My Viewsonic VX924 monitor died!

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

OK PlainBill, I'll order a replacement cap for C320. Before I do though, since you suggest that you replace all caps except the filter cap, do you think I should replace the caps I have marked with a green dot, or should I leave well alone?

Many here suggest replacing all caps.

You already had 2 bad caps, possibly 3 (C320). There are 6 caps (if I count the big one) so a 50% failure rate would warrant a "blind" recap of everything.

The big cap rarely, but not never, fails so that one is up to you. But the other ones should be recapped. It should not cost more than $2.00 for the remaining caps. Remember to get low ESR caps.

If caps were $5 each, then it might be a different story, but when they are 65 cents or less and this monitor has 50% failure cap rate ....

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

OK PlainBill, I'll order a replacement cap for C320. Before I do though, since you suggest that you replace all caps except the filter cap, do you think I should replace the caps I have marked with a green dot, or should I leave well alone?

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

All the other combinations of pins on each of the transistors flashed a figure on the dmm then defaulted back to 1.

I wasn't sure if I was measuring correctly, so I took a pic of the setting I had on the dmm.

"1" when it is on the left hand side of the dmm, means "out of range" or "infinity". So if you had the dmm on 200ohm scale and saw "1", it means the resistance is greater than 200 ohm.

1.0 ohms, on the right hand side of the dmm display, means one point zero ohms.

Many autorange dmms, say 0L to indicate out of range which makes it less confusing to read.

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Here we go, answers!

Retiredcaps, the cap marked C320 is a 10uF @ 50v CapXon. The SMPS controller has 2 lines of text on it. These are as follows, SG6841SZ - top line, KA2674605150 - bottom line.

PlainBill, the resistance on the 12v DC out on the board reads as 00.5 on my dmm. The only reading I could get from the all of the transistors on the board were between pins 2+3 of each transistor reading from 96.5 to 97.4. All the other combinations of pins on each of the transistors flashed a figure on the dmm then defaulted back to 1.

I wasn't sure if I was measuring correctly, so I took a pic of the setting I had on the dmm.

You were using the DMM correctly. The voltage readings indicate the SMPS was trying to start, and failing. The most likely cause is C320. This cap is responsible for filtering the DC supply to SMPS controller.

The SG6841SZ is notorious for failing, but in this case I don't think that is the problem here. The problem you are having is why I usually replace ALL electrolytic caps except the big one.

PlainBill

[newbie1]

Re: My Viewsonic VX924 monitor died!

Here we go, answers!

Retiredcaps, the cap marked C320 is a 10uF @ 50v CapXon. The SMPS controller has 2 lines of text on it. These are as follows, SG6841SZ - top line, KA2674605150 - bottom line.

sorry if i 'jump the gun' here.. i am obviously not in a position to solve the problem that you are having because i am quite new like you as well.. however, i am just wondering whether what i am facing now is the same as you.. see link > https://www.badcaps.net/forum/showthread.php?t=11584..

in summary, my faulty HP1905 is also having no 12+v output, no LED, and guess what, it is also having the same IC which is SC6841S, which Plainbill, in particular, have pointed out that it is likely to be faulty,, here is what he wrote>

The prime suspect is IC901 - the SMPS controller. It is a prone to failure. With the monitor plugged in points on the power supply are at over 300 volts, so be careful. Plug in the monitor and using pin 1 of IC901 as ground measure the voltage at pins 3 and 7. IC901 will not start until the voltage at pin 7 is about 16 volts.

If pin 3 is above 5 volts, below 16 volts, and steady, replace IC901. If it reads 0 volts check R912, R916, and R918. If it is pulsing, replace C917 and C918.

PlainBill

and here are my measurements
With monitor plugged and DMM set at 20V DC,
- the reading of Pin 3 with respect to Pin 1 of IC901 is 0.04V (stable)
- the reading of Pin 7 with respect to Pin 1 of IC901 is 0.07V (stable)
which is pointing to a suspected faulty SG6841S, which i am waiting for its arrival,

& as jetadm123 requested i further measured the below, (WITH POWER UNPLUGGED!)
with DMM set at 2000 ohms i measure on the IC SG6841S,
the reading of Pin 7 (VDD) with respect to Pin 1 shows no reading, BUT
the reading of Pin 3 (VIN) with respect to Pin 1 shows 229 ohms!!!
Pin 6 (Sense) is also shorted to Gnd

hope by sharing this, while waiting for further confirmation from the more experienced guys (Plainbill, retiredcaps, jetadm123, Dgtech, Wrog, etc) you can match the readings of your SG6841S to my faulty SG6841S and to see whether your IC is ok..
for the record SG6841S datasheet is here http://pdf1.alldatasheet.com/datashe...C/SG6841S.html

Good luck!

[jetadm123]

Re: My Viewsonic VX924 monitor died!

Here we go, answers!

The only reading I could get from the all of the transistors on the board were between pins 2+3 of each transistor reading from 96.5 to 97.4. All the other combinations of pins on each of the transistors flashed a figure on the dmm then defaulted back to 1.

I wasn't sure if I was measuring correctly, so I took a pic of the setting I had on the dmm.

When it flashes a "figure", what is that figure? And does that figure correspond to anything in the user manual that would indicate a short? Do you get this same figure if you touch the tip of the negative probe to the tip of the positive probe with the DMM on the same setting?

Also, there are tons of YouTube videos that are short and get to the point on how to use a DMM. Under search, just type in "using multimeter" and select the video that most interests you.

[bonney]

Re: My Viewsonic VX924 monitor died!

Dont these have a 220uf 25v that cause trouble?

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Here we go, answers!

Retiredcaps, the cap marked C320 is a 10uF @ 50v CapXon. The SMPS controller has 2 lines of text on it. These are as follows, SG6841SZ - top line, KA2674605150 - bottom line.

PlainBill, the resistance on the 12v DC out on the board reads as 00.5 on my dmm. The only reading I could get from the all of the transistors on the board were between pins 2+3 of each transistor reading from 96.5 to 97.4. All the other combinations of pins on each of the transistors flashed a figure on the dmm then defaulted back to 1.

I wasn't sure if I was measuring correctly, so I took a pic of the setting I had on the dmm.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Quick update. I rechecked the DC output voltage. The readout on the dmm bounces between 0.02v and 0.08v and weirdly, the minus readout flickers on and off while measuring.

I'll be back asap with the answers to every ones questions.

[PlainBill]

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.

I've attached a marked-up version of the picture you supplied.

Here's some things to check in no particular order.

The IC circled in yellow is the SMPS controller. I believe retiredcaps asked for the part number.

I've labeled the +12 and Gnd pins at the output connector. Set the DMM on the 200 ohm range and measure the resistance between those points. If it more than 200 ohms it is highly unlikely the inverter drivers are shorted.

I've numbered the pins on the transistors I suspect. Again, with the DMM on the 200 ohm range, measure the resistance between pins 1 & 2, 1 & 3, and 2 & 3 of Q8. Repeat for the other 3 transistors. If all pins of a particular transistor show a low (<10 ohms) resistance, the transistor is shorted.

PlainBill

[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.

[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

[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!

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!

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.

[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!

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!

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.

[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

[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.