My Viewsonic VX924 monitor died!

[Th3_uN1Qu3]

Originally Posted by NeiltheDruid

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.

[NeiltheDruid]

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!

[NeiltheDruid]

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.

[bonney]

Well done

[BDS303]

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?

[retiredcaps]

Originally Posted by BDS303

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

http://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.

[kaboom]

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

[jfr1326]

Originally Posted by PlainBill

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?

[PCBONEZ]

Now change the battery in you dmm.

[jetadm123]

Originally Posted by jfr1326

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.

[PlainBill]

Originally Posted by jfr1326

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?

A denture? No, but it will be an adventure. There are several possible causes for the shorted transistors. Check all points for bad solder joints, resolder the transformers, check all diodes, etc.

I have one of these but all replacement transistors short as soon as power is turned on. The transformer passes a ring test, no diodes are shorted. I haven't been able to identify the cause YET, but it's on my 'to do' list.

PlainBill

[jfr1326]

Originally Posted by jetadm123

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

Thanks a lot Jetadm123. I'll go buy them.

[jfr1326]

Originally Posted by PlainBill

A denture? No, but it will be an adventure. There are several possible causes for the shorted transistors. Check all points for bad solder joints, resolder the transformers, check all diodes, etc.

I have one of these but all replacement transistors short as soon as power is turned on. The transformer passes a ring test, no diodes are shorted. I haven't been able to identify the cause YET, but it's on my 'to do' list.

PlainBill

Sorry about the missing 'v' PlainBill. I've got a sticking v key on my laptop. I have found something that looks suspicious; the capacitor C24 is not an electrolytic but it looks bulged to me. It has the markings H154JH 250MPP1. Any ideas what this is? If this had blown, would it have caused the shorting of the two tr's? I have not managed to find a circuit diagram for this board, so I'm groping in the dark a bit.

[jetadm123]

Originally Posted by jfr1326

Sorry about the missing 'v' PlainBill. I've got a sticking v key on my laptop. I have found something that looks suspicious; the capacitor C24 is not an electrolytic but it looks bulged to me. It has the markings H154JH 250MPP1. Any ideas what this is? If this had blown, would it have caused the shorting of the two tr's? I have not managed to find a circuit diagram for this board, so I'm groping in the dark a bit.

If it's the orange cap, then it's a poly cap. Replace both as these have been known to fail.


Hopefully, this makes sense. The "154" is capacitor code for:

15 is the cap value followed by the multiplier (in this case 4 zeros) and the value is in pf. Therefore, we have:

150,000pf = 150nf = .15uf

J is the tolerance: +/- 5% and +/- 10%

250=250 Volts

[PlainBill]

Originally Posted by jfr1326

Sorry about the missing 'v' PlainBill. I've got a sticking v key on my laptop. I have found something that looks suspicious; the capacitor C24 is not an electrolytic but it looks bulged to me. It has the markings H154JH 250MPP1. Any ideas what this is? If this had blown, would it have caused the shorting of the two tr's? I have not managed to find a circuit diagram for this board, so I'm groping in the dark a bit.

Bingo!!! A bad poly cap will cause the transistors to fail.

If I recall correctly, the Belinea 101910 service manual is available, and is an exact match for this monitor.

PlainBill

[jfr1326]

Originally Posted by jetadm123

If it's the orange cap, then it's a poly cap. Replace both as these have been known to fail.


Hopefully, this makes sense. The "154" is capacitor code for:

15 is the cap value followed by the multiplier (in this case 4 zeros) and the value is in pf. Therefore, we have:

150,000pf = 150nf = .15uf

J is the tolerance: +/- 5% and +/- 10%

250=250 Volts

Thanks Jetadm123. I have ordered caps and tr's from Digikey. I'll let you know how it goes when they arrive.

[Scenic]

Originally Posted by PlainBill

If I recall correctly, the Belinea 101910 service manual is available, and is an exact match for this monitor.

http://bambooz.pytalhost.net/badcaps...nea_101910.rar

[PlainBill]

Originally Posted by Scenic

http://bambooz.pytalhost.net/badcaps...nea_101910.rar

Vielen Dank.

PlainBill

[jfr1326]

Originally Posted by PlainBill

Bingo!!! A bad poly cap will cause the transistors to fail.

If I recall correctly, the Belinea 101910 service manual is available, and is an exact match for this monitor.

PlainBill

Thanks PlainBill and Jetadm123. The transistors and poly caps arrived with the usual exorbitant postal costs from DigiKey. They are now soldered in and ........a lovely bright, colourful monitor. Thanks guys.

[retiredcaps]

Originally Posted by jfr1326

the usual exorbitant postal costs from DigiKey.

What country do you live in? Shipping for USA customers is around $2.75 USD.