Benq BL3200PT with a cooked PSU.

[cheeky2]

A friend dropped one of these round today as its faulty however I am struggling to get it open as it appears sealed up!! Are these glued together as normally I would just separate them at the corner, but appears to be no edges to unclip?
Help would be appreciated from someone who has had them apart before. I have taken plenty of different size monitors apart but this seems to evade me. There are 4 screws that are behind the monitor fixing plate and two on the bottom of the base plastic (all removed ok).
My issue I hope is with the connections to the screen as the unit powers on but the screen goes black briefly (backlights still on) and then image comes up normal and then just cycles through the same all the time.
Thanks

[valendorn]

Re: Benq BL3200PT with a cooked PSU.

I found why i gat the short circuit for the capacitor 120uF 450V, the reason - is not isolated Drain mosfet 11N80C3.
I give a photo of the difference between what was and what I bought, and how to solve this problem.
Now i get standby voltages on the low voltage side of the load board.

[R_J]

Re: Benq BL3200PT with a cooked PSU.

Here is the a/c path from the a/c inlet to the bridge rectifier, You must have an open coil or a broken trace

[valendorn]

Re: Benq BL3200PT with a cooked PSU.

parameters for replaced parts
R616 - 10k - tanning
R623 - 0,1 Ω - tanning
R620 - 15k - tanning
C615, C625 - 22uF 50V - seems ok but has been replaced
Q601 - 11N80C3 - short circuit to all pins
IC601 - TEA1751T - there is a hollow in the microcircuit housing, 100% died, for replacement
The resistor R616, R620 was tanned and the marking could not be read. But I explored a typical drawing and realized that these must be the correct resistance parameters.
I have tested all the other components on the board and they seem to work. But I can't figure out some one... Why is the test lamp always on? and in theory, it should go out if the board is working, right? I switched on the lamp instead of the fuse to avoid global tanning for the board, this is standard procedure...
and yet, if I dropped out the Q601 mosfet for a while, then the short circuit on the capacitor 120uF 450V disappears. Although mosfet it still remains serviceable, I check its RLC tester. I also checked the ESR for this capacitor 120uF 450V, and it is also normal.
...and by the way, the voltage of 220 volts does not go further than the inductors, L601, L602. 220 V does not come to the BD601 diode bridge, or how can this be, all inductances are working.

[valendorn]

Re: Benq BL3200PT with a cooked PSU.

Hi guys. I am a radio amateur, and basically I solder serviceable boards, repairs are not my element. but... I have a burned out power supply 4H.2CR02.A21
this is very similar to what has already been discussed in this thread, and i need your help
spoiled R616, R623, R620, C615, C625, Q601, IC601
All these parts have been replaced with new ones.
but after replacement, I turn it on through an incandescent lamp, and the lamp is on and does not go out
what else could be the problem?
on a large capacitor C651 120 uF 450 V, I see a short circuit, but this is most likely the transformer windings

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

By chance, do you have a spare power board that works? Mine has not worked after numerous fixes.

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

I have not had any success fixing my cooked power board, does anyone have a working power board that they are willing to sell?

[the.norbulus]

Re: Benq BL3200PT with a cooked PSU.

What was interesting with my monitor was that there was a tiny inspection hole cut in the metal plate covering the circuit boards. The hole let you see whether R616 had burnt out. This resistor was between the transformer and pin 16 of the IC. I'm guessing it was there to let the technician know that the IC had failed without having to further disassemble the monitor. Maybe they don't bother when they see that. I'm just curious to know if your monitor has a similar design?

[R_J]

Re: Benq BL3200PT with a cooked PSU.

If the resistors etc check ok, then like I said I suspect the ic is bad.

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

i checked the resistors and other components between the ic and the mosfet, but everything was working. Do you know what the problem could be?

[R_J]

Re: Benq BL3200PT with a cooked PSU.

Also check R651, it is a low value (likely under 1Ω). It is connected between the modfet source and the primary (hot) ground, if this resistor is open, the ic would have been damaged.

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

Thanks! I'll take a look at it and get back to you!

[R_J]

Re: Benq BL3200PT with a cooked PSU.

Ok. Where would I find any of the resistors, diodes, or any other part that needs to be replaced? Additionally, which ones would need to be replaced?

The components are connected between the ic's output and mosfet gate. You need a multimeter to check them to see if they are good or bad. as for the ic, it can't be tested but is likely bad.

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

Ok. Where would I find any of the resistors, diodes, or any other part that needs to be replaced? Additionally, which ones would need to be replaced?

[R_J]

Re: Benq BL3200PT with a cooked PSU.

Q651 is the main switching mosfet, if the pins were shorted, then the gate saw a very high voltage, so it likely destroyed the resistors and diodes in the gate circuit and also the driver ic located on the trace side of the board

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

The board number is 4H.2CR02.A21. Transistor 651 was broken because the pins shorted when they weren't supposed to.

[R_J]

Re: Benq BL3200PT with a cooked PSU.

I am also having a problem with a BenQ power board, but for a BL3201 rather than a 3200. I found that there was a dead transistor, which is now replaced, but I couldn't find the problem anywhere on the board. Nothing looked visually broken, and I tested major components which were all good.

What is the board number? which transistor? what do you mean the transistor was "dead"? was it shorted?

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

Sadly, no. My monitor is still not working and I have no clue what the issue is.

[the.norbulus]

Re: Benq BL3200PT with a cooked PSU.

Well done finding that! I assume your monitor is back up and running? I was only able to find the failed IC on my PSU because it had affected other components around it. Also I never would have got anywhere without help from people on this forum. I hope this thread helps other people out of similar problems.

[Noodles]

Re: Benq BL3200PT with a cooked PSU.

I am also having a problem with a BenQ power board, but for a BL3201 rather than a 3200. I found that there was a dead transistor, which is now replaced, but I couldn't find the problem anywhere on the board. Nothing looked visually broken, and I tested major components which were all good.