.
The 3.3v L.Regulator (IN) varied from a high of 5.1v to a low of 2.4v (Low being when the blue light was off).
The 3.3v L.Regulator (OUT) varied from 3.5v to 1.4v
The 1.8v Regulator (IN) varied from 3.4v to 1.4v.
The 1.8v Regulator (OUT) varied from 2.4v to 1.0v
The 8-Pin connector varied from 5.3v to 2.4v

budm, I am unclear as to what you mean by the pin 5 ON/OFF (Backlights command pin).
I would be glad to measure its voltage if I knew where to find it.
This is the first time that I have ever opened up an LCD monitor.

You also asked for a photo of the power supply board bottom side.
In fact I had attached this to my last post (as the third of four attachments).
However, I attach it here again for your convenience.

Attached Files

**budm** · 12-08-2015, 04:56 PM

Re: Acer LCD monitor on-off repeatedly

'The 8-Pin connector varied from 5.3v to 2.4v'
'I also want to say thanks to budm who asked me to measure the voltage on the 8 pin connector between the two boards. Here I got 4.8v which was again very steady.' I do not understand your measurement, steady reading or not steady reading. 4.8V for VCC is also on the low side.

The 8-pin connector on the power supply board are labeled with pin names to tell you what they are, see picture attached.
The 5VCC should be steady 5VDC.
To make it less confusion, you should report the reading of each pin and followed by the pin names so we can see what each pin has.

Attached Files

**paulfxh** · 12-09-2015, 02:09 AM

Re: Acer LCD monitor on-off repeatedly

Originally posted by budm View Post

'The 8-Pin connector varied from 5.3v to 2.4v'
'I also want to say thanks to budm who asked me to measure the voltage on the 8 pin connector between the two boards. Here I got 4.8v which was again very steady.' I do not understand your measurement, steady reading or not steady reading. 4.8V for VCC is also on the low side.

The 8-pin connector on the power supply board are labeled with pin names to tell you what they are, see picture attached.
The 5VCC should be steady 5VDC.
To make it less confusion, you should report the reading of each pin and followed by the pin names so we can see what each pin has.

Hi budm
Thanks for the clear explanation.
I've made the measurements suggested and present the results here.
Note, however, that all voltages on that 8-pin connector vary as the blue power light (and the backlights on the screen) switch on and off (without any intervention from me).
Typically, the blue light stays on for 5-6 seconds and off for 3 seconds. In between these two events, the orange power light comes on for about 1 second.
For the few seconds that the blue light in on, the voltages are, indeed, steady.
Therefore, I present two voltage reading for each pin. One when the blue power light (on the keyboard) is on, and the other when the blue light (and backlights) are off.

Pin 1: vcc5v: Blue on 5.2v, Blue off 2.8v
Pin 2: vcc5v: Blue on 5.2v, Blue off 2.6v
Pin 3: gnd: Blue on 1.8v, Blue off 1.8v
Pin 4: gnd: Blue on 1.6v, Blue off 1.6v
Pin 5: On-Off: Blue on 4.1v, Blue off 1.6v
Pin 6: Brightness: Blue on 3.9v, Blue off 1.9v
Pin 7: Vol: Blue on 1.7v, Blue off 2.0v
Pin 8: Mute: Blue on 1.4v, Blue off 1.9v

**budm** · 12-09-2015, 09:58 AM

Re: Acer LCD monitor on-off repeatedly

Pin 1: vcc5v: Blue on 5.2v, Blue off 2.8v
Pin 2: vcc5v: Blue on 5.2v, Blue off 2.6v
Pin 3: gnd: Blue on 1.8v, Blue off 1.8v
Pin 4: gnd: Blue on 1.6v, Blue off 1.6v
Pin 5: On-Off: Blue on 4.1v, Blue off 1.6v
Pin 6: Brightness: Blue on 3.9v, Blue off 1.9v
Pin 7: Vol: Blue on 1.7v, Blue off 2.0v
Pin 8: Mute: Blue on 1.4v, Blue off 1.9v
That must be measurement error, What did you for GND ref for your meter? You must be doing it wrong because you are showing 1.8V, 1.6V on the GND (Ground pins). All your readings are wrong.
GND pin should be the GND ref for your meter black probe.

