Benq T701 17" 1 second on - then black.

[Richie_asg1]

Thought I would start a thread on this one for easier reference rather than hyjack another one.





Monitor flashes on for 1 second displaying logo, then goes out and turns off - no lights.

First prod around the inverter board showed 2 of the C5707 to be open circuit.(Q808 & Q809)

Transformer output resistances seem very similar, and solder pads seem ok.

What was deffinately not ok was the 220nf box capacitor (C826) between the transistors - which had cold solder joints. As the two of them are coupled through this - could this have caused them to go ?

Will be replacing all 3 as soon as the parts arrive. Prices of C5707's has come down recently - just hoping they aren't knock-offs from china

I will try the tubes in turn on start-up to make sure nothing funny is going on with them.

The board seems to have most of the components it should have, but 2 links have PF701 and PF702 on them which I presume should have pico-fuses instead. These look like they protect the 2 large Fets(?) D701 & D702 (far right on heatsink) and stop them catching fire if they fail.
It's missing an electrolytic at C712, and resistor at R701 & R702, IC 701 is missing also. I think all these are to do with another connector that is not fitted for this model, so nothing to worry about.

I just want to say what a help this forum has been to me, as I have self- taught my way to identifying common failures and more importantly the reason for them.

Will post again with results of the replacments.
Thank you.

[retiredcaps]

Re: Benq T701 17" 1 second on - then black.

First prod around the inverter board showed 2 of the C5707 to be open circuit.(Q808 & Q809)

Transformer output resistances seem very similar, and solder pads seem ok.

Don't forget to check Q805 and Q812.

You must resolder all the transformer pins. They may look okay, but they cause the C5707s to fail due to poor solder joints which may not be visible to the eye.

[Richie_asg1]

Re: Benq T701 17" 1 second on - then black.

Don't forget to check Q805 and Q812.

You must resolder all the transformer pins. They may look okay, but they cause the C5707s to fail due to poor solder joints which may not be visible to the eye.

As those are Mosfets, I'm not too sure about testing them in circuit. I did probe them - getting first a reading of 25K and 2k, Grounded the pins with my finger then tried the second reading again, and it was in 26Mohm range - so I suppose this proves I'd latched it on for the first reading, then off for the second right ?

Can't seem to find replacements online for those two D10PF06.

I've bought a pocket microscope to check solder joints. Very handy for getting the dust off laser lenses too I'd recommend getting one.

Thanks for the info.

Took apart a small SMPsu wall-wart today - was dropping out under load, and the output was down by a volt or two. No ripple though.
You guessed it - CapXon blown it's vent and sprayed gunk all over inside. No wonder these things catch fire.

[retiredcaps]

Re: Benq T701 17" 1 second on - then black.

Can't seem to find replacements online for those two D10PF06.

Likely FU9024 is suitable since a lot of Benq jobs have this, but not 100% sure.

[hoadle]

Re: Benq T701 17" 1 second on - then black.

I had same problem on Dell & BenQ + HP monitor here the repair kit
http://cgi.ebay.com/REPAIR-KIT-E172F...efaultDomain_0

[Richie_asg1]

Re: Benq T701 17" 1 second on - then black.

Thanks for the links.
I've replaced the C5707 pair today, and reflowed the transformer joints. It only takes 2 mins to do - so I agree it's worth doing anyway. A couple of the pins were under tension - so it could easily be bad design leading to stresses at the pins when warm.
Original cap tested and put back. Would prefer to replace it, but no spares yet.

Anyway - it works now, and I'm using it now to burn-in and check it's not going to black out anytime soon.

Will get some FU9024 as spares. That number rings a bell as I think I've done those before.

Cheers guys.