My Viewsonic VX924 monitor died!

[NeiltheDruid]

So, I was just watching some youtubes and my monitor powered off completely with no warning. No flickering, no flashing lights, no smells, no sounds. I tried the power button, nothing. Switched the mains cable, nothing. I'm guessing it's something to do with the power circuitry, so I opened the monitor up and fished out the board. There are a couple of 470uf @25v caps that are bulging quite a bit (pic below). On the other hand I am wondering if the instant powering off has more to do with a fuse blowing rather than bad caps, what do you think?

[Dgtech]

Re: My Viewsonic VX924 monitor died!

Check the fuse but definitly change the caps. If there are any more capxon's on that board (470 and above) I'd change em out too.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Ok, I've swapped out the two caps that dgtech suggested. I have checked the fuse and that seems to be ok. Unfortunately, I'm still getting nothing. Anyone got any ideas?

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Follow the links in my signature for suggestions on taking and directions on attaching pictures. Take good pictures of both top and bottom of the power supply. Make them as close to 2000 x 2000 as possible. Attach them using 'Manage Attachments'. Do NOT post the picture in-line, it slows down loading the thread and irritates many of us. At this stage I don't need dozens of pictures, I need two good quality pictures, one of the top and one of the bottom.

A couple of additional points. If troubleshooting gets difficult it helps if I can quickly refer to top and bottom pictures. Most boards have an orientation - most of the component identifiers on the top, the manufacturer's name, etc are all oriented in one direction. Take the picture of the top so it is easy to reed that information. Then flip the board over sideways to take the bottom picture.

PlainBill

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Gotcha PlainBill. I'll get some pics asap.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Here you go PlainBill. Pics, top and bottom.

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Unfortunately, I'm still getting nothing.

Do you have a multimeter? If yes, check for 5V DC on the connector from the power board to the main/logic board.

If that is not present, carefully check for mains * 1.4 on the main filter capacitor. This is high voltage so be careful checking.

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Do you have a multimeter? If yes, check for 5V DC on the connector from the power board to the main/logic board.

If that is not present, carefully check for mains * 1.4 on the main filter capacitor. This is high voltage so be careful checking.

Note the label on the power supply. 12V output. Otherwise, an excellent starting point.

NeiltheDruid, do you prefer to be called Neil, or Dru? Those are excellent pictures.

Check the voltage as suggested by retiredcaps, except expect 12V.
You may find no or very low output voltage, but input DC is present. In that case, look at the back of the board. You will find four transistors. Two, near the white label are Q8 and Q9, the other pair on the left side of the board are (I believe) Q10 and Q11. One pair are probably shorted.

If so, you are in for an adventure.

PlainBill

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Check the voltage as suggested by retiredcaps, except expect 12V.

Arrggh, I didn't see the markings for 12.0V earlier as I was trying to read the connector CN110.

Yes, this board clearly says 12.0V DC. So look for that instead of 5V DC.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

If that is not present, carefully check for mains * 1.4 on the main filter capacitor. This is high voltage so be careful checking.

Retiredcaps, I'm a bit of a noob when it comes to working with electronics like this. When you say "* 1.4 on the main filter capacitor" what do you mean? I know what the filter cap is, but what do you mean by * 1.4?

NeiltheDruid, do you prefer to be called Neil, or Dru? Those are excellent pictures.

Check the voltage as suggested by retiredcaps, except expect 12V.
You may find no or very low output voltage, but input DC is present. In that case, look at the back of the board. You will find four transistors. Two, near the white label are Q8 and Q9, the other pair on the left side of the board are (I believe) Q10 and Q11. One pair are probably shorted.

If so, you are in for an adventure.

PlainBill

PlainBill, Neil is fine. Just to warn you, I am a little bit of a noob. I have only fairly basic knowledge of electronics, so be gentle with me if I ask a nooby question. I'm very interested in learning from you guys though.

I checked the power-main board connector and I'm getting nothing on the dmm.

Also, when I had the board plugged in for testing, I could hear a repeating click, once a second or so. This stop a couple of seconds after the mains lead is removed.

[Wrog]

Re: My Viewsonic VX924 monitor died!

Retiredcaps, I'm a bit of a noob when it comes to working with electronics like this. When you say "* 1.4 on the main filter capacitor" what do you mean? I know what the filter cap is, but what do you mean by * 1.4?

He means 1.4 times your input line voltage. So if you are on 120V mains, you should read ~168V on the filter cap.

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Retiredcaps, I'm a bit of a noob when it comes to working with electronics like this. When you say "* 1.4 on the main filter capacitor" what do you mean? I know what the filter cap is, but what do you mean by * 1.4?