**paulfxh** · 12-09-2015, 05:08 PM

Re: Acer LCD monitor on-off repeatedly

Originally posted by budm View Post

Pin 1: vcc5v: Blue on 5.2v, Blue off 2.8v
Pin 2: vcc5v: Blue on 5.2v, Blue off 2.6v
Pin 3: gnd: Blue on 1.8v, Blue off 1.8v
Pin 4: gnd: Blue on 1.6v, Blue off 1.6v
Pin 5: On-Off: Blue on 4.1v, Blue off 1.6v
Pin 6: Brightness: Blue on 3.9v, Blue off 1.9v
Pin 7: Vol: Blue on 1.7v, Blue off 2.0v
Pin 8: Mute: Blue on 1.4v, Blue off 1.9v
That must be measurement error, What did you for GND ref for your meter? You must be doing it wrong because you are showing 1.8V, 1.6V on the GND (Ground pins). All your readings are wrong.
GND pin should be the GND ref for your meter black probe.

Thank you for that clarification.
Yes, for the readings in my last post I used an external (outside of either of the monitor pc boards) ground for the black probe on my multimeter.
I've repeated this exercise and this time I used Pin-3 (GND) on the 8-Pin connector (main board side) between the two boards for the multimeter black probe.
Once again, I took the readings both when the blue power light was on and when it was off.

Pin 1: VCC5v: Blue on 4.86v, Blue off 1.4v
Pin 2: VCC5v: Blue on 4.86v, Blue off 1.5v
Pin 3: GND: Blue on -0.15v, Blue off -0.15v
Pin 4: GND: Blue on -0.14v, Blue off -0.14v
Pin 5: On-Off: Blue on 2.98, Blue off 0.0
Pin 6: Brightness: Blue on 2.97v, Blue off -0.1
Pin 7: Vol: Blue on 0.25v, Blue off 0.7v
Pin 8: Mute: Blue on -0.04v, Blue off 0.5v

Note that when the blue power light is on, the voltage readings are very steady. However, when the blue power light goes off the voltage falls gradually over about a three second period and then rises quickly when the blue power light and screen turn themselves on again.
The 'Blue off' reading that I provide above is the lowest voltage that the meter reading reaches as it falls while the screen is off.
So, the voltages from the power supply board to the main board vary significantly in unison with the screen powering itself on and off.
But which is the 'prime mover'?
Does some defective component cause the screen to trip which necessarily causes the voltage to drop?
Or does something cause the voltage to drop which brings down the screen?

Also, now that I can actually see the pc boards as they operate, I should mention that I can neither see nor smell any evidence that any component on either of the boards is overheating which may have been a reason for the observed on-off behaviour. However, I don't have a non-contact thermometer to confirm or quantify this observation.

**budm** · 12-09-2015, 07:51 PM

Re: Acer LCD monitor on-off repeatedly

'Pin 1: VCC5v: Blue on 4.86v, Blue off 1.4v
Pin 2: VCC5v: Blue on 4.86v, Blue off 1.5v'
That is not right at all, VCC5V is the standby 5VDC which should be at 5Vdc at all time when the monitor is ON or OFF.
And I cannot see how you get Voltage readings on the GND pin especial on pin 3 since you have black probe on pin 3 which is GND pin.
Pin 3: GND: Blue on -0.15v, Blue off -0.15v
Pin 4: GND: Blue on -0.14v, Blue off -0.14v
Something is not done right.
At this point you have standby VCC problem if that is what you are seeing, may be bad startup/running cap.
Try heating the board with hair dryer for a couple minutes then plug the monitor into the outlet to see if VCC5V is steady at 5VDC.

