My Viewsonic VX924 monitor died!

[jetadm123]

Re: My Viewsonic VX924 monitor died!

My vx924 has the normal flashing led symtoms and I've followed this thread with interest until I reached the above. I then found two of the four transistors marked DKQR have shorted causing the smps to keep dropping out. Any ideas what these transistors are and where to get them? Why will this be an adenture?

The transistors marked "DK" are 2SC4672 manufactured by Rohm and are readily available at digikey.com. Also, remember to replace them in pairs.

[PCBONEZ]

Re: My Viewsonic VX924 monitor died!

Now change the battery in you dmm.

[jfr1326]

Re: My Viewsonic VX924 monitor died!

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

My vx924 has the normal flashing led symtoms and I've followed this thread with interest until I reached the above. I then found two of the four transistors marked DKQR have shorted causing the smps to keep dropping out. Any ideas what these transistors are and where to get them? Why will this be an adenture?

[kaboom]

Re: My Viewsonic VX924 monitor died!

Let's fault trace...

Those CrapXons were gradually deteriorating. Ripple increasing, capacitance decreasing. Once the filter caps after the rectifier get bad enough, we are suddenly left with no place to dump the current from the power transformer secondary.

Doing what inductance must, the voltage across that rectifier was forced higher and ever higher, until its PRV was exceeded. On top of it, only one section shorted, unbalancing the transformer. Diode becomes wire, and primary side current limit gives 'ticking.' Consistent with the unit 'going out suddenly,' and the readings going from positive to negative repeatedly- you had AC w/ shorted rectifier. This is why it's always good to peek inside equipment and note any craps _before_ the 'earth shattering kaboom.'

I had one of those Manson power supplies with all capxons. It would always 'hiccup' when starting. It seemed to be worse at colder ambients- electrolyte less active. Pulled 'em all out and replaced with Nichicons. Two CDE SLPX for the rectifier/doubler.

Starts instantly now, and ripple is less.

"What ESR/capacitance values you got? Anything you want (printed on the sleeve.)"
-Paul

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

The two 470's were popped, but measured OK with a VOM (ramped from low ohms to infinity over 2-3 sec).

Locally, I found all the parts for 50cents each on the caps, and $1.50 for the diode.

I mentioned above that I used a VOM to judge my caps. Obviously, I don't think this method works. Short of buying a cap bridge, does anyone have a more reliable way to test a cap?

A bulged cap is a bad cap regardless what your VOM says.

Capacitors die from age, heat, and shoddy build quality. Capacitors DO NOT have to be visibly bloated in order to bad. They can be out of tolerance uF (a 1000uF measures 20uF) and/or have high ESR (ohm). A multimeter will be insufficient to test for ESR. For that you need an ESR tester which costs between $50 and $300. Some multimeters can measure uF, but no multimeter that I know of can measure ESR.

Most members here will recommend that you replace ALL capacitors with reputable brands from reputable sellers. Brands like Rubycon, Panasonic, and United Chemicon are suggested. A list of recommended caps can be found at

https://www.badcaps.net/forum/showthread.php?t=2280

If the caps you bought at your local store are general purpose caps rather than high quality brand name low ESR (ohms) caps, you may be changing the caps again in the future. SMPS applications require low ESR caps.

[BDS303]

Re: My Viewsonic VX924 monitor died!

Thanks to everyone that supported this thread.

I have this same VX924, and replaced the 2- 470 uF, 1- 330 uF, 1- 10 uF. The two 470's were popped, but measured OK with a VOM (ramped from low ohms to infinity over 2-3 sec). I struggled with what else could be wrong, and this forum pointed me to the MBR10100CT Schottky Diode. I found one half of this diode pair shorted. Replacing this and everything is working great again. Locally, I found all the parts for 50cents each on the caps, and $1.50 for the diode.

I found a manual for this unit also at http://elektrotanya.com/?q=keres.

I mentioned above that I used a VOM to judge my caps. Obviously, I don't think this method works. Short of buying a cap bridge, does anyone have a more reliable way to test a cap?

[bonney]

Re: My Viewsonic VX924 monitor died!

Well done

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

WOO SUCCESS!

It was the MBR10100CT schottky that was defective. Now I'm back basking in LCD goodness!

Thank you everyone who helped me identify what was wrong with my PSU, I really appreciate it. This experience has taught me a lot and I'm getting a bit of a taste for it, so maybe you might see me around the forum a bit more often.

Thanks again guys.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Right, I'll pick up a MBR20100 they are fairly cheap and I can get one from within the UK pretty easy. Actually, now you mention heat, there does seem to be quite a lot of discolouring in the area of the schottky and its heatsink.

Hopefully, this is the last thing I have to swap!

[Th3_uN1Qu3]

Re: My Viewsonic VX924 monitor died!

Ok, to save hassle I removed the diode from the board.

This time the diode test looks like this

Black pin 1, red pin 2 = 1 (out of range)
Red pin 1, Black pin 2 = 190 stable
Black pin 3, red pin 2 = 000 stable
Red pin 3, black pin 2 = 000 stable

Resistance looks like this:

Pins 1+2 = 1 (out of range)
Pins 1+3 = 1 (out of range)
Pins 2+3 = 00.5

Does that make any sense?

Yes it does. FYI you ALWAYS connect the black lead to the middle pin. Double diodes are marked like this: >||< which designates that the cathode of both diodes is joined into the middle pin. That allows two diodes to act as a larger, higher current one.

