Acer LCD monitor on-off repeatedly

[paulfxh]

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.

OK, I tried this but am unsure which pins are S1, S2 etc.
I see the pin denotation in the pdf but only the corner pins are marked on the actual board (1,4,5 and 8).
For both units (U501 and U502), I will assume that Pin 1 is S1.

Then for U501 I get the following resistance readings:
S1-D1 1,755 Ohms (1.755 kOhms)
S2-D2 1,754 Ohms (1.754 kOhms)

For U502, I get
S1-D1 1,750 Ohms (1.750 kOhms)
S2-D2 1,750 Ohms (1.750 kOhms)

These tests were done with no power applied to the board other than what the multimeter supplied.

[budm]

Re: Acer LCD monitor on-off repeatedly

So Sources (pin 1 (S1), pin 3 (S2)) and Drains (pin 5 (D2), pin 8 (D1) do not have low Ohms to cause overload of the power supply.

[budm]

Re: Acer LCD monitor on-off repeatedly

See the marked Rectifier diodes.
Also try lifting one end or the fuse and then connect the main logic back back in place to see if the 5V is steady.

[paulfxh]

Re: Acer LCD monitor on-off repeatedly

See the marked Rectifier diodes.
Also try lifting one end or the fuse and then connect the main logic back back in place to see if the 5V is steady.

Thanks.
OK, I removed the end of the fuse as you suggested (photographic "evidence" available but I was unable to attach it as keep getting "server error" message ) and hooked up the power board to the screen backlights and the main board and plugged in the mains power supply.
The voltage on both sides of the fuse F802 was rock steady at 5.22v over a long period.
However, as I assume you expected, the voltages at pins 1 and 2 of the 8-Pin connector between the boards were barely measurable at about 0.05v.

[momaka]

Re: Acer LCD monitor on-off repeatedly

OK, I removed the end of the fuse as you suggested (photographic "evidence" available but I was unable to attach it as keep getting "server error" message ) and hooked up the power board to the screen backlights and the main board and plugged in the mains power supply.
The voltage on both sides of the fuse F802 was rock steady at 5.22v over a long period.

Basically, this power supply has two outputs: a 5V standby (VCC5V) for the control/video board and T-con (board that connects to the TFT panel), and another output that is usually 10-20V for providing power away from the inverter (monitor backlights). The purpose of fuse F802 is to protect the power supply from an overload condition if there is something wrong with the inverter. By lifting one end of this fuse, you are essentially cutting power away from the inverter to see if the inverter is (likely) the problem or not (which is excellent advice from budm, BTW).

Since you say the monitor does run for a very long time with one leg lifted on fuse F802, then that likely indicates a fault with the inverter when it heats up. My advice would be to re-solder the joints on the inverter transformers (those black boxes with T501 and T502 designators on the board), resistors R506 and R525, as well as those 8-pin ICs (MOSFETs?) U501 and U502. The re-solder the lifted leg on F802 back to the board and see if the monitor runs or has problems again.

Also...

The ones that I have not changed are the filter cap (120uF/450v), C701 (10uF/50v) and C708 (220uF/25v).

That's fine.
The filter cap rarely fails. I would be very surprised if this is the cause of the issue.
C701 and C708 are likely for the amplifier IC, so *no* need to change them either.

The start-up/run cap that budm talks about in post #13 is C807, which you have changed, so you are good there.

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.

Now that you brought my attention to these again, I had a second look and I think they should indeed be 1.5 MOhm with color code BROWN-GREEN-GREEN-GOLD. I don't think they are 2.5 MOhm (Red-Green-Green-Gold), because if I remember correctly, that's not a standard value for E12/E24 4-band resistors.
So, in short, the readings you got are fine. However, those resistors seem to have discolored the traces on the board, so that might indicate further trouble with the inverter. But we will see.

Anyways, in many cases, it is normal for resistors to read lower value in circuit than their rated value. However, if you find a resistor that reads a higher resistance than its stated value (say for example, a 1 KOhm resistor reading 3.4 KOhms in circuit), you should definitely check it out of circuit, because that resistor could be bad (almost always open-circuit).

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.

That's a decent meter, so no need to worry about it.

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.

Nothing wrong there, actually. In fact, this is very typical for the heatsinks of many power supplies. So when probing with a device plugged it, ALWAYS BE CAREFUL and AVOID touching any non-grounded metal parts and the primary side. The primary side is the one that has mains voltage on it. It is marked on some PSUs, but not on many. For this reason, it is usually a good idea to take a moment and observe the power supply before plugging it in and working on it.

I guess I need apologize for the late warning now .

[paulfxh]

Re: Acer LCD monitor on-off repeatedly

The purpose of fuse F802 is to protect the power supply from an overload condition if there is something wrong with the inverter. By lifting one end of this fuse, you are essentially cutting power away from the inverter to see if the inverter is (likely) the problem or not (which is excellent advice from budm, BTW).

Since you say the monitor does run for a very long time with one leg lifted on fuse F802, then that likely indicates a fault with the inverter when it heats up. My advice would be to re-solder the joints on the inverter transformers (those black boxes with T501 and T502 designators on the board), resistors R506 and R525, as well as those 8-pin ICs (MOSFETs?) U501 and U502. The re-solder the lifted leg on F802 back to the board and see if the monitor runs or has problems again.
.

