Benq MP721c projector repair - internal blown fuse but no damage and lamp OK. - Page 1
Forum section: electronics / repair

author: jammo
author_rank: Newbie
timestamp: July 09, 2018, 04:44:26 pm
content: Hi all.I'm a student in the UK, and having just finished Secondary school, I've been trying to find ways to spend my 3 month holiday. I'm really interested in electronics and technology, so I thought I'd explore that.I've got two identical Benq MP721c projectors. One I'd had for a year and works fine, which I intended to refurbish. Another I'd bought on eBay (described as not powering on) and intended to use for parts for said refurbishment.I took the eBay projector and attempted to power it on - lights flashed, but the lamp didn't light. I then noticed ahugeballs of dust within the projector. I removed it with compressed air, and then attempted the power-on again; to my surprise, it worked! I then decided I would refurbish both projectors, starting with the (now working) eBay machine.I fully disassembled the projector, washing all plastic parts and dusting off PCBs in the process. Following reassembly, the projector started (with lamp working) on the first attempt. Everything appeared to be working, so I thought.After displaying the 'No Signal' image for about 10 minutes, I decided I needed to try an input. I took a (known good) VGA cable and connected it to my Lenovo T440. However, the projector still said 'No Signal', despite the laptop recognizing the output device. Odd, but unlikely to be a fault with the projector, I thought. So I took another laptop from approximately the same time period, a Dell D620. The Dell also recognized the output device, and briefly displayed the desktop on the screen, but after a few seconds the projector only displayed a blue screen with no messages.During this time, I noticed a slight burning smell, but I attributed this to residual dust or moisture from the clean being heated by the lamp.A few seconds after the projector began displaying the blue screen, I heard a pop. I immediately thought that the lamp must have blown. The fuse on the plug had blown, and the circuit breaker in the garage had tripped, too. I abandoned the projector over the weekend and investigated some more today.I've fully disassembled this problem projector again, and I'm stumped. There is no visible damage the internal electronics at all. No scorch marks, no blown chips, no bad smells. Furthermore, the lamp is also good - I tested it with its working twin! The only sign of failure I can find is a blown internal fuse on the power supply board for the incoming mains. It is 5x20mm and labeled "T6.3AH 250V".Barely any documentation exists about this projector at all, unfortunately. I'm hoping someone can shed some light on what may have caused the fuse to blow. I'm suspecting it had more to-do with the slight burning smell rather than video input. I imagine the previous dust buildup within the projector causes heat issues. Can fuses fatigue over time?PhotosProjector PSU board (4H.J2C40.A00):http://u.cubeupload.com/jammo/IMG20180709164007.jpghttp://u.cubeupload.com/jammo/IMG20180709164024.jpgLamp PSU board:http://u.cubeupload.com/jammo/IMG20180709163847.jpghttp://u.cubeupload.com/jammo/IMG20180709163804.jpghttp://u.cubeupload.com/jammo/IMG20180709163820.jpghttp://u.cubeupload.com/jammo/IMG20180709163834.jpgMain control board:http://u.cubeupload.com/jammo/IMG20180709164055.jpghttp://u.cubeupload.com/jammo/IMG20180709164108.jpgInternal fuse:http://u.cubeupload.com/jammo/IMG20180709164358.jpgApologies for the wall of text and thanks in advance for any help,James

author: Relayer
author_rank: Contributor
timestamp: July 10, 2018, 12:45:14 am
content: Hello jammo,QuoteYes, they can. But they would go open circuit quietly, you wouldn't hear a thing.But since you heard a pop, that only means that the fuse blew violently.Since the fuse in question is the ceramic body type, you cannot see the violenceof the blow. Where-as, at least, with glass fuses, you can.Therefore you've definitely got a short-circuit somewhere.For starters, you need to measure with an ohm-meter across the main filter capacitor/sfor a short. If so, you need to de-solder or better still, remove the bridge rectifier fromcircuit and measure it for shorts between the pins.If it measures OK, you need now pay attention to the switching MOSFETS and diodesand any other semi-conductor devices.Lastly, DON'T bother replacing the fuse without eliminating the short. And also, makesure you replace the fuse with an identical type. i.e. a M205 6.3A Time delay.Regards,Relayer

author: jammo
author_rank: Newbie
timestamp: July 10, 2018, 07:53:21 am
content: Thanks a lot for your reply. I'll get busy with that today and let you know the results.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 10, 2018, 10:04:15 am
content: Hi!Will this MP611 Service Manual help?https://www.google.co.uk/url?sa=t&source=web&rct=j&url=https://elektrotanya.com/benq_mp611_level2_sm.pdf/download.html&ved=2ahUKEwjj6Pb0oZTcAhXRZFAKHVfsCQMQFjABegQIARAB&usg=AOvVaw0ch1FdnJUydgnAWJBwG719I believe it uses the same main power supply board as your MP721 projector – there'll be general fault–finding, disassembly and setup information that will be similar, if not identical to, the MP721!Chris Williams

