ViewSonic VA712b goes black after power on

[crash-cyberdrive]

Hi Forumers

I have a ViewSonic VA712B Monitor that it powers but as soon as it receives a signal the led goes green but i only see a flicker and goes black. By turning it off and on it works but with whine sound that its been like that for a year now. If I turn off the monitor the whine still there. Got a new monitor, but want to fix this one for my kid new old computer.

What i have done so far - Open the back case to find very little regular capacitors, most are surface mounted and the PCB looks very clean for the age this have. Check connections and solder joints

I posted a video of the whine it makes just hope YouTube don't mess with the already crappy audio. VA712 Inverter

Btw I cannot find anything related to the inverter transformers they have a number SIT36350-1736 G534T

[budm]

Re: ViewSonic VA712b goes black after power on

Can I see the straight shot pictures of the whole board and also the inverter transformer?
I can see the number on one of the MOSFET (4410, N-CHANNEL)) that drives the inverter transformer, another one is FDS4435. Check the resistance between D and S, D and G, S and G.
Check the resistance of the transformer between 2 and 1, 2 and 3.

[crash-cyberdrive]

Re: ViewSonic VA712b goes black after power on

Whole Inverter Power Board and The whole assembly ( logic and power inverter board).

To take reading do I remove the transformer and MOSFET. Read about that somewhere here on the forum but don't recall.

[budm]

Re: ViewSonic VA712b goes black after power on

You can measure the resistance with parts still in place, we are looking for really low Ohms reading on the MOSFET if they are bad, then we will remove it to verify that part off the board again. The transformer reading should be within 3% from each other.

[crash-cyberdrive]

Re: ViewSonic VA712b goes black after power on

Hi Budm
Finally got the readings of the components.
Readings on
T1: 2-1= OL 2-3=1.55kΩ
T2: 2-1= OL 2-3=1.577kΩ
T3: 2-1= OL 2-3=1.564kΩ
T4: 2-1= OL 2-3=1.581KΩ
I think not too large a difference.

FDS4435-1 MOSFET
D-S=489Ω
D-G=1.46kΩ
S-G=742Ω

FDS4435-2 MOSFET
D-S=487Ω
D-G=1.43kΩ
S-G=722Ω

SI4410-1 MOSFET
D-S=466Ω
D-G=OL
S-G=610Ω

SI4410-2 MOSFET
D-S=488Ω
D-G=OL
S-G=605Ω

[crash-cyberdrive]

Re: ViewSonic VA712b goes black after power on

Had to take new readings of the components, the mosfet's were tested wrong. Im a newbe on testing mosfet components.
Readings on Transformers stayed the same
T1: 2-1= OL 2-3=1.55kΩ
T2: 2-1= OL 2-3=1.577kΩ
T3: 2-1= OL 2-3=1.564kΩ
T4: 2-1= OL 2-3=1.581KΩ
I think not too large a difference.

FDS4435-1 MOSFET Specified Value on Datasheet –8.8 A, –30 V RDS(ON) = 20 mΩ @ Vgs = –10 V
D-S=651Ω
D-G=1725Ω
S-G=930Ω

FDS4435-2 MOSFET
D-S=651Ω
D-G=1713kΩ
S-G=935Ω

SI4410-1 MOSFET Specified Value on Datasheet Rds(on)Ω 0.0135 @ Vgs = 10V My DMM uses a 9V battery so it should be close.
D-S=612Ω
D-G=OL
S-G=793Ω

SI4410-2 MOSFET
D-S=614Ω
D-G=OL
S-G=804Ω

I dont know much about Mofset but aren't those readings a bit too high?