Hi momaka
Many thanks for that great reply.
It's a tutorial in itself.

Just to avoid misunderstandings, when one of the F802 pins was desoldered from the board and the power board then connected to the main board and mains power, the power reading at the F802 fuse remained unvarying at 5.22 volts for about 10 minutes which is as long as I left it running.
The screen backlights, however, did not light up as (I think) was expected.

OK, I carried out all of the resoldering as suggested.
Note, however, that I did not remove any of the old solder on the joints but used the soldering iron to remelt the old solder and then applied a very small amount of new solder. This was the case for the inverters T501 (8 pins) and T502 (8 pins) and the resistors R506 (2 pins) and R525 (2 pins).
For the mosfets U501 (8 pins) and U502 (8 pins), however, I was more cautious because the solder joints were so very close together I was concerned about creating a short between adjacent pins.
Therefore, for U501 and U502, I merely remelted the old solder and allowed it to resolidify.
Then I reconnected the desoldered leg of F802 and connected everything back together and switched on mains power.
However, the behaviour of the system was unchanged. The screen and blue power light cycled on and off exactly as before.
I will also mention that after about 10 minutes running, I could feel (finger) no heat in either T501 nor T502 which suggests that overheating of these transformers may not be a factor in the malfunction of the monitor.

Incidentally, after budm had pointed out the rectifier diodes D804 and D805 to me, and following the suggestion in this thread by stj that the problem in my monitor may be due to a faulty rectifier diode, I tested D804 and D805 but without taking them out of the circuit.
For this I followed the procedure found in this thread on this forum.
With the multimeter in Diode mode, I got the following readings:
D804 (1N4148)
Pin1-Pin2: 252
Pin3-Pin2: 252

D805 (BAW56)
Pin1-Pin2: 128
Pin3-Pin2: 128

I really do not know how to interpret these results and am reluctant to take these units out of the board as they are screwed onto the largest heatsinks with almost inaccessible screws as well as having solder joints that are very close together.
However, I provide links to the data sheets for 1N4148 diodes and BAW56 diodes.

[stj]

Re: Acer LCD monitor on-off repeatedly

your tests are pointless if it only fails when its running.

you need to meter the voltage coming from the rectifier and see if it slowly starts to drop as it warms up.

[paulfxh]

Re: Acer LCD monitor on-off repeatedly

your tests are pointless if it only fails when its running.

you need to meter the voltage coming from the rectifier and see if it slowly starts to drop as it warms up.

Thanks for the suggestion.
Well, I tried that on D804 but due to a last minute twitch of the hand I seem to have shorted out the diode with a resultant
Not quite that dramatic but enough to render the board inoperable.
So, whether the rectifier diode was deficient before or not, it certainly is now.
Nevertheless, for a little more than the cost of a replacement diode, I can get a new power supply board.
So, that's my next step.
I want to thank everybody who contributed to this thread from which I learned enormously.

[stj]

Re: Acer LCD monitor on-off repeatedly

if you shorted the pins together on the rectifier then it should be fine - unlike some of the stuff after it.

or did you short it to something else?

[paulfxh]

Re: Acer LCD monitor on-off repeatedly

if you shorted the pins together on the rectifier then it should be fine - unlike some of the stuff after it.

or did you short it to something else?

Well, the rectifier certainly looks OK but the fact that it exudes a strong smell of burnt plastic doesn't suggest it escaped unscathed from this incident.
In any event, I've ordered a new power supply board for a very reasonable €13 so I'll just wait for that.
Just hope that the main board and the screen which were attached to the power board were not damaged.
I'll know soon.

[momaka]

Re: Acer LCD monitor on-off repeatedly

In any event, I've ordered a new power supply board for a very reasonable €13 so I'll just wait for that.

Let us know how it works out.

If the monitor works fine after this, at least we know where the problem was. And then you can even try repairing the original power supply board.

[paulfxh]

Re: Acer LCD monitor on-off repeatedly

Let us know how it works out.

Well, after six weeks the ILPI-139 power board finally arrived all the way from China and it works perfectly.
So, I now have a fully operational 21.5" monitor for a total repair cost of €13 and some cents (a little over US$14).
The board was purchased as a used board but it arrived in perfect condition other than being a little dusty.
Here's where I got it.
Although I still don't know which component on the power board malfunctioned, I can at least confirm that the problem was indeed on the power board.
Thanks again to everybody who contributed to this thread.

[cyjun1990]

Re: Acer LCD monitor on-off repeatedly

where did you order the board?

[stj]

Re: Acer LCD monitor on-off repeatedly

http://www.aliexpress.com/item/Free-...355792752.html

[momaka]

Re: Acer LCD monitor on-off repeatedly

Well, after six weeks the ILPI-139 power board finally arrived all the way from China and it works perfectly.

Cool. Thanks for coming back and letting us know .

So the problem was with the power supply board, after all. Might be worth investigating if more people show up with same problems with this particular model Acer (X223HQ).