BENQ mp720 projector

[madan1]

Hello everyone.
I'm trying to fix the projector from the title. When I got it, it was stuck in power-boot. Fortunately, the faulty component was losing its magic smoke and managed to find it almost immediately ( a SMD cap ).
After I replaced it, the projector started to show signs of life - LEDs staying lit, fans turning on when the power button pressed, color wheel turning... but no light.
On the input of the ballast, the expected V is present, but on the output - nada.
Just to mention - the PSU has no stand-by mode. All rails are present - 2.69V, 11.9V, 5.07V ( values when the unit is ON ) + the high ballast voltage, as soon as it is plugged in the socket.

I found the pinout of the ballast control cable from another model and managed to start it, by shortening pin 1 and 2. So the ballast starts with little hesitation, but after that works without any porblems. The HG lamp seems to be working also fine. Unfortunately, this workaround works only in "stand-by" mode. Several seconds after I press the power-on button and the fans start to spin, the ballast turns-off ( even with the shorted pins ).
The control cable ends(?) with 3 optocouples. The Voltages on the pins are:

no ballast
stand-by -> on
1 - 4.7 -> 5
2 - 0 > 0
3 - 4.7 -> 5
4 - 0 -> 0
5 - 0 ->0

with ballast
stand-by -> on
1 - 4.7 -> 5.0
2 - 0 -> 0
3 - 4.7 -> 5.0
4 - 4.0 -> 4.3
5 - 4.0 -> 4.3

This is my first projector repair at this level, so I'm really not sure how to proceed...

[stj]

Re: BENQ mp720 projector

this may be a good item to try an led conversion - those lamps are expensive and have a short life.

btw, the fans must be running to have the lamp on - check any tacho signals.

[madan1]

Re: BENQ mp720 projector

With shorted pins, the ballast turns and stays on only when the mainboard is in stand-by.
When the power button is pressed, all fans + the DLP wheel start spinning, but the ballast turns off. After about 30 seconds the fans stop spinning ( the DLP wheel continues to spin ) and the lights on the motherboard still indicate that the projector is on.... go figure.
I tried to see with a flashlight if the mirror matrix reacts to interface commands ( menu, source, etc. ), but with no success .
I also noticed that an IC A8904slpt on the mainboard is getting too warm. Did not have time to dig it up, but it looks like this is the step motor driver ( for the DLP wheel ). Unfortunately before being able to see if the motor is causing the problem, the low-voltage SMPS controller literally exploded.. good thing I had it covered with its metal shield...

By the way, can you provide details about converting projectors to LED?

p.s. in my drawing from the first post, the traces of one of the two upper octocouples are wrong.. pin4 is common for both of them

[stj]

Re: BENQ mp720 projector

there are no details, it's new territory.
but there are leds now you wouldnt believe.

[eccerr0r]

Re: BENQ mp720 projector

Wouldn't the LEDs need to be near the same wattage as the original bulb if it where HID? I thought that the efficiency of HID was not that far off from LED.

If they were incandescent/halogen, then that's a different story, and LED could be an easy upgrade...

I have two DLPs that may one day need new bulbs...

[stj]

Re: BENQ mp720 projector

no, the led is directional so you have avoided losses from a reflector.

[eccerr0r]

Re: BENQ mp720 projector

I would still think that it needs to be fairly close, and an even larger problem of having to deal with designing a heatsink to fit. With HIDs usually at least 80W or so, heatsinking can get a bit tricky to fit in these small projectors.

I'd say one would still need at least 50-70% of original wattage to get enough brightness, and the custom heatsink to match... HID bulbs can stand higher temperatures and don't need heatsinking as the temperature differential between the forced air is sufficient to keep it from melting.

[stj]

Re: BENQ mp720 projector

a cpu heatsink would work

[madan1]

Re: BENQ mp720 projector

Well, I got the projector with a "new lamp" ( original philips ) according to the seller. The lamp indeed has nice white flood light, no flickering or spots. The bulb housing looks fine and has no signs of usage ( discoloration, dust, etc. ). I believe it also had some markings, but have not looked for production date.
Anyway, with an "exploded" PSU I'm again back to square one.
By the way, I'm starting to get an idea about how the mainboard communicates with the ballast. Basically there are two ways - UART or high/low signal ( this is an old model, so I hope it's the second... the UART seems to be a real pain in the.. ). High lampEN should start the ballast, but I'm not sure if the ballast logic can overwrite it.
Also, I'm not sure if bad ballast response can cause the CPU to shutdown.
Reviewing the service manuals for other benq projectors, noticed that on many models, the same ballast control is used, but with different cable pinout ( great consistency, benq! ). I assumed the pinout is the same through the whole projector line, so when I get back to this, I will definitely have to check again the pinout and which optoc. is which ( can anyone find a SM for NEC EUC 220d A/21 ? ).

I briefly check what LEDs are offered at aliexpress, and indeed there are cheap 100W LEDs ( I do not know if this is enough ). The power requirements are ~30V @ 3.5A. I do not think any of the low-V rails can support that consumption, so the only other options are the 380V rail or the 220 mains. I guess it will be easier to find 220 -> 30V psu, but then how to emulate the ballast<->cpu communication and sync the soft on/off function?

[stj]

Re: BENQ mp720 projector

not COB plates, they arent efficent.
look here:
https://www.cree.com/led-components/products/xlamp-leds