author: jammo
author_rank: Newbie
timestamp: July 10, 2018, 03:30:18 pm
content: Thanks for that manual! It'll come in handy.I have an update. I decided to swap the PSU, ballast and control board from the known-good projector to the eBay projector. I made sure to only swap these parts, leaving the housing, fans, etc in situ.To my surprise, the fuse on the known-good board has also popped within the first ten minutes. I can't say if this is coincidental, but it popped a few seconds after I instructed the laptop to 'clone' displays to the projector to show an image.I tested the capacitors and bridge rectifiers, and I couldn't see anything odd. I think this means I can rule out power and the control board, leaving me with:Lamp 'PC style' fanPSU 'PC style' fanLamp 'blower style' fanTemperature sensor mounted to blower fanSpeakerColour wheelThe odd thing is that this occurs a few minutes after startup and (perhaps) when trying to input video. The fans appear to be delayed upon startup, so I'll look there first.

author: jammo
author_rank: Newbie
timestamp: July 10, 2018, 05:54:18 pm
content: Narrowed it down slightly further.All the fans spin fine when tested with a 12v source. My logic was that the blower might have been damaged and was shorting out during the warm up.I doubt it'd be the DLP mirror chip or the colour wheel, otherwise there'd be visual defects being projected.I'm now looking at what appears to be a temperature sensor/thermal cut-out located on the blower. However, documentation surrounding it appears to be a bit scarce. It's a KLIXON 250V/7A YS11A125B-C6 E7AB. Some photos:http://u.cubeupload.com/jammo/IMG20180710184046.jpghttp://u.cubeupload.com/jammo/IMG20180710184056.jpgIt's connected directly to the PSU board, the connector adjacent to the lamp door kill-switch. This makes me believe it is a thermal cutout.I'd be grateful if someone could help me identify this part and how I could test it.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 11, 2018, 11:17:25 am
content: Hi!It's a simple normally-closed temperature-sensitive switch - if it measures open-circuit you need a replacement!New ones come as a ceramic element only so you need to remove the old one from the bracket first, and solder the leads/connector from the old one onto the lead-out wires of the new device.They're a bit hard to find but you might be able to order here:-https://www.yoycart.com/Product/555408785587/https://www.ebay.com/itm/KSD9700-Temperature-Switch-Thermostat-Thermal-Protector-Normally-Closed-Open/292110026379?hash=item44031d468b%3Am%3AmLtQZk5fCWDJCewAbYAMwUg&var=591072371182. . .if you order from eBay use the "dropdown" box to select "100C" from the range available - altho' these are 5A rather than 7A I think you should still be OK with them!BEWARE:- the light-intensity from a projector-bulb close-up is enough to cause potentially permanent damage to your eyesight - make sure you're wearing arc-welding-rated safety glasses when powering up a projector lamp housing, or use a dummy load resistor!Chris Williams

author: jammo
author_rank: Newbie
timestamp: July 11, 2018, 05:36:20 pm
content: Thanks for the reply again, Chris.Testing both the known-good and eBay thermostats, both appear to be normally open.I've ruled the thermostat out as a candiate for the fault. I'm now looking at the optics (inc. DMD & colour wheel), speaker and IR reciever. I did attempt to clean the colour wheel, but I wouldn't have thought it'd cause problems?

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 12, 2018, 08:35:43 am
content: Hi!Do you have a lab. or bench–type PSU with current control?I'm beginning to think you could have a problem with the colour–wheel drive motor or the electronics feeding it – old motors can suffer from partially siezed bearings or carbon–deposits from the motor brushes causing a partial short–circuit, elther fault causing the motor to take a much higher current than it should do!You can use a lab PSU to compare the current taken by the drive motor/colour wheel in your suspect projector and compare it with the current taken by the first one – I assume the first projector doesn't blow the power board fuse!If you've no suitable PSU of the type I suggest, or the motor is a.c. operated or "brushless" type, then try the colour–wheel and motor assembly from your first projector complete!Chris WilliamsPS!Simply cleaning a colour–wheel disc shouldn't have any effect whatsoever on any electical fault symptoms, unless excess dust–deposits, etc., are preventing it rotating freely! Do be aware that these colour–wheels are very, very fragile!

author: jammo
author_rank: Newbie
timestamp: July 12, 2018, 02:35:04 pm
content: Thanks again for the reply, Chris.I swapped the colour wheels around from the eBay projector to the known-good projector. Everything seems to work normally, I'm even writing this reply using the projector.So that eliminates the colour wheel as the suspect. At the moment, I'm thinking the DMD chip? Before that, I'll do a swap test with the fans, just in case.

author: jammo
author_rank: Newbie
timestamp: July 13, 2018, 03:29:24 pm
content: So now I've swapped everything around, and I can't replicate the issue. That makes me think it might have been a fluke or an issue with the housing. Would bad grounding on the internal metal chassis cause this sort of thing?