BenQ T705, no backlight

[munuainen]

Hi there.

I just recently got a T705 that was reported having no image. I've been searching for information on the issue and found plenty. However, I'm pretty much a beginner in the field of monitor repair so I could use some advice.

What I've discovered/done so far:

Power turns on and the power led green/amber states seem to be ok. The display is recognized when connected (maybe there's even a proper image but it's impossible to say without backlight working). There was no voltage in backlight connector pins when tested.

El. caps looked fine but I've replaced most of them just in case. Caps C703, C704, C818 and C821, each 470uf 35V are yet to be replaced. The huge C605 is original as well.

Of C5707 transistors Q808, Q809, Q815 & Q816, three were faulty (shorted or open) and one gave ohm readings Base-Emitter 2,1k, Base-Collector 450, Emitter-Collector 2,6k. I assume those are not exactly healthy readings.

Of FU9024N mosfets Q805 & Q812, one was shorted and the other gave ohm readings: Gate-Drain 160, Gate-Source 160, Drain-Source 3. That's to be replaced too, right?

The fuse F601 was fine, PF801 ohms were 11k. Isn't that 11k too much?

I've also replaced solder in T801-T804 as instructed by many. I think I'll order new C824 and C826, too, as one of them was partially melted by Q808 and had a cracked soldering. Anything else I should do while waiting for replacements to arrive?

[PlainBill]

You've got it pretty well covered. There are some (I'm one of them) that replace all caps except the large (100µF, 450 volt?) one. There are also some who will replace that cap also, I don't claim they are wrong.

The 4 C5707 transistors, the two FU9024N, and the fuse must be replaced. And of course, resoldering the transformers should prevent a repeat failure.

PlainBill

[el3ctroded]

Originally Posted by munuainen

El. caps looked fine but I've replaced most of them just in case. Caps C703, C704, C818 and C821, each 470uf 35V are yet to be replaced. The huge C605 is original as well.

Of C5707 transistors Q808, Q809, Q815 & Q816, three were faulty (shorted or open) and one gave ohm readings Base-Emitter 2,1k, Base-Collector 450, Emitter-Collector 2,6k. I assume those are not exactly healthy readings.

The 470 35v caps you might as well replace, especially if it doesn't work after replacing the C5707 transistors. Yes, you were likely right about those readings, especially if it was read out of circuit.

Of FU9024N mosfets Q805 & Q812, one was shorted and the other gave ohm readings: Gate-Drain 160, Gate-Source 160, Drain-Source 3. That's to be replaced too, right?

I don't see Q805/Q812... In any event, note that sometimes a transistor/fet/diode will read as shorted, but it will actually be the resistance of a inductor or transformer you are reading. Remove it from the circuit and retest if you read it in-circuit. If you get similar readings, then you know if it's bad.

The fuse F601 was fine, PF801 ohms were 11k. Isn't that 11k too much?

Yes. A fuse should be < 1 ohm.

I've also replaced solder in T801-T804 as instructed by many. I think I'll order new C824 and C826, too, as one of them was partially melted by Q808 and had a cracked soldering. Anything else I should do while waiting for replacements to arrive?

If you haven't already, do diode tests on all diodes, in both directions.

[munuainen]

First of all, thanks for quick replies. I forgot to mention in my opening post that all ohm values were indeed out-of-circuit readings. What kind of caps can I replace C824 and C826 with? All 22uf 160Vs I found by googling were radial electrolytics. Are those ok?

[jetadm123]

Hello! There's a guy on ebay selling a repair kit for a very reasonable price:

http://cgi.ebay.com/REPAIR-KIT-E172F...item3355ff735c

If the link doesn't work, do a ebay search for c5707.

He's sold over 700 of these kits! It would be funny, if it wasn't so sad.

[munuainen]

Originally Posted by jetadm123

He's sold over 700 of these kits! It would be funny, if it wasn't so sad.

I actually bid on a similar item being sold by this person. I'll save $1 compared to using buy it now...am I stingy or what?

[smason]

Originally Posted by jetadm123

Hello! There's a guy on ebay selling a repair kit for a very reasonable price:

He's sold over 700 of these kits! It would be funny, if it wasn't so sad.

I bought 2. So far....

[retiredcaps]

A tip for those buying these "kits".

I found that some of these ebay sellers will do "custom" kits. So if you need more c5707 or less/more picofuses, send them an email and ask for a custom kit and some will do it for you.

I bought my own c5707/fu9024/3a 125v fuse (4 kits) from an ebay seller for about the same price as one kit.

Originally Posted by smason

I bought 2. So far....

[munuainen]

Originally Posted by el3ctroded

If you haven't already, do diode tests on all diodes, in both directions.

I assume you mean only the diodes visible in the front side of the PCB.

Again, I'll be delighted if someone can assist me with with whether box-shaped caps C824 and C826 can be replaced with radial electrolytic ones of equal specs.

[retiredcaps]

Post pictures from all sides on the C824 and C826. There should be a value on the top or side. On one of my Benqs (FP71g+), it lists .22K 160. You should have something similar to that?

Also, Mouser, Digikey, etc will sell these type of caps.

Originally Posted by munuainen

Again, I'll be delighted if someone can assist me with with whether box-shaped caps C824 and C826 can be replaced with radial electrolytic ones of equal specs.

[munuainen]

Originally Posted by retiredcaps

Post pictures from all sides on the C824 and C826. There should be a value on the top or side. On one of my Benqs (FP71g+), it lists .22K 160. You should have something similar to that?

Thanks for the reply. Yes, both caps are .22uF 160V, similar to those in this discussion. The cap type was what I was mainly after before I found that thread. So they are metallized polypropylene film caps. Will some of these do? Is it a big crime to have a cap with voltage rating greater than the original if you can't find an exact replacement?

[retiredcaps]

Originally Posted by munuainen

Is it a big crime to have a cap with voltage rating greater than the original if you can't find an exact replacement?

You can go higher in voltage, but some of those caps on ebay are 650V and 2000V.

79 cents each at Digikey (part# 495-1278-ND)

http://search.digikey.com/scripts/Dk...me=495-1278-ND

BTW, I think the packaging of these caps can take a lot of abuse. While soldering, I accidentally touched one of these types of caps with the barrel of the iron. The plastic is slightly melted, but it still works AFAIK.

I don't have any experience in these types of caps, but if one of the more knowledgable members say replace, then replace them. If they say leave it alone, I would leave it alone.

[munuainen]

Thanks for the digikey link. Unfortunately, international shipping costs are like $30 no matter how small the order.

That's why I prefer ebay as a source.

[retiredcaps]

Fill out your country profile. That way, we know where you are and can suggest more appropriate suppliers.

Originally Posted by munuainen

Thanks for the digikey link. Unfortunately, international shipping costs are like $30 no matter how small the order.

That's why I prefer ebay as a source.

[lucky13]

Originally Posted by munuainen

I think I'll order new C824 and C826, too, as one of them was partially melted by Q808 and had a cracked soldering. Anything else I should do while waiting for replacements to arrive?

Can you post pix showing damages to these 2 caps? If just cosmetic damage, you cna leave them alone.

[munuainen]

Originally Posted by lucky13

Can you post pix showing damages to these 2 caps? If just cosmetic damage, you cna leave them alone.

Only one of them had suffered damage from heat. The other is fine as far as its appearance is concerned. I just thought it'd be a minor investment to replace them as well, just in case.

[munuainen]

Replacement parts arrived. After soldering c5707s, fu9024s and the fuse the backlight began to work. Image was perfect. Oh joy! Thanks to everyone who contributed to this thread.