Viewsonic VX900 Bad Inverter

[ZappedAgain]

Trying to save a Viewsonic VX900 from the landfill. Monitor will turn on with a green light but there is no backlight. Flashlight reveals that the image is there. Was able to find a service manual and was able to follow a troubleshooting chart which says I have a bad inverter. I believe I agree with this diagnosis. Here is what I have done.
1. Checked all connections to the inverter.
2 Checked for 5v and 12v to the inverter at the proper locations.
3. Checked Q2 Q3 Q11 Q12 no shorts appear to be good.
4. No bad solder joints
5. 2 fuses near CN1 are both good 0 ohms
6. Checked D1 and D2 no shorts.

A new inverter is available for 35.00 but that is probably about what the monitor is worth. Would love to fix it if I can. Could it be bad caps? They look Fine 100uf 35v KMF x 2

I am willing to put the time in just too cheap to spend the money on a new inverter.

[budm]

Re: Viewsonic VX900 Bad Inverter

So, you do not notice the backligts ever flash on for seconds if you at it in the dark room?

[Rtech]

Re: Viewsonic VX900 Bad Inverter

Transformer secondary resistances ???

[Davi.p]

Re: Viewsonic VX900 Bad Inverter

After that if all ok open the screen and watch for damaged solderings at the lamps or darkened ends.

[ZappedAgain]

Re: Viewsonic VX900 Bad Inverter

Budm

Yes if i Turn the monitor off and back on with a PC connected I get a display for about half a second.

Rtech

Secondaries measure .2 ohms between all 4 pins both transformers.

Davi.P

I have a ccfl tester. It has the small ccfl connectors. I will get out the alligator clip leads and connect and test the ccfl's

[Davi.p]

Re: Viewsonic VX900 Bad Inverter

lamp connectors are protected, i think there's no space for alligators.. you can solder wires..

[selldoor]

Re: Viewsonic VX900 Bad Inverter

Budm


Secondaries measure .2 ohms between all 4 pins both transformers.

That doesnt sound correct did you have your meter on 2000 ohms.
If not try again - can you see a part number on a transformer

[ZappedAgain]

Re: Viewsonic VX900 Bad Inverter

Ok First Readings on the Transformer Secondaries where made with an autoranging fluke 116.

Remeasured with a Wavetek
on 2000 range readings between all four connectors is 0.0
on 200 range readings are 1.2 ohms on all connectors both transformers.

Ken

[selldoor]

Re: Viewsonic VX900 Bad Inverter

Cant believe they are all shorted. Must be using the wrong tabs
Can you make out the part number.

[Davi.p]

Re: Viewsonic VX900 Bad Inverter

Selldoor if you watch the picture you see that pins on output are united by a track (don't ask me why) , i think this is not new in my memory, maybe lamp connectors are connected 1 pin to the transf. & 1 pin to chassis ground.

[Davi.p]

Re: Viewsonic VX900 Bad Inverter

To the OP: after the ccfl test i think if you have initially worn lamps you can continue to use them by re-trim the feedback, locate D3 and D4 diodies , D3 is to the right of Big L1.. from them 1 leg arrives to a resistor that goes to gnd, that must be swapped with 2 trimmers for measurements, set it to the same value of smd resistor then mount them, switch on the mon. see if lamps works, if not increase a bit the value of both, switch off and on, if it don't stay on repeat, if yes decrease both until ccfl goes off, then increase a bit, see if is a good value then dismount them measure them and replace with same value or a bit higher smd resistor.

[budm]

Re: Viewsonic VX900 Bad Inverter

The inverter transformer drive is ROYER Self Oscillating circuit, the amount of lamp brightness is adjusted by using variable DC feeding the OSC, it is done by the MOSFET Q8, 2, D2 (Q1, L1, D1 for the other channel) the Pulse width signal from the PWM IC is used to drive the Gate of the MOSFETs. If the MOSFET Q1/Q8 has shorted S to D, the lamp will be fully on all the time. The PWM will be not be sending the signal to turn on the MOSFET if it detects open lamp (lamp not connected or will not stay on after firing voltage is applied), or circuit draws too much current. So anything goes wrong in the first 3 seconds, the circuit will go into shutdown (this 3 second time out can be bypass for troubleshooting purpose by grounding one of the pin on the IC).
One end of the lamp is connected to the ballast cap then the transformer winding, the other end of the lamp is connected to the resistor/diode network for the lamp sensing circuit.
So, did you check or replace the two electrolytic caps C1, C24 yet? Try heating them up with hair dryer.
C2, C3 of the Royer Osc also need to be checked.
You can also have bad lamp assembly that needs to be verify.
Bad lamps:
http://s807.photobucket.com/user/bud...?sort=3&page=1
Test lamps:
https://www.badcaps.net/forum/showthread.php?t=19987

[selldoor]

Re: Viewsonic VX900 Bad Inverter

Selldoor if you watch the picture you see that pins on output are united by a track (don't ask me why) , i think this is not new in my memory, maybe lamp connectors are connected 1 pin to the transf. & 1 pin to chassis ground.

Like I said, must be using the wrong tabs?

[Davi.p]

Re: Viewsonic VX900 Bad Inverter

Like I said, must be using the wrong tabs?

i don't know what do you mean..

[tron]

Re: Viewsonic VX900 Bad Inverter

secondary windings for backlight
have high resistance, 200mohm
is a low resistance reading

tabs are where
the transformer
pins come out
of the secondary
(high voltage/resistance side)
and go to light up the tubes

01 and V1...06 and V6

is the row where
the secondary side is in this
picture:

https://www.badcaps.net/forum/attach...chmentid=13909

[ZappedAgain]

Re: Viewsonic VX900 Bad Inverter

Here is a picture to show where i measured the inverter transformers. Hope this clarifies things. I will work on making sense of the other post to give it a try.
I am not a professional at this.

[selldoor]

Re: Viewsonic VX900 Bad Inverter

You have still not advised the part numbers off the inverter transformers.
and I cant quite read it -its in black on that orangey transparent plastic.

[budm]

Re: Viewsonic VX900 Bad Inverter

Yes, we need the P/N as printed on the transformer to look up the pin out.

[budm]

Re: Viewsonic VX900 Bad Inverter

Try this reading, it is similar to one of the transformer I worked on before:

[ZappedAgain]

Re: Viewsonic VX900 Bad Inverter

Transformer has the following Numbers on top in two lines
2801947600
DC6M 0313

Thanks Guys and Merry Christmas.

Ken