PlainBill, Neil is fine. Just to warn you, I am a little bit of a noob. I have only fairly basic knowledge of electronics, so be gentle with me if I ask a nooby question. I'm very interested in learning from you guys though.

I checked the power-main board connector and I'm getting nothing on the dmm.

Noobs are OK, as long as they are willing to ask questions.

On retired caps request, there are several possible failures. One is a failure of the input fuse. Input voltage x 1.4 describes the DC voltage you will get across the large cap when we do not know the AC voltage in your area. For example, if you look at my profile, you will see I live in Phoenix, AZ. Our input voltage is 120 VAC. I should read 165 VDC (give or take 5 volts) across the main filter cap when the monitor is plugged in.

If you do not see any voltage, or a very low (under 5 VDC) voltage, it means there is a problem on the AC input side of the power supply - often a blown fuse. If you measure 165 VDC across the large filter cap, but do not see any output voltage either the DC - DC converter is defective, or there is a short on the 12V line.

In this case, the monitor is very similar to one which I have (and is currently kicking my butt). If the transistor pairs I mentioned earlier are shorted, the result will be a very low voltage out of the supply.

NOW, we've gotten burned on this before, and I'm not certain as to your level of nooberyness (I don't think that's a real word), so will you please attach a picture of your DMM, showing the position of the selector switch when you tried to measure the voltage.

PlainBill

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

I checked the filter cap and got 333-335, which is about right since our mains is 240v.

To check the transistors I put the probes of the mm on the collector and emitter, right?

I'll get some pics of the mm tommorrow PlainBill.

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

I checked the filter cap and got 333-335, which is about right since our mains is 240v.

To check the transistors I put the probes of the mm on the collector and emitter, right?

333 to 335V DC is correct as long as it is stable and not fluctuating.

With power off, LCD unplugged, and testing "in circuit", I like to check transistors by using ohms and testing base to collector, base to emitter, and emitter to collector.

Any reading under 20 ohms suggests a possible short. I say "possible" because components around the transistor may cause false readings.

If the readings are under 20 ohms, you will have to desolder the transistor and test them out of circuit to verify the initial ohms readings.

PS. Do you see 12V DC on the connector from the power board to the main/logic board?

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

PS. Do you see 12V DC on the connector from the power board to the main/logic board?

No, I didn't get any voltage. But I'm not sure I'm measuring correctly. The connector is six pins, arranged in a layout of 3x2, I don't know if this is a standard connector, but I'm not sure which pins to check.

The first set of pins is marked +12v, the second set of is marked ground and the third set is marked "AOJ". I tried the multimeter on the top +12v pin and top ground pin. Is that right?

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Also, when I had the board plugged in for testing, I could hear a repeating click, once a second or so. This stop a couple of seconds after the mains lead is removed.

The clicking might be the SMPS trying to start, but can't because either the startup/run cap is bad or the SMPS controller chip is bad. Or something else (to hedge my bets).

The startup/run cap is right above R112. I think it is labeled C320. The little caps can often fail without visible distress.

Once you provide the results on the 12V DC measurements on the connector, we can determine if one or both of the above is relevant or not.

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

No, I didn't get any voltage. But I'm not sure I'm measuring correctly.

Here is how I do it.

Set your multimeter to 20V DC if it is not autoranging.

Put your black lead in COM and red lead in Volts.

Put your black tip probe on a screw head near the power board for GND. Put your red tip probe on the pins marked +12V.

When you say "didn't get any voltage", do you mean 0.0 V DC?

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Here you go PlainBill. Pics, top and bottom.

I don't have PlainBill's 20/20 vision, so help me out and please report the part number for chip U320 (backside).

I'm guessing this 8 pin IC is the SMPS or PWM controller. Once the part number is listed, we can then search for the datasheet and compare the stated voltages to what you are actually getting.

Also, what is the uF and voltage rating of that startup/run cap (above R112)? And do you have a replacement handy?

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Just to warn you, I am a little bit of a noob. I have only fairly basic knowledge of electronics, so be gentle with me if I ask a nooby question. I'm very interested in learning from you guys though.

Re: Noob, here are a few posts that helped me when I started to get an overall "big picture".

PlainBill's "quick walk-through on the power supply"

https://www.badcaps.net/forum/showthread.php?t=9872

PlainBill's 'divide and conquer' troubleshooting approach

https://www.badcaps.net/forum/showpo...76&postcount=8

PlainBill's Cliff's Notes on SMPS

https://www.badcaps.net/forum/showpo...6&postcount=30

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Just back to report that there appears to be nothing coming out of the 12vdc-out power-main board connector.

Right! It's midnight, I'm off to bed, I'll pick this up in the morning.

Thanks for all the help guys.