LCD 19" BENQ T905 problems

[David59400]

Re: LCD 19" BENQ T905 problems

Hello there,

Here are my symptoms and then the question

After some 7 or 8 years of faithful service and it failed to come on one morning.
I power off every night.

The green light flashed as already spoken about and sometimes the benq logo flashed.
I unplugged the VGA cable and the logo would hang on for 2 or 3 secs. With constant trying I could get it going and so I left my pc and monitor running. After power off, nothing , morte !!

Question --- How do you get inside.??? I have taken off every screw but the back will not come off.

[Rtech]

Re: LCD 19" BENQ T905 problems

Dear Selldoor,

Thanks for your quick reply. The voltage on the 400v cap c605 = 316V.
the 2 fuses measured on board are 0 ohm for f 601, and 0 ohm for the picofuse fp 801 respectively. I attached here the pictures of board both sides. Please help me to find the problem and give me instruction for resolving it.

The voltage looks ok.On the right side of the board is the connector to the logic board,which is normally fed with 5V.The Board is usually marked as to what is on each pin on that connector,so see if you have 5v between the 5v pin and ground,which will also be on that connector.measure this with the logic Board not connected,and then try with it connected,and advise.

[Rtech]

Re: LCD 19" BENQ T905 problems

Hello there,

Here are my symptoms and then the question

After some 7 or 8 years of faithful service and it failed to come on one morning.
I power off every night.

The green light flashed as already spoken about and sometimes the benq logo flashed.
I unplugged the VGA cable and the logo would hang on for 2 or 3 secs. With constant trying I could get it going and so I left my pc and monitor running. After power off, nothing , morte !!

Question --- How do you get inside.??? I have taken off every screw but the back will not come off.

The surround and case are also held together by plastic connectors.Try putting your fingers under the top of the surround and try and gently pull up from the screen,that should then let you work all the way around the monitor.If the top does not give way,then one of the sides or bottom will.

[hua3]

Re: LCD 19" BENQ T905 problems

The voltage looks ok.On the right side of the board is the connector to the logic board,which is normally fed with 5V.The Board is usually marked as to what is on each pin on that connector,so see if you have 5v between the 5v pin and ground,which will also be on that connector.measure this with the logic Board not connected,and then try with it connected,and advise.

Thanks very much for your king advise. I checked the pin connector voltage: before connecting to the logic board, pin 1 to 6 is 0V to ground, and pin 7,8 is 5V, and pin9,11 is 3.3V, . After connecting to the logic board, only pin 1 changed to 1.3V, others remain unchanged. In this case, can we say that the power board is OK? And then what causes the power off (no orange led light, no green light)? Or the power switch no function?
I remember one month ago the orange led cannot change to green light. After unplug and plug the power cable and press the power switch many times, it finally recovered. I tried the same this time, but no way. So should I check the power switch, and how to do?

[selldoor]

Re: LCD 19" BENQ T905 problems

In all this thread I cant actually see a decent picture of the main board.
Can you post one - both sides of the board. Is the power switch cable connected to the main board?

You should also do this - if you have not already
https://www.badcaps.net/forum/showpo...2&postcount=21

[hua3]

Re: LCD 19" BENQ T905 problems

I have resolved the power led off problem with very simple way. Considering that no any caps blown and all fuses are ok, moreover the output of 5V and 3,3 V from the main board to the AD board are correct, so I think no any problem from the main board. Checked the switch resistance, I found it is variable by itself in the order of hundred ohms. It is clear an electric leaks problem. So I just clean the switch by using WD-40 in a few minutes, the resistance goes to normal. Then plug the electric power cable and turn on the switch. The power led is ON and the monitor works perfectly as before!!! Hope it is helpful to others with same problem.

[selldoor]

Re: LCD 19" BENQ T905 problems

Well done ! - I was working towards a power switch problem.
These boards do however have a repeated problem with bad joints
so you should still resolder as suggested.

[manthi]

Re: LCD 19" BENQ T905 problems

hi guys
i have a benq t905, it has been sitting in my basement for several years now, it display the very common problem many benq monitor have, which is when you turn pc on the light on monitor goes from orange to green, but it doesn't display anything (black screen).
i have never tried soldering, but i think it could be a good opportunity, trying to resuscitate that monitor.

[Rtech]

Re: LCD 19" BENQ T905 problems

1st step is open it and take photos of the boards.

[manthi]

Re: LCD 19" BENQ T905 problems

1st step is open it and take photos of the boards.

ok lets go then

[manthi]

Re: LCD 19" BENQ T905 problems

here they are

[selldoor]

Re: LCD 19" BENQ T905 problems

First test the fuse PF801

[budm]

Re: LCD 19" BENQ T905 problems

Check the black fuse PF801 which supplied the B+ to the backlights inverter circuit, it should be <1 Ohm.
This board is also know for bad solder joints, especially around the Inverter transformers, C5707 transistors and MOSFET q805/812, etc. Those transistors should also be checked for shorts or low resistance leakage.

[manthi]

Re: LCD 19" BENQ T905 problems

Check also the polyester capacitors at the switching transformers. C824 and C826 (220 nF, 160V) usually have dry solder joint in 90% of the cases due to ultrasonic vibration from the switching current.

Chris

i think it's what i have here no? it is one of c826 leg

[manthi]

Re: LCD 19" BENQ T905 problems

ok sorry guys i didn't see there was already answers, i am going to check what you ask

[manthi]

Re: LCD 19" BENQ T905 problems

ok i set my multimeter on 200 ohm, and when i check pf801, it stays at 1

[budm]

Re: LCD 19" BENQ T905 problems

1 Ohm? or over range? Touch the two probes together and tell us what you read, the fuse should gives about the same reading, it sounds like the fuse is open.

[manthi]

Re: LCD 19" BENQ T905 problems

Some browning is normal.



Here is how I test transistors in circuit.

a) put black probe (com) on pin 1 and red probe on pin 2 - read ohm
b) put black probe (com) on pin 1 and red probe on pin 3 - read ohm
c) put black probe (com) on pin 2 and red probe on pin 3 - read ohm

If any ohm reading is less than 50 ohm you have a shorted transistor and it will need to be replaced.

edit:

Some boards will have a fuse or picofuse and they may be open because a transistor was shorted or some other component went bad.

You can test for open fuses by (with power off and unplugged)

a) put black probe (com) on one pin and red probe on other pin - read ohm

If the reading is 0.3 or 0.4 ohm you have a good fuse. Anything higher is a sign of an open fuse. Do NOT replace fuses and turn on the power back on without checking for what caused the fuse to go open otherwise you will just be replacing the fuse again.

so
q808 and q809 give me reading of
leg1 to leg 2 03.4
leg1 to leg 3 03.0
leg2 to leg 3 00.8

q815 and q816
leg1 to leg 2 1
leg1 to leg 3 1
leg2 to leg 3 1

q805 and q812
leg1 to leg 2 1
leg1 to leg 3 1
leg2 to leg 3 1

so if i understand this correctly, my pico fuse pf801 is dead so are q808 q809
and c826 has a broken soldering ,
am i right, what else?

[manthi]

Re: LCD 19" BENQ T905 problems

1 Ohm? or over range? Touch the two probes together and tell us what you read, the fuse should gives about the same reading, it sounds like the fuse is open.

if i touch the prob together i get 00.3, but on the fuse it says 1 so i think it means over range, check picture

[budm]

Re: LCD 19" BENQ T905 problems

That fuse is open, then you need to check those transistors/MOSFETs for shorts/low Ohm, it looks like you do have some with low Ohms readings. Remove them and test them off the board.