Your readings say that one of its two internal diodes is shorted. Replace the MBR10100. Preferably with a MBR20100 which has double the current capability thus will heat up less. One thing i do with those is cut away the pin of the shorted diode (in this case pin 3) and use the remaining good diode in a lower current application.

[bonney]

Re: My Viewsonic VX924 monitor died!

Black probe to middle pin (2) red to 1
Black probe to middle pin (2) red to 3

Red probe middle pin (2) black to 1
Red probe middle pin (2) black to3

what are the readings

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

I rechecked. There's no change on the multimeter.

[alexanna]

Re: My Viewsonic VX924 monitor died!

Recheck your measurements with the diode test
Red pin 1 black pin 2
Red pin3 black pin 2
Do not let pin 2 come into contact with pins 1 our 3 when testing.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Ok, to save hassle I removed the diode from the board.

This time the diode test looks like this

Black pin 1, red pin 2 = 1 (out of range)
Red pin 1, Black pin 2 = 190 stable
Black pin 3, red pin 2 = 000 stable
Red pin 3, black pin 2 = 000 stable

Resistance looks like this:

Pins 1+2 = 1 (out of range)
Pins 1+3 = 1 (out of range)
Pins 2+3 = 00.5

Does that make any sense?

[bonney]

Re: My Viewsonic VX924 monitor died!

Do you get a reading set at 20K

[alexanna]

Re: My Viewsonic VX924 monitor died!

Ok, I dug out the manual to my dmm. This is how it reads:

"DIODE MEASUREMENT

1. Red lead to "V (OHM symbol) mA". Black lead to "COM". (Check)
2. RANGE switch to (diode symbol) position. (Check)
3. Connect the red test lead to the anode of the diode to be measured and the black test lead to the cathode.
4. The forward voltage drop in mV will be displayed. If the diode is reversed, figure "1" will be shown."

To be honest I am utterly confused by this, result are variable when I touch the pins, dropping to 000 with each pin combination. These are the results of the diode test that I have collected anyway:

Black pin 1, red pin 2 = -254 - drops to 000
Red pin 1, Black pin 2 = 230 - drops to 000
Black pin 3, red pin 2 = -242 - drops to 000
Red pin 3, black pin 2 = 372 - drops to 000

I also did the resistance tests that retiredcaps asked for with the resistance range set to 200 ohms:

Pins 1+2 = 00.3
Pins 1+3 = 00.2
Pins 2+3 = 00.3

When you check the 5 and 12 voltage, do not use the mains ground. Use the low voltage ground, a ground screw.On the diode check and the ohm reading of the MBR10100CT you may have to lift one leg of it out of the circut to get a accurate reading.
Al.

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

I decided to recheck everything. Starting from the 12vdc out I checked the voltage and noticed something strange. If I check the voltage using the 12v and ground pins I get no reading whatsoever. If I do the same I but I use the grounding around the mains socket or around a screw hole, I get the fluctuating tiny voltage of 0.02 - 0.08 that I mentioned in post #22. What does this mean?

[NeiltheDruid]

Re: My Viewsonic VX924 monitor died!

Ok, I dug out the manual to my dmm. This is how it reads:

"DIODE MEASUREMENT

1. Red lead to "V (OHM symbol) mA". Black lead to "COM". (Check)
2. RANGE switch to (diode symbol) position. (Check)
3. Connect the red test lead to the anode of the diode to be measured and the black test lead to the cathode.
4. The forward voltage drop in mV will be displayed. If the diode is reversed, figure "1" will be shown."

To be honest I am utterly confused by this, result are variable when I touch the pins, dropping to 000 with each pin combination. These are the results of the diode test that I have collected anyway:

Black pin 1, red pin 2 = -254 - drops to 000
Red pin 1, Black pin 2 = 230 - drops to 000
Black pin 3, red pin 2 = -242 - drops to 000
Red pin 3, black pin 2 = 372 - drops to 000

I also did the resistance tests that retiredcaps asked for with the resistance range set to 200 ohms:

Pins 1+2 = 00.3
Pins 1+3 = 00.2
Pins 2+3 = 00.3

[retiredcaps]

Re: My Viewsonic VX924 monitor died!

Correct, does that mean I am doing it wrong? Because no matter what I do, the meter just drops to 000.

I have only used 2 multimeters in my life. Both are autoranging and both give different results than what you are seeing.

I don't know what 000 means on your meter. Maybe check your manual to see how to do diode test. BTW, 220 could me 220mV. I won't have access to my boards or a multimeter this weekend to see if 220mV is a good measurement or not for that type of diode.

You can try a resistance test to see if the diode is shorted or not.

With power off and LCD unplugged, measure pins 1-2, 1-3, and 2-3 using 200 ohms scale. If the reading is less than 20 ohms, that suggests the diode is shorted.

[PlainBill]

Re: My Viewsonic VX924 monitor died!

Unfortunately, I don't have an analogue multimeter.

Hmmmmm. I'm a bit stuck. What do you wizards think?

I'm puzzled. The easy approach would be to hook up a source of electricity from Gnd to the 12V output of the power supply and monitor the current as the voltage is increased. You should be able to increase it to 5V or so with very little current draw. I have a 25V variable supply, so that's easy for me to suggest. If you don't, I'd take a 6 or 9 V battery and hook it up (Remember polarity!!!) and see if the battery voltage stays at 9V. If it drops, there is a problem.

PlainBill