ViewSonic VA903b

[820uF]

Re: ViewSonic VA903b

got the same model monitor.
power button does nothing, no powerup when power cable attached, no caps bubleing.
transformer seconderies seem to be ok one 789 second 791.
what could be the problem?

[820uF]

Re: ViewSonic VA903b

hi-res pic of it

[ben7]

Re: ViewSonic VA903b

Check the main fuse. If the fuse is good, check the voltage on the biiig black filter capacitor.

[820uF]

Re: ViewSonic VA903b

main fuse is ok.
The 400V cap you mean while power is connected ?

[Rtech]

Re: ViewSonic VA903b

Yes, measure DC volts with power on ...'BE CAREFUL'.
At the top of your picture there are 3 diodes,with Power OFF put your meter on Diode Test and measure across each.If you have not got Diode test then put on Resistance.At least one of these tend to go short circuit,which results in no voltage out.

[820uF]

Re: ViewSonic VA903b

318V it shows.
big yellow transformer makes hearable static.
can there be soething?

[selldoor]

Re: ViewSonic VA903b

318V is OK can you check the volages on the connector to the main board.
(Black and red wires) You need to have the boards connected and the lights if possible. Set meter on 20 VDC
Black probe on a ground screw then check each pin on the socket and post the results.

Did you test the 3 diodes as per Rtechs post no 25

[820uF]

Re: ViewSonic VA903b



Measures with beep function:
1. 530
2. 232
3. 232

Measures with 2k diod function:
1. .629
2. .269
3. .269




Measures between ground screw and these pins:

1. 0,01V
2. 0.01V
3. 5,03V
4. 5,03V
5. 0,00V
6. 0,00V

[selldoor]

Re: ViewSonic VA903b

Ok for diodes you need to measure in both directions

Most tests we use are in this guide written by retiredcaps start at post 19
https://www.badcaps.net/forum/showthread.php?t=10419

On the voltage picture I notice a 24v on the board. If that is a list of what you are supposed to get on that connector, please can you list it.

Also if the top one is 24v you need to test it with meter on higher than 20vDC - sorry.


I have been reading the rest of this thread. It seems that on your board the 1000uf
caps generally fail and they recommend replacing the startup cap which is a small one on the primary side. 50v 10uf

[820uF]

Re: ViewSonic VA903b





Measures with 200 ohm function:
1. nothing both ways
2. 198,0 back on cathode side, second way nothing
3. 198,0 back on cathode side, second way nothing

I think the markings on PCB belong to another model.
Tere is marked 9 pins on PCB but it only has 6


replacing that small capiside the corner of that heatsink ?
ok i'll try

Thanks!

[selldoor]

Re: ViewSonic VA903b

[
Measures with 200 ohm function:
1. nothing both ways
2. 198,0 back on cathode side, second way nothing
3. 198,0 back on cathode side, second way nothing

,

As it measured more than 200 previously you need to increase the scale
please check them again on 2000.
When you say "Nothing" do you mean 0 zero or 1 as in off scale (same as probes apart.)

Yes since I asked about the board label I saw it on another picture so we can only guess what we should have.-
Hopefully changing the start up cap will help. How a bout the 1000uf you can use a 16v or 25v- I can only see the mains fuse on the board is that the only one?

[820uF]

Re: ViewSonic VA903b

i ment 1 - off the scale

Measures with 2k ohm function:
1. .625 back on cathode side, second way off scale (on 20k function it shows 7,53)
2. .267 back on cathode side, second way it shows 1,639
3. .267 back on cathode side, second way it shows 1,635

Changeing that 50v 10uf cap didn't do much


Only raised voltage 5,03V to 5,04V

[Rtech]

Re: ViewSonic VA903b

Your Post 28 shows:-
1. 0,01V
2. 0.01V
3. 5,03V
4. 5,03V
5. 0,00V
6. 0,00V
which actually can be confusing as it looks as your numbering is not corect based on the Legend showing the voltages expected.Your pin 1 is actually pin 3,your pin 2 is 4 etc etc,So what you have got is
Adj.....0.1 v
On/Off..0.1v
5v..5v
5v..5v
Gnd..0v
Gnd..0v
So does the 5v vary when everything is plugged in and switched on, with an input signal ?? Have you changed the 1000uf capacitor ?? do you get a voltage on the on/off pin when you initally switch on ??

[820uF]

Re: ViewSonic VA903b

5v pins are soldered togeter on the other side. the do not vary.

Replaced 16V 1000uf with new one - still all same
tried 25v 1000uf cap (as one did here) - still same.

[selldoor]

Re: ViewSonic VA903b

Ok Can you disconnect the main board and with the lights plugged in and the board
plugged into the mains can you apply 3 volts to the on/off pin. Use 2x 1.5v batteries.
negative to a ground pin. Or if you have a 10k resistor connect the 5v to the on/off pin.

We are also going to need pictures of the Main Board - can you post them
(max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed

https://www.badcaps.net/forum/attach...7&d=1280167246

https://www.badcaps.net/forum/attach...6&d=1280167246

https://www.badcaps.net/forum/attach...4&d=1280167246

[820uF]

Re: ViewSonic VA903b

Used some not so full 1,5 batteries on on/off pin. Got 2,7V outof these.
It powered up the LCD - all pixels white.

Here's some pictures:

[budm]

Re: ViewSonic VA903b

If the backlights stay on without going black using external force on power source, then your power supply/inverter board is OK. The screen is showing white due to the main board is not getting the power. Now you will have totroubleshoot the main board.

[selldoor]

Re: ViewSonic VA903b

I hate it when we get it down to the mainboard - it does not seem that we get them fixed very often.

Start with testing the voltage regulators - think I see two but look for more.
One is at U3 and one at U7.

Retiredcaps has prepared a guide on how to find ,test and post results.
https://www.badcaps.net/forum/showthread.php?p=261329

[budm]

Re: ViewSonic VA903b

Here is the service manual.

[820uF]

Re: ViewSonic VA903b

U3
1. 5.04v
2. 3,30v
3. 0,00v - is this normal ?
4. 3,30v

U7
1. 3,30v
2. 1,80v
3. 0,00v
4. 1,80v

U4
1. 3,20v
2. 3,20v
3. 3,23v
4. 0,00v
5. 0,00v
6. 0,00v
7. 0,00v
8. 0,00v