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Old 04-08-2019, 06:54 AM   #1
the.norbulus
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Default Benq BL3200PT with a cooked PSU.

Well from what I can see I seem to be the first person who's 32 inch BL3200PT has lost power. A couple of months ago it went completely blank (including lights) and I could smell some hot smokey electronics.
The first problem I came up against is that they aren't a very common monitor so there's no useful information about them online. I haven't been able to find a service manual (on electrotanya) and once I got it open I couldn't find any reference to the power supply model online either!
The caps look fine visually. There's a guy fixing benq psus on youtube who says they are the only ones he frequently sees go bad elsewhere before the caps do. I can believe it. I will test and replace the caps for good measure if I can fix the current problem.
The obvious observable fault is an overheated mosfet HS4 Q601 (11N80C3) and a charred resistor R616 on the HV side of the board. The resistor lies between a transformer T601 and pin 16 on an undermounted chip IC601. I'm hoping that this will be enough context to identify the resistor but so far I can't identify either the transformer or the IC.
So as you can probably tell I'm no electrical engineer. I am keen to learn though if given a nudge in the right direction. I hope I have provided enough information for people to be able to see what's going on. If I've missed anything I'll add it of course.
Any help with this one would be much appreciated.

Note: I put the hi-res photos of the boards on Dropbox because they are too big to upload to the forum.

Hi-res Board Photos:
https://www.dropbox.com/sh/nozzv360u...aGg4usfQa?dl=0

PSU Board: 4H.2CR02.A00

Capacitors (just in case any stick out as particularly bad)
- CapXon 120uf 450v 105C
- Rubycon 470uf 35v 105C
- Lelon 33uf 100V 105C
- Elite 22uf 50V 105C
Attached Images
File Type: jpg PSU_FRONT_resize.jpg (887.4 KB, 46 views)
File Type: jpg PSU_BACK_resize.jpg (1,011.7 KB, 28 views)
File Type: jpg LOGIC_FRONT_resize.jpg (911.1 KB, 20 views)
File Type: jpg LOGIC_BACK_resize.jpg (914.7 KB, 17 views)
File Type: jpg R616 path.jpg (1.00 MB, 26 views)
File Type: jpg HS4 Q601.jpg (389.6 KB, 23 views)
File Type: jpg IC601.jpg (2.32 MB, 33 views)
File Type: jpg PSU1.jpg (234.8 KB, 17 views)
File Type: jpg PSU2.jpg (148.3 KB, 15 views)
File Type: jpg R616.jpg (141.3 KB, 23 views)
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Old 04-08-2019, 09:58 AM   #2
R_J
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Default Re: Benq BL3200PT with a cooked PSU.

I suspect the fets are ok, check the resistors R623 & R651 (0.1Ω) if the resistors are ok its likely the fets are ok
I would also check the other fet Q601? (pfc circuit). The resistor seems to be a 10K based on other schematic with this ic
If the fets check ok ( which I suspect) and not shorted, check the resistance of pin 16 of the ic to hot ground (primary cap [-])
If there is no shorts, try replacing the resistor, maybe the supply will work or the resistor will burn up again, If the resistor burns again its likely the ic is bad.

It could be that the resistor just decided to burn up on its own

I would replace c615 & c625 also, if they are bad as in open that may have led to the ic going bad (those small caps almost always LOOK fine but are bad)
Attached Images
File Type: png schematic.png (53.4 KB, 45 views)
File Type: jpg ULA-12_Shema_primary.jpg (46.5 KB, 40 views)

Last edited by R_J; 04-08-2019 at 10:56 AM..
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Old 04-09-2019, 06:28 AM   #3
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

Hi R_J, thanks for your help, I really appreciate it. I went googling for the IC but what I was reading off it was 'A17511' which is only partial and also wrong! Knowing what the chip is demystifies things and makes the problem seem almost solvable. Not looking forward to de-soldering the little bugger though if I have to. I see from the datasheet that pin 16 controls high-voltage start-up which I guess makes sense.
I will get busy testing the components you've mentioned and post my findings.
Thanks again
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Old 04-09-2019, 09:42 AM   #4
R_J
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Default Re: Benq BL3200PT with a cooked PSU.

If you do deside the chip is bad and need to remove it, use a sharp knife and cut the pins along the side of the ic, the pins are easy to cut that way, then you remove the plastic ic and then you can easily remove each pin.
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Old 04-23-2019, 10:11 AM   #5
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

Ok, results time. I have tested the following components and replaced some of the caps. After that I replaced the burnt resistor with a 10k 1/4 watt metal film and powered the board up. The resistor instantly started smoking and I cut the power. I guess at least I haven't created any new problems?