**paulfxh** · 12-10-2015, 09:10 AM

Re: Acer LCD monitor on-off repeatedly

Originally posted by budm View Post

'Pin 1: VCC5v: Blue on 4.86v, Blue off 1.4v
Pin 2: VCC5v: Blue on 4.86v, Blue off 1.5v'
That is not right at all, VCC5V is the standby 5VDC which should be at 5Vdc at all time when the monitor is ON or OFF.
And I cannot see how you get Voltage readings on the GND pin especial on pin 3 since you have black probe on pin 3 which is GND pin.
Pin 3: GND: Blue on -0.15v, Blue off -0.15v
Pin 4: GND: Blue on -0.14v, Blue off -0.14v
Something is not done right.
At this point you have standby VCC problem if that is what you are seeing, may be bad startup/running cap.
Try heating the board with hair dryer for a couple minutes then plug the monitor into the outlet to see if VCC5V is steady at 5VDC.

Many thanks once again for your comments and suggestions.
I really am learning an awful lot from this thread.
I tried your hairdryer suggestion and a number of interesting observations came out of this.

1. First I should mention that my multimeter is not a top-of-the-range model. It's a Veyron VL-890D as shown in the attachment. Also, the manual provided is only in Chinese, which I cannot read, and I was unable to locate an English manual online.

2. The "zero-reading" on this multimeter (when set to Volts and with neither probe touching anything) is typically -0.15v at normal room temperature.
However, as I was heating the power supply board with the hairdryer, I noticed that the zero-reading on the multimeter, located close to the board, was increasing and eventually reached 0.00 without by any means excessive heating. I think this explains why the GND readings I reported yesterday were close to -0.15v

3. While repeating the voltage tests on the 8-Pin connector, my hand accidentally touched off one of the heatsinks on the board and I felt a moderate shock in my hand. So I checked the voltage on the heatsink against one of the GND pins and got -160v.
With that reading, I'm surprised I didn't get a bigger shock.
Certainly, there's something very wrong right there.
The heatsink concerned is shown circled in red in the attached photo.

4. I applied the hairdryer to the power board for about 5 minutes (I would guess the air temperature from the dryer was no higher than 45ºC) and began the voltage reading on the 8-Pin connector.
However, now I noticed that the a third "phase" had entered into the cycle as the monitor cycled on and off as before.
Now I see:
i) Blue light off, backlights off
ii) Blue light on, backlights off
iii) Blue light on, backlights on
iv) Phase ii again
v) Phase iii again
vi) Phase i again

...and all of this within about 8-10 seconds and repeated over and over.

5. So, after heating the power board with the hairdryer and ensuring that the multimeter zero-reading was 0.0, I carried out the voltage readings on the 8-Pin connector with the black probe touching against Pin 3 (GND) on the main board connector.
I took reading for Phases i, ii and iii as defined above.
Because this is getting complicated now, I present the results in spreadsheet form as an attachment.
Besides the 8 pins, I also measured the voltage variation on the heatsink and the filter capacitor (Blue cap in image of board 120 uF/450v) each of which varies through the phases mentioned here.

6. You mentioned that the fact that the standby voltage is not invariant may indicate one or more problems capacitors.
I have aready changed 12 of the 15 caps on the power board for Panasonic brand 105º caps. The ones that I have not changed are the filter cap (120uF/450v), C701 (10uF/50v) and C708 (220uF/25v).
None of the caps on the main board have been changed. There are six caps in total on the main board (3x22uF/16v, 2x100uF/16v and 1x220uF/10v)

Attached Files

**budm** · 12-10-2015, 10:03 AM

Re: Acer LCD monitor on-off repeatedly

'3. While repeating the voltage tests on the 8-Pin connector, my hand accidentally touched off one of the heatsinks on the board and I felt a moderate shock in my hand. So I checked the voltage on the heatsink against one of the GND pins and got -160v.
With that reading, I'm surprised I didn't get a bigger shock.
Certainly, there's something very wrong right there.
The heatsink concerned is shown circled in red in the attached photo' The heatsink in the primary side and it is at LINE potential! That is why you should plug the monitor into the GFCI outlet so you do not get kill when you accidentally touch anything in the primary side.
What are the P/N of the power MOSFET U501, U502?
What do you get for DCV of the 8-pin connector of the power supply board without the main board connected?

