Benq FP731

[W1ll]

Hi all,

My first post and straight in with a question you're all probably thoroughly fed up with but is still a huge b***ache to me.

It's the very dim screen from start off. I've bought the kit with the fuse and 6 trannies. But still the same. I've re-soldered the t/formers and power trannies. Everything is just the same.

I'm on the edge of my understanding, but am real good at following instructions.

Regards,
Will.

[PlainBill]

Re: Benq FP731

Hi all,

My first post and straight in with a question you're all probably thoroughly fed up with but is still a huge b***ache to me.

It's the very dim screen from start off. I've bought the kit with the fuse and 6 trannies. But still the same. I've re-soldered the t/formers and power trannies. Everything is just the same.

I'm on the edge of my understanding, but am real good at following instructions.

Regards,
Will.

Define 'very dim'. Do the backlights come on?

PlainBill

PlainBill

[W1ll]

Re: Benq FP731

Very dim as in can just about see what's on the screen with a bright light being shone onto it.

[PlainBill]

Re: Benq FP731

Very dim as in can just about see what's on the screen with a bright light being shone onto it.

OK, no backlights.

I'm glad you are good at following directions. The first step will be to take two pictures of the power supply / inverter; one of the top, one of the bottom. It is best if they are sharp, well lit, and at least 1000 x 1000 pixels. Try to take them in natural light (no flash) from directly above the board. Attach the pictures using 'Manage Attachments' - above the text entry area.

PlainBill

[W1ll]

Re: Benq FP731

Hi,

Here are the images as requested.

[PlainBill]

Re: Benq FP731

Hi,

Here are the images as requested.

Thanks. I'm going to have to revise my estimation of the quality of pictures possible. These are excellent.

If you look at both top and bottom views you will see a pair of lines running across the board. This designation is arbitrary, but I will refer to everything above those lines as the power supply area, everything below as the inverter area.

Straddling the boundary is PF751. Use either of the two lower heat sinks on the left side of the power supply area as ground. What are the voltages at the upper and lower ends of PF751?

Next, in the upper right corner of the power supply is the connector to the signal card. The two right hand pins on that connector are the brightness and On/Off control lines. What are the voltages on those pins?

Last, the lime green caps. What is the brand and series of those caps?

PlainBill

[PlainBill]

Re: Benq FP731

Argh!!! Test 2 refers to the picture of the bottom of the power supply board. The other two refer to the picture of the top.

PlainBill

[W1ll]

Re: Benq FP731

Hi,

There is 36.8V on the power supply side of the fuse, 0V on the inverter side.
0.04V on the first lead (black).
3.8V on the second lead (brown).
Caps are Lelon A410(M) A419(M)

[PlainBill]

Re: Benq FP731

Hi,

There is 36.8V on the power supply side of the fuse, 0V on the inverter side.
0.04V on the first lead (black).
3.8V on the second lead (brown).
Caps are Lelon A410(M) A419(M)

That confirms - the fuse is open.

I've marked up a picture. Set your DMM to the 200 ohms range.

Perform the following tests on the transistors circled in yellow (note that each lead has a letter next to it).

1. Black probe to B; red probe to C
2. Black probe to B; red probe to E
3. Black probe to C; red probe to E

All of these should show open.

Perform the following tests on the transistors circled in orange (note that the leads have no identification, so letter them S, D, G, starting with the leg closest to the top of the picture).

1. Black probe to S; red probe to D
2. Black probe to S; red probe to G
3. Black probe to D; red probe to G

These should also show open.

If all of the above show open, measure the resistance from the lead circled in red to the lead circled in black.

PlainBill

[W1ll]

Re: Benq FP731

Hi,

These 6 components plus the fuse have all been renewed.

All tests show as being open.
No reading from the fuse to diode either.

Items marked in orange were CHN D10PF 06 31W417, but were replaced with FU9024N.
Told these were compatible.

[PlainBill]

Re: Benq FP731

Hi,

These 6 components plus the fuse have all been renewed.

All tests show as being open.
No reading from the fuse to diode either.

Items marked in orange were CHN D10PF 06 31W417, but were replaced with FU9024N.
Told these were compatible.

The service manual shows FU9024N, so that should not be a problem.

Since the fuse is open, there are a couple of options.