R623 & R651 tested OK
Q601 tested OK
C615 & C625 tested OK (replaced with Rubycon YXF)
Pin 16 to hot ground - no continuity

R_J, when you said test the other mosfet you mentioned Q601 which was the one I thought might be faulty. I'm guessing you meant Q651 (K4087). Assuming it's ok given that R651 is, should I go ahead with replacing the IC? Thanks for the tip about removing it. I bought two replacements. One to screw up and one to hopefully get right
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Old 05-02-2019, 10:14 AM   #6
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

This is just a quick update if anyone's interested. I've decided to go ahead with replacing the IC as I'm not sure what else I can do. I neglected to follow R_J's sensible advice for removing the old IC and things didn't go quite as planned. I didn't know they glue the the chips down! I wicked of the solder as much as possible and then used a heat gun to soften what remained. One side must have been hotter as it came off cleanly then the glue must have broken and then the other side came up with the tracks. I think the damage can be fixed though. I've never done smd work before. I'm not convinced it's work for humans.
Attached Images
File Type: jpg 2019-05-02 20.09.23.jpg (483.5 KB, 29 views)
File Type: jpg 2019-05-02 23.35.53.jpg (284.6 KB, 23 views)
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Old 05-03-2019, 10:13 AM   #7
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

She rides! I've just tested the board after replacing the TEA1751T and the resistor didn't burn up. The output voltage is 11.98V and nothing else went on fire
Next stage will be replacing the remaining caps and reassembly.
Thanks again for your help R_J. You were right about the IC.
Attached Images
File Type: jpg IMG_6137.jpg (3.35 MB, 33 views)
File Type: jpg 2019-05-04 01.59.01r.jpg (656.2 KB, 23 views)
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Old 05-04-2019, 10:26 AM   #8
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

Well I thought I was done! After reassembly I got intermittent vertical lines on the whole screen when I powered it on with no source connected. They went away though so I tried not to think about them. When connected to a PC everything was fine for about 30 minutes and then they came back and now they won't go away. If these look familiar to anyone I'd appreciate any advice.
I will try re-seating the connectors and maybe cleaning the logic board with flux remover but beyond that I'm stumped.

The lines are:
- vertical, light coloured tightly spaced
- intermittent and present with no source
- react to the content on the screen (disappearing in places)
Attached Images
File Type: jpg 2019-05-05 01.52.56.jpg (1.77 MB, 23 views)
File Type: jpg 2019-05-05 01.55.02.jpg (3.25 MB, 15 views)
File Type: jpg 2019-05-05 01.57.51.jpg (1.12 MB, 11 views)
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Old 05-04-2019, 12:26 PM   #9
R_J
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Default Re: Benq BL3200PT with a cooked PSU.

Check the lvds cable connections between the main and the t-con, they could also be caused by bad caps on the main board regulator ic's, but it looks more like a poor lvds connection

Last edited by R_J; 05-04-2019 at 12:32 PM..
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Old 05-04-2019, 12:58 PM   #10
the.norbulus
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Default Re: Benq BL3200PT with a cooked PSU.

Thanks R_J. I saw a similar problem in another thread (but vertical) and the suggestion was the LVDS. I'll have a look.
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Old 06-22-2020, 08:26 AM   #11
BusError
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Default Re: Benq BL3200PT with a cooked PSU.

Hi, turns out I have /exactly/ the same problem with a BL3201PT screen -- same damage, same location after 5 years of use.

I've JUST managed to decode the label on the driver, which lead me to this thread; but I spent some time trying to probe around the board and discovered that 2 of the SMT diodes underneath the board are shorted in my case. As well as the crispy resistor, and very likely the driver itself. FET looks OK but definitely looks a bit sunburnt.

I found some replacement ICs, replacement FET, but I still need to find the value of that resistor, and also more importantly, the diodes themselves has they have no markings.

Thanks to everyone who already contributed to this, I've been following the NXP application note on the TEA1751 but the more schematics, the merrier!

https://www.nxp.com/docs/en/user-guide/UM10391.pdf
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Old 06-24-2020, 05:32 AM   #12
BusError
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Default Re: Benq BL3200PT with a cooked PSU.

So I coulnt find a SOD-80 packaged fro the diode suggested by the application note, so I just picked another "small signal node" in that package with slightly better characteristics, and I'll just have to hope it works!
Resistor I've 'decided' is 10K, more or less inspired by a schematic from another BenQ supply. it seems these supplies change the DC-Dc and various bits, but most of the drivers and passive stay the same, so I'll just have to hope it works!
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