**stj** · 12-10-2015, 12:13 PM

Re: Acer LCD monitor on-off repeatedly

wouldnt surprise me if it was a rectifier diode.

**budm** · 12-10-2015, 01:22 PM

Re: Acer LCD monitor on-off repeatedly

Originally posted by stj View Post

wouldnt surprise me if it was a rectifier diode.

I expect the OP to already tested them such simple components, or may be I should not assume that.

**stj** · 12-10-2015, 02:58 PM

Re: Acer LCD monitor on-off repeatedly

failing under load i mean,
i had an LG tv like that - to220 double-schottky looked fine till it got hot.
then it went high-resistance.

**paulfxh** · 12-10-2015, 03:15 PM

Re: Acer LCD monitor on-off repeatedly

Originally posted by budm View Post

'3. While repeating the voltage tests on the 8-Pin connector, my hand accidentally touched off one of the heatsinks on the board and I felt a moderate shock in my hand. So I checked the voltage on the heatsink against one of the GND pins and got -160v.
With that reading, I'm surprised I didn't get a bigger shock.
Certainly, there's something very wrong right there.
The heatsink concerned is shown circled in red in the attached photo' The heatsink in the primary side and it is at LINE potential! That is why you should plug the monitor into the GFCI outlet so you do not get kill when you accidentally touch anything in the primary side.
What are the P/N of the power MOSFET U501, U502?
What do you get for DCV of the 8-pin connector of the power supply board without the main board connected?

OK, disconnected the Main Board and measured again the voltage on pins 1, 2, 4 and 5 with the black probe on Pin 3.
All pins gave absolutely steady readings.
Pin 1: VCC5V: 5.18v
Pin 2: VCC5V: 5.17v
Pin 4: GND: 0.00v
Pin 5: On-Off: 0.00v

The part numbers on the MOSFETs you mentioned are:
U501: 9971GD 938466
U502: 9971GD 938466
I've attached close-up photos of each if you want to check.

Where I am, we don't have GFCI outlets. However, the outlets are surge protected to avoid problems with lightning strikes and all plugs are fused.
To protect against shocks, however, we're basically limited to taking care or wearing gloves.

Originally Posted by budm View Post
I expect the OP to already tested them such simple components, or may be I should not assume that.

Simple or not, I haven't tested anything other than the resistors on the power supply board.
Anything that I've done I have mentioned in the thread.
As of now, I could not identify a rectifier diode nor know how to test it.
But I would gladly do it if given some guidance.

Originally Posted by stj View Post
failing under load i mean,
i had an LG tv like that - to220 double-schottky looked fine till it got hot.
then it went high-resistance.

Thanks for your input.
Do you mean your TV behaved exactly like my monitor with the power light and screen powering on and off with a periodicity of about 7-10 seconds?
If so, tell me more.
I'm willing to test or change any component if shown where to look.
Right now, I would not know how to recognise a rectifier diode but am very willing to learn.

Attached Files

**budm** · 12-10-2015, 04:17 PM

Re: Acer LCD monitor on-off repeatedly

Lets do the quick resistance test between S1 and D1, S2, and D2 to see if any of them show low resistance.
AP9971GD Dual-MOSFET.

Attached Files

AP9971GD.pdf (325.1 KB, 13 views)

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Acer LCD monitor on-off repeatedly

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