Viewsonic VE710b Power on black screen

[Guti904]

Hi everyone, i want to ask you about my Viewsonic lcd monitor, when i power it on backlights blinks one or two times and after that screen remains black. When I take out the mainboard and the power supply i haven´t not notice any puffed capacitor, but the power supply was noisy. I paid attention and figured out a transistor overheated next to 470mF caps (see picture). The problem was i cannot identify what kind of transistor it's because i cannot read the identification code...I attached a pic of the power supply and transistor to see if someone can help me! Thanks.

[selldoor]

Re: Viewsonic VE710b Power on black screen

Hi you perhaps need to read this: start at post 19
https://www.badcaps.net/forum/showthread.php?t=10419

Your component is a diode from the board description
In the above it tells how to test it with a multimeter.

[Guti904]

Re: Viewsonic VE710b Power on black screen

Thanks selldor, but assuming diode was bad, i cannot read the code... so, what replacement can I find or what diode can I buy to repair it?

[Guti904]

Re: Viewsonic VE710b Power on black screen

The board code is slv03150450 and the diode is marked in the attached pic of the first post.

[selldoor]

Re: Viewsonic VE710b Power on black screen

The board code is slv03150450 and the diode is marked in the attached pic of the first post.

Yes I spotted that and have been looking but cant find much.
Other members might be able to work out what is needed from the surrounding circuit - please post a picture of the back of the board as well.
Best to test it first to save us all looking for a replacement.
Try and clean it by rubbing with isopropyl alcohol- even some of the letters/numbers would help.

[Guti904]

Re: Viewsonic VE710b Power on black screen

Thanks again selldoor, i will post a pic of the back of the board in a few minutes!!

[Guti904]

Re: Viewsonic VE710b Power on black screen

I tried to read the diode (or transistor) code and this is what i've read:
STM DGH10 DCT
75ADG VW or (7SADG VW)
606

the first line is the most unreadable, the other two lines seems to say that.

The input of the power supply is 100-240V and ouput: 12V 1.25A

[selldoor]

Re: Viewsonic VE710b Power on black screen

Hi - I think it may have been ST ( That is the manufacturer ) M60H100CT.

Which might make it close to
http://www.digikey.com/product-detail/en/STPS60H100CT

I will keep looking - wait and see what anyone else discovers
- have you tested it yet?

EDIT unfortunately it could equally have been STM 40H100CT
Which for some reason or other is now 41!!!?

http://www.digikey.com/scripts/dksea...s=STPS41H100CT

Need some guidance as what would happen if you used a 60 as against a 40

[Guti904]

Re: Viewsonic VE710b Power on black screen

Thanks for the tip I have not tested it yet, i'm go out now from work and when I arrive home i will test it!

here is a pic of the back of the board:

[Rtech]

Re: Viewsonic VE710b Power on black screen

If you can see 100, then that is the voltage....normally the current would be in the region of 10-20Amps.so something like MBR20H100CT or similar should be adequate.

[budm]

Re: Viewsonic VE710b Power on black screen

Make sure the replacement is Isolated Tab TO-220 since the Common Cathodes are tied to the center pin, non-isolated tab the tab will be tied to the center pin, since the heat sink is grounded, you will ending up short out the Diode if you do not use the Isolated tab TO-220.
You should verify by measuring the resistance between the center pin and the mounting tab. If the heat sink is not tied to ground then you will be OK.

[selldoor]

Re: Viewsonic VE710b Power on black screen

While you are testing I notice the board has been hot under
the inverter transformers so perhaps you can test the transformers. No power to the board, meter on 2000 ohms. I will add a pic showing what I think are the secondaries.
Measure S1>S2 and S3>S4 and post the results.
Do you have any spare working lcd bulbs or another panel (broken or laptop screen might do) to substitute for the current bulbs.

[Guti904]

Re: Viewsonic VE710b Power on black screen

Make sure the replacement is Isolated Tab TO-220 since the Common Cathodes are tied to the center pin, non-isolated tab the tab will be tied to the center pin, since the heat sink is grounded, you will ending up short out the Diode if you do not use the Isolated tab TO-220.
You should verify by measuring the resistance between the center pin and the mounting tab. If the heat sink is not tied to ground then you will be OK.

The resistance of the diode between the center pin and the mounting tab is 0 (have continuity). With the other two pins don't.

[Guti904]

Re: Viewsonic VE710b Power on black screen

While you are testing I notice the board has been hot under
the inverter transformers so perhaps you can test the transformers. No power to the board, meter on 2000 ohms. I will add a pic showing what I think are the secondaries.
Measure S1>S2 and S3>S4 and post the results.
Do you have any spare working lcd bulbs or another panel (broken or laptop screen might do) to substitute for the current bulbs.

The resistance between S1 and S2 is about 160KOhms and between S3 and S4 is also 160KOhms...

[budm]

Re: Viewsonic VE710b Power on black screen

So it is not Isolated tab, is the resistance between the heat sink and the negative leg of the electrolytic caps (470uF) next to it shows open circuits? Just want to make sure the heat sink is not grounded.

[Guti904]

Re: Viewsonic VE710b Power on black screen

So it is not Isolated tab, is the resistance between the heat sink and the negative leg of the electrolytic caps (470uF) next to it shows open circuits? Just want to make sure the heat sink is not grounded.

Between the heat sink and the negative leg of the caps there's open circuit. I test the diode off the board and this are the results (see pic for leg numbers)

Multimeter in 2K Ohms

(black probe - red probe)

1-2: infinite resistance
2-1: 5.45 KOhms
1-3: infinite resistance
2-3: infinite resistance
3-1: infinite resistance
3-1: infinite resistance
3-2: infinite resistance
2-4: continuity (0 Ohm)
4-2: continuity (0 Ohm)
3-4: infinite resistance
4-3: 5.57 KOhm

[retiredcaps]

Re: Viewsonic VE710b Power on black screen

Perform a diode test for verification.

1) Set your multimeter to diode test.
2) Put your red probe on pin 1, black on pin 2. Report reading.
3) Put your red probe on pin 2, black on pin 1. Report reading.
4) Put your red probe on pin 3, black on pin 2. Report reading.
5) Put your red probe on pin 2, black on pin 3. Report reading.

[Rtech]

Re: Viewsonic VE710b Power on black screen

The resistance between S1 and S2 is about 160KOhms and between S3 and S4 is also 160KOhms...

160Kohms indicates that those points are not the secondaries you are looking for between 400-1200 ohms,so suggest you number all the pins,and then test 1-2,1-3,1-4 etc...then 2-3, 2-4, etc.then 3-4,3-5 etc,until you have measured, them all,although most boards do get hot,some far worse then this, and I suspect they are probably fine.

[selldoor]

Re: Viewsonic VE710b Power on black screen

Well that is odd - it looks very much like this

[Rtech]

Re: Viewsonic VE710b Power on black screen

There are mutiple VE710b versions.the monitor should also have a model number on,and they are all different as far as I can tell.As different manufacturers can use different components, different layouts and circuits to obtain the same inputs and outputs,anything goes.So unless you got the above tfmr details from exactly the same board as this unit has then that is probably the difference