Benq Q9w5 lcd monitor query regarding transistors

[Outoflipsync]

Hi All

Have read a lot of information on this site regarding the capacitor and transistor problems with Benq lcd monitors. I have two of them, one is not working, the other is starting to fail. They are both turning off 2 seconds after turning on. I have removed the card out of one and found all the caps to be swollen and some heat discolouration on the board around the transistors. It lokks (from my attempts with my meter), that two of the transistors are toast (1 & 2 on the attached picture) and one still okay (No.3 on attached picture). I tested with my meter set at 20k as it just 1's out otherwise. I tested between each transistor leg and on transistor 1 and 2 only got readings between the right hand leg and the middle cut off leg. The reading started at around 1.5 and then kept rising. Testing between the right and left, or middle and left I got no reading at all. On transistor No.3 I got readings between each of the legs, again starting around 0.5 and rising.

Looking at the part numbers on the top of the transistors I can see that 1 & 2 are 19N10L, No.3 is a bit more difficult but I can make out L1117 a3.3 on it. I can get 19N10L's from RS Components (671-0970) as well as the capacitors. I can't however find any of the L1117 type transistor. I thought that while I was changing things out I may as well replace it as well. Is there an equivalent transistor that I can order from RS?

I'm a newby when it comes to electronics, so if my explanation above is a bit dodgy, then that's why.

Thanks for your help

John

[sabre504]

Re: Benq Q9w5 lcd monitor query regarding transistors

Hi your number 3 is a voltage regulator 3.3v output

Have you tested it for power output

[Outoflipsync]

Re: Benq Q9w5 lcd monitor query regarding transistors

Hi your number 3 is a voltage regulator 3.3v output

Have you tested it for power output

Hi Sabre

No I haven't tested it for output. I'm a little hesitant to plug it into 240v to test it with all the bulging capacitors on it. One even has a small hole in the top (C701). I did test it for resistance and I get readings across each of the legs.

Should I wait till I have replaced the caps before testing it?

[retiredcaps]

Re: Benq Q9w5 lcd monitor query regarding transistors

It lokks (from my attempts with my meter), that two of the transistors are toast (1 & 2 on the attached picture) and one still okay (No.3 on attached picture). I tested with my meter set at 20k as it just 1's out otherwise.

I'm a newby when it comes to electronics, so if my explanation above is a bit dodgy, then that's why.

If you have a manual ranging multimeter, set it to 200 (two hundred) ohms. Touch the black and red probe together. It should read 0.3 or 0.4 ohms. If it is higher than 1.0 ohm, there is something wrong with your multimeter.

If your multimeter reads "1" or "OL", it means the measurement is outside your chosen range. Don't confuse "1" on the left hand side of this display with 1.0 on the right hand side. The first means out of range and the second means 1.0 unit of your measurement.

======

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

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

If any ohm reading is less than 30 ohm you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify. A reading of "1" means the resistance is 201 ohms or more -> i.e. it is NOT shorted.

=====

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test a mosfet "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any ohm reading is less than 30 ohm you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

[jeanbo]

Re: Benq Q9w5 lcd monitor query regarding transistors

Any followup on this matter? Many Thanks!

[luketanti]

Re: Benq Q9w5 lcd monitor query regarding transistors

Hi. I have the same power board in my Benq FP91GX. But the problem is not the same. It will sometimes work fine for and hour or sometimes for only seconds. The monitor would shut off completely even the LED. and then it turns on by it self again. This happens very often. I changed all the caps on the power board but left the main cap the same. Can some one direct me to the problem please.

[PlainBill]

Re: Benq Q9w5 lcd monitor query regarding transistors

Hi. I have the same power board in my Benq FP91GX. But the problem is not the same. It will sometimes work fine for and hour or sometimes for only seconds. The monitor would shut off completely even the LED. and then it turns on by it self again. This happens very often. I changed all the caps on the power board but left the main cap the same. Can some one direct me to the problem please.

What brand and series of caps did you use for replacements?

The symptoms you describe sound more like a bad solder joint than a bad capacitor. I suggest you take pictures of your power supply / inverter (top and bottom) and of your signal card (top only) and attach them using 'Manage Attachments' - below the text entry area.

PlainBill

[luketanti]

Re: Benq Q9w5 lcd monitor query regarding transistors

Here are the pictures of both cards. Also a picture of the replacement caps. I tried to resolder everything on the inverter board but same symptoms.

[PlainBill]

Re: Benq Q9w5 lcd monitor query regarding transistors

Here are the pictures of both cards. Also a picture of the replacement caps. I tried to resolder everything on the inverter board but same symptoms.

I'm not familiar with the replacement caps you used, but I'd bet a very small amount of money they are not low ESR.

PlainBill

[UniT3d]

Re: Benq Q9w5 lcd monitor query regarding transistors

[cloaking mode off]

Here are the pictures of both cards. Also a picture of the replacement caps. I tried to resolder everything on the inverter board but same symptoms.

Definitely not low esr capacitors. Those are www.yg-elec.com stuffs .... Capxons are way better than that ...

[cloaking mode on]

[luketanti]

Re: Benq Q9w5 lcd monitor query regarding transistors

But I do not think that is a cap problem. May be the big filter cap is?? I do not know. The only monitor I have ever repaired was a benq with a blown fuse and 2 c5707 burned.

[PlainBill]

Re: Benq Q9w5 lcd monitor query regarding transistors

But I do not think that is a cap problem. May be the big filter cap is?? I do not know. The only monitor I have ever repaired was a benq with a blown fuse and 2 c5707 burned.

You are correct - it is probably not a cap problem. As I said earlier, it is probably a bad solder joint. The problem lies in isolating the fault. It could be on either the power / inverter board or on the signal board.

The usual way of locating a bad solder joint is to tap and flex the board in hopes of identifying a sensitive area. Given the voltages the power supply / inverter operates at, this could range from unpleasant to dangerous.

Also, it would be useful if you could determine if the voltage out of the power supply drops, or is the problem occurring on the signal card. That might not be too hard. Hook an LED and resistor in series from 5V to ground at the input of the signal card. If the LED goes out when the monitor goes blank you can be sure the problem is either on the signal card, or possibly the connector between the cards.

Intermittents are a real pain to isolate.

PlainBill