1. Assume you overheated it when you soldered it in and it went open. I don't like assuming that.

2. Rig a current limiting jumper across the fuse. My favorite is an 1157 (dual filament 2.1 /.6 A @12V) automotive taillight (I'm not sure what the European equivalent is). If the light glows at full brightness with the fiilaments in parallel, there is definitely a problem.

3. Buy a pigtail fuse holder and some fuses, solder the leads in place of the picofuse and try again.

I recommend the light bulb. If it glows brightly, remove one FU9024N and retest. If the light still glows brightly, you have a problem on that part of the inverter. If the backlights come on for two seconds, you know the side with the FU9024N is good.

PlainBill

[alexanna]

Re: Benq FP731

Hi,

These 6 components plus the fuse have all been renewed.

All tests show as being open.
No reading from the fuse to diode either.

Items marked in orange were CHN D10PF 06 31W417, but were replaced with FU9024N.
Told these were compatible.

I am not sure about the compatibility of the transistors.
When you installed the kit, did the backlights work for a while or did the fuse go open immediately?

[W1ll]

Re: Benq FP731

Hi,

After testing the bulb does glow brightly at first for a couple of seconds then goes out followed by one brief flash, then nothing.

I will remove 1 mosfet FU9024N, but is there a test I can do with my DMM?
Not sure about these little beauties.

[W1ll]

Re: Benq FP731

Hi,

After removing both FU9024N. One at a time and retesting.

The bulb lights for 2 seconds when the middle one is removed but doesn't light when the one near the edge of the board is removed.

Are we getting any nearer to solving this?

[alexanna]

Re: Benq FP731

Is Q 743 installed correctly? Looks like it's reversed

[PlainBill]

Re: Benq FP731

Hi,

After removing both FU9024N. One at a time and retesting.

The bulb lights for 2 seconds when the middle one is removed but doesn't light when the one near the edge of the board is removed.

Are we getting any nearer to solving this?

Yes. First of all, having an intelligent assistant doing the actual work is a big help. The results show that something different is happening on the two sections. One possibility is one is operating normally, but has no load, so it draws very little current; while the other has a fault that causes it to draw full current. The other possibility is one is operating normally and lighting a CCFL while the other isn't even working.

Now for some questions - Did you have the CCFLs hooked up when you did this test? Did they light up? What is the current rating of the lamp you are using?

Here is the problem. The best way to do this testing is with a current limited bench supply. Even an inexpensive one is too expensive for the casual user. It might be possible to use a DMM set on the current range, but if the fuse in the DMM blows you haven't saved any money. My DMM has a limit of .2A on the fused current range; using a higher range MIGHT damage the DMM. That isn't doing you any favors either.

PlainBill

[W1ll]

Re: Benq FP731

Hi,

Although I hate to agree with you, I think Q743 was reversed when the picture was taken!
Both FET's aren't in the board at present, due to the latest tests from PlainBill.

I think I'll start over again from scratch with the advice given.

[W1ll]

Re: Benq FP731

Did you have the CCFLs hooked up when you did this test? Did they light up? What is the current rating of the lamp you are using?

Hi,

Both CCFL's were hooked up during the test and they both lit up along with the auto bulb that I used to bridge the fuse.

It's rated at 21W/5W. (Stop & tail), between 0.5A & 2.4A.
Only the lower rated filament - 5W would light.

[PlainBill]

Re: Benq FP731

Did you have the CCFLs hooked up when you did this test? Did they light up? What is the current rating of the lamp you are using?

Hi,

Both CCFL's were hooked up during the test and they both lit up along with the auto bulb that I used to bridge the fuse.

It's rated at 21W/5W. (Stop & tail), between 0.5A & 2.4A.
Only the lower rated filament - 5W would light.

That is excellent information. The reason the higher wattage filament wouldn't light is the current draw of the inverter was too low. It may be a combination of an open fuse and the reversed FET. I can't visualize the consequences of the reversed FET well enough to say if that would have blown the fuse, however.

PlainBill

[W1ll]

Re: Benq FP731

Hi,

The Fet's were correctly fitted when I conducted your latest tests.
I'm still left with exactly the same problem though, 2 seconds to lights out.
If I have to remove them again I think the tracks are going to break up.