Viewsonic VX2255WMB *New caps still not working*

[Solder Boy]

Re: Viewsonic VX2255WMB *New caps still not working*

Well, since you asked, you are wrong twice.

First, a fuse does not 'short' - it is normally a very low ( less than one ohm) connection. It blows, goes open, or fails.

Second, this fuse supplies power to the inverter. A defect in the power supply would not cause this fuse to blow.

The good news is your description of a diode's action is correct.

In baseball a .333 batting average is quite good. In hand grenades it's not.

PlainBill

I stand corrected... I'm just a newbie when it comes to fixing LCD's..haven't done much actually.... I repair computer power supplies, and the fuse is near the main filter caps. Thanks for your corrections.

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

I set my digital meter to auto and here are the results.
Q205:
Pin 1&2: 2.6 Mohm
Pin 1&3: 10 Mohm
pin 2&3: 4.5 Mohm
Q208:
Pin 1&2: 2.6 Mohm
Pin 1&3: 10 Mohm
pin 2&3: 4.5 Mohm

Diode test works as well.

Top on the transistor near pin 3 says: 9973GH
Lower says: 625191

Those readings indicate the transistors are not shorted. That is good news; they would have been a bear to replace.

Also the diode in front D105 (the one between the 2 small caps) have failed since I am getting readings on both directions.

The 16 pin IC to the left of the transistors part number is
Top: BIT3501
Bottom: FA23P G0643

If you use your DMM on the resistance ranges I would expect you to get the same reading in both directions. Most modern DMMs use a very low voltage so as to not cause any interference from semiconductors. The question is 'What was the reading?' If the RESISTANCE measures over 1K ohms in both directions, the diode is probably good. If it measures under 1 ohm, it is definitely bad. If it measures the same in both directions on the diode test range, it is bad.

PlainBill

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

Is the diode test in circuit or out of circuit? I believe your in circuit may not give accurate results depending on the layout.

It was tested in circuit, yes it may be inaccurate.

Good news is I was able to eliminate back light issues today by using one of my old laptop inverter to test it.

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

Those readings indicate the transistors are not shorted. That is good news; they would have been a bear to replace.


If you use your DMM on the resistance ranges I would expect you to get the same reading in both directions. Most modern DMMs use a very low voltage so as to not cause any interference from semiconductors. The question is 'What was the reading?' If the RESISTANCE measures over 1K ohms in both directions, the diode is probably good. If it measures under 1 ohm, it is definitely bad. If it measures the same in both directions on the diode test range, it is bad.

PlainBill

Good thing they are not short because I can imagine how hard it would be to remove the 3rd pin with solder holding it underneath. But SMD are a pain the the a** in general to remove/install without proper tool.

The reading I got from the diode in the opposite direction was 1.993 and correct direction was 0.488. They are probably still good since this was tested in circuit.

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

Good thing they are not short because I can imagine how hard it would be to remove the 3rd pin with solder holding it underneath. But SMD are a pain the the a** in general to remove/install without proper tool.

The reading I got from the diode in the opposite direction was 1.993 and correct direction was 0.488. They are probably still good since this was tested in circuit.

Removing SMT devices is quite easy with the proper equipment. A product called 'Chip-Quik', and some solder braid are all it takes. The Chip-Quik forms a low melt 'solution' with the solder so it melts at very low temperature; you can easily remove three terminal devices. The solder braid is used to clean up the site before soldering the new part in place.

I would suggest the next step is to measure the resistance from the inverter (load) end of the fuse to ground. If it is over 10 ohms, try running the inverter from a variable power supply, using an automotive light bulb instead of the fuse to limit current, or just replace the fuse. If you don't have a 3A, 24V SMT fuse handy, hook a pair of leads to a conventional fuse. Or use a Picofuse.

PlainBill

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

Removing SMT devices is quite easy with the proper equipment. A product called 'Chip-Quik', and some solder braid are all it takes. The Chip-Quik forms a low melt 'solution' with the solder so it melts at very low temperature; you can easily remove three terminal devices. The solder braid is used to clean up the site before soldering the new part in place.

I would suggest the next step is to measure the resistance from the inverter (load) end of the fuse to ground. If it is over 10 ohms, try running the inverter from a variable power supply, using an automotive light bulb instead of the fuse to limit current, or just replace the fuse. If you don't have a 3A, 24V SMT fuse handy, hook a pair of leads to a conventional fuse. Or use a Picofuse.

PlainBill

Just ordered the fuse, will see how it goes after replacing it.

[lucky13]

Re: Viewsonic VX2255WMB *New caps still not working*

F200 fuse does not pass the continuity test. Does that mean its blown?

Say, hat's the marking on this fuse? There should be a single letter printed on it.

BTW, do what PlainBill suggests in bypassing the fuse with a fuse in a fuse holder jumpered in. I get myself a pair of PCB fuse clips and solder wires to the end. Then I slip in whatever current size fise the original one was. Then I solder across the blown SMD fuse to test. No need to waste money on expensive SMD fuse until you are sure everything else is good.

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

Say, hat's the marking on this fuse? There should be a single letter printed on it.

BTW, do what PlainBill suggests in bypassing the fuse with a fuse in a fuse holder jumpered in. I get myself a pair of PCB fuse clips and solder wires to the end. Then I slip in whatever current size fise the original one was. Then I solder across the blown SMD fuse to test. No need to waste money on expensive SMD fuse until you are sure everything else is good.

There is. It's 'P'. And on the board it says 3A/24V. Pretty easy to figure out.

PlainBill

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

There is. It's 'P'. And on the board it says 3A/24V. Pretty easy to figure out.

PlainBill

Monitor works great now with new fuse. The fuse I ordered was extremely small (about 1.5 mm in length, original was 3.5mm) which was the only size digikey had available so it was a pain in the a** to install. Special thanks goes out to Plain Bill and all other member who helped me repair this monitor.

[Peter Horse]

Re: Viewsonic VX2255WMB *New caps still not working*

Hello, I'm a newbie and I have exactly the same problem, same screen, blue light but no backlight...

My fuse F200 P seems to be perfectly working...

I had a few bad caps that I replaced, but I still have no backlight.

What would you recommend me to test next ?

[selldoor]

Re: Viewsonic VX2255WMB *New caps still not working*

Can you post good clear pictures of your board/s - Use manage attachments
JPgs less than 2mb should be good

[Peter Horse]

Re: Viewsonic VX2255WMB *New caps still not working*

Hello,

Thank you a lot for your response and your help. I just noticed it today.

Meanwhile, I ordered this to test the backlight :

http://www.befr.ebay.be/itm/200987005288

But I did not yet received it.

Here are the pictures of the board you asked for, I hope it's gonna be Ok!