Viewsonic VX2255WMB *New caps still not working*

[xclusiveplayer]

I am repairing a Viewsonic VX2255WMB for a friend that had new Nichicon HE series caps installed. The monitor turns on but no back light and stays on blue light for about 5 sec then turn red. The backlight/ccfl transformer tested within spec with my fluke. I also noticed the yellow rectangular transformer making noticeable buzzing/hissing noise. Does anyone know how I can tell if the yellow rectangular transformer is defective? Also is there any way that I can test the backlight without having to buy expensive equipment?

[pedro]

Re: Viewsonic VX2255WMB *New caps still not working*

Goes to show that there is more to repairing a monitor than simply replacing all the caps!

First up, have you checked (the "flashlight test") whether there is a video image presented to the screen? From the symptoms you describe I suspect there isn't but it's always handy to have that done and out of the way, as it helps direct further investigation. Any sound from that SMPS transformer suggests the switcher is in trouble or affected by a fault elsewhere. Certainly run the Mk1 eyeball over all the solder joints especially the transformers.

Your pics aren't clear enough to make it out, but on that 8-pin connector on the left edge (where the video board plugs on) is there any legend indicating pin voltages? The will be typically two signals from the video board back to this PSU/inverter board (for backlight dimming and shutoff), as well as various power supply-generated voltage for the video board - maybe as few as just one. You need to measure that it is/isn't there before/after the front panel LED changes colour. If the voltages are present and remain so, then I'd be checking the fundamentals of the SMPS section - voltage on the main cap, voltages at the SMPS controller chip's Vcc pin, etc etc. If they all measure up, post again - I'm sorta tired right now ...

[retiredcaps]

Re: Viewsonic VX2255WMB *New caps still not working*

I am repairing a Viewsonic VX2255WMB for a friend that had new Nichicon HE series caps installed.

Were all the caps replaced? By all, I mean ALL (big and small).

The monitor turns on but no back light and stays on blue light for about 5 sec then turn red.

Do you see a picture if you shine a flashlight on it?

The backlight/ccfl transformer tested within spec with my fluke. I also noticed the yellow rectangular transformer making noticeable buzzing/hissing noise. Does anyone know how I can tell if the yellow rectangular transformer is defective? Also is there any way that I can test the backlight without having to buy expensive equipment?

If you don't see a picture with the flashlight, you probably have a logic card problem. If you do see a picture, then the problem is somewhere in the inverter section.

You can also test the transistors to make sure none are shorted.

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

Were all the caps replaced? By all, I mean ALL (big and small).

Do you see a picture if you shine a flashlight on it?

If you don't see a picture with the flashlight, you probably have a logic card problem. If you do see a picture, then the problem is somewhere in the inverter section.

You can also test the transistors to make sure none are shorted.

I apologize for the crappy pictures shot with my phone as my digital camera suddenly stop working. I cannot provide better pics until I can afford a decent camera.

No, the small ones rarely fail and they tested fine with my ESR meter so I didn't bother to replace them.

When shined with the flashlight, I only see an image on the far right side about a quarter of the screen.

The transistor tested fine (not shorted).

I am suspecting back light/inverter issue.

[seanc]

Re: Viewsonic VX2255WMB *New caps still not working*

It'd be worth replacing the other small caps for the small expense, rules them out.

Seeing an image on only part of the screen isn't right... wouldn't be an inverter issue there.

[retiredcaps]

Re: Viewsonic VX2255WMB *New caps still not working*

No, the small ones rarely fail and they tested fine with my ESR meter so I didn't bother to replace them.

Well at least you tested them.

When shined with the flashlight, I only see an image on the far right side about a quarter of the screen.

I am suspecting back light/inverter issue.

Do you have a spare or extra CCFL? You could disconnect one CCFL and connect it to the extra and retest. And then proceed to test each one in a similar fashion.

[lucky13]

Re: Viewsonic VX2255WMB *New caps still not working*

When shined with the flashlight, I only see an image on the far right side about a quarter of the screen.

That might well be a panel issue. :-(

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

Guess I just have to keep this monitor for spare parts, as the cost of replacing the back light is too expensive. I do not have any spare CCFL to further test this monitor. The only CCFL I have are in my working monitors, but its not worth the trouble to take them out just to test it.

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

Guess I just have to keep this monitor for spare parts, as the cost of replacing the back light is too expensive. I do not have any spare CCFL to further test this monitor. The only CCFL I have are in my working monitors, but its not worth the trouble to take them out just to test it.

Why do you assume the CCFLs are the problem? It is far more likely that the problem is with the inverter. Have you even done the very basic step of checking the fuse (F200) to the inverter? How about checking the inverter drivers (Q205 and Q206?) for shorts?

PlainBill

[lucky13]

Re: Viewsonic VX2255WMB *New caps still not working*

Guess I just have to keep this monitor for spare parts, as the cost of replacing the back light is too expensive. I do not have any spare CCFL to further test this monitor. The only CCFL I have are in my working monitors, but its not worth the trouble to take them out just to test it.

You don't have to physically remove the lamps from the panel. I just hook them up side by side. Plug the good lamps from your other monitor into this unit's inverter output. If the lamps can sustain for several minutes, then you know the inverter is working.

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

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

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

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

Yes. Before replacing it, check the transistors I indicated above; they may be shorted. Ordinarily fuses blow for a reason.

A brief explanation: There are two possible failure symptoms for the backlight system. The most common is called 'Two seconds to black' The monitor turns on' the display appears, then the screen goes black but the power LED stays on. That can be caused by a number of things, including bad CCFLs. The other one is what you saw - no backlight at all. It is very unlikely bad CCFLs would cause that.

PlainBill

[Solder Boy]

Re: Viewsonic VX2255WMB *New caps still not working*

Yes it means the fuse is blown.. so something caused the fuse to short out... I'd check the power supply area of the board for bad diodes etc. With diodes you are supposed to get a reading in one direction only.. if you get a reading in both directions it means the diode is bad.

Perhaps other LCD veterans here will chime in

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

Yes it means the fuse is blown.. so something caused the fuse to short out... I'd check the power supply area of the board for bad diodes etc. With diodes you are supposed to get a reading in one direction only.. if you get a reading in both directions it means the diode is bad.

Perhaps other LCD veterans here will chime in

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

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

How about checking the inverter drivers (Q205 and Q206?) for shorts?

PlainBill

How do I know if these are short using a multimeter?

[retiredcaps]

Re: Viewsonic VX2255WMB *New caps still not working*

How do I know if these are short using a multimeter?

With power off and everything unplugged, put black probe on pin 1 and red on 2. Then black on pin 1 and red on 3. Then black on pin 2 and red on 3.

If they are not shorted, they should read very high resistance.

If they are not shorted, you can then do a diode test on them.

[PlainBill]

Re: Viewsonic VX2255WMB *New caps still not working*

How do I know if these are short using a multimeter?

Set the meter to the lowest resistance scale - probably 200 ohms.

I've identified the pins I want you to check on each transistor. On Q205 measure the resistance between pin 1 & 2; 1 & 3; 2 & 3. Repeat for the other transistor. In each case, I would expect a reading of over 200 ohms.

Next, what are the numbers on each transistor? And there is a 16 pin IC to the left of the lower transistor. What is the part number of that?

PlainBill

[xclusiveplayer]

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

[xclusiveplayer]

Re: Viewsonic VX2255WMB *New caps still not working*

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

[retiredcaps]

Re: Viewsonic VX2255WMB *New caps still not working*

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

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