My Viewsonic VX924 monitor died!

[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!

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.

[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.

[bonney]

Re: My Viewsonic VX924 monitor died!

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

[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.

[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!

[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

[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.

[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!

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!

Small question, how can I tell if capacitors are low ESR caps? I am looking at some caps on ebay, they are advertised as "low impedance" is that the same thing?

[PlainBill]

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?

Yes to both. You will have to slice the heat shrink tubing off the one to identify the value. I'm not sure which part of the world you are in, but in the USA those three caps would cost less than $5.00, including shipping. With a new set of caps I'd expect the monitor to last another 5 years, or until the CCFLs die. At that time you might be able to buy a 26" wide screen with LED backlights for $75.

PlainBill

[jetadm123]

Re: My Viewsonic VX924 monitor died!

Yes, low impedance is the same as low-esr. On Ebay, just be careful who you order from (especially overseas), since there are sellers that deal in counterfeit caps. Look at the seller's feedback rating carefully.

[Wrog]

Re: My Viewsonic VX924 monitor died!

Small question, how can I tell if capacitors are low ESR caps? I am looking at some caps on ebay, they are advertised as "low impedance" is that the same thing?

If the ESR (or impedence) is listed on it's datasheet, then it's probably low ESR (impedence is different, but for this purpose it is close). Honestly, avoid eBay for caps unless you know the seller is a good one (referrals through badcaps or other forums) and even then you may get fakes. I'd check out getting Panasonic FC or FM through either Farnell or Radio Spares in the UK, whichever has cheaper shipping.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Honestly, avoid eBay for caps unless you know the seller is a good one (referrals through badcaps or other forums) and even then you may get fakes. I'd check out getting Panasonic FC or FM through either Farnell or Radio Spares in the UK, whichever has cheaper shipping.

I would definitely order from digikey, farnell or radio spares, but the cost of the cost of delivery is at least £10 from each of them. I wish they could do a low cost delivery option.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Hi guys. Just posting back with results of a full recap. Unfortunately, I am getting the same results off the 12vdc out. I'm now guessing that the next suspect would be the SMPS controller?

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Hi guys. Just posting back with results of a full recap. Unfortunately, I am getting the same results off the 12vdc out. I'm now guessing that the next suspect would be the SMPS controller?

Darn!!! I'm sorry to hear that.

Before replacing the SMPS controller, there are three tests I would like you to perform.

1. Double and triple check the orientation of the capacitors you installed.

2. Try the power supply without the logic card connected

3. I've circled the ends of the jumper that supplies power to the inverter. Disconnect either end of that jumper. (I believe it is obstructed by a heat sink on the top side). With the logic card connected, try the power supply again.

PlainBill

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Caps are all in correct. Checked the power board and logic board together, nothing. With the jumper removed, it's also the same.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Also, is there a way to test the SMPS controller before I do order a replacement? Since it looks like I'm going to have to order one from way out East.

[bonney]

Re: My Viewsonic VX924 monitor died!

have you checked D660?