GNR TS700 2 seconds to black

[Jasgriff]

This monitor was part of the bunch that I got today.

The backlight would turn on and off every second or two. I took it apart and tried another set of CCFL's connected to the power supply ans it still did the same. 2 CCFL's would light up and 2 of them would not light at all.

When I took it apart it was obvious that the 2 mosfets that drive one of the inverters were burnt out. I gave them a clean and tested them. They are both shorted.

I have removed them and cleaned up the area ready for some new ones but I am struggling to find these transistors? anywhere.

DK
QJ

Is all I can see on them but looking in all of the normal places returns no results.

[Nocta]

I got the same problem with a Viewsonic VX724. DKQS was the name of my part and it was a 2SC4672.
You should look for the datasheet of your board and make sure that is the same transistor (or bipolar).

In my case, it was almost the same price of 5 transistors (because I had to import them) and a used (but working) board. So I obviously bought the board.

Anyways, you can check this guide about 2 seconds to black: http://www.badcaps.net/forum/showpos...4&postcount=19 & http://www.badcaps.net/forum/showpos...4&postcount=20.

Cheers.

[Jasgriff]

Just tried swapping the 2 working transistors to the other inverter and the CCFL attempts to light up on that side so hopefully that will mean that a couple of transistors will do the trick.

[retiredcaps]

Originally Posted by Nocta

I got the same problem with a Viewsonic VX724. DKQS was the name of my part and it was a 2SC4672.

Made by Rohm. See

http://www.badcaps.net/forum/showpos...80&postcount=9

http://www.badcaps.net/forum/showpos...7&postcount=10

2 for $2.50 + free shipping or 5 for $4.99 + free shipping on ebay. I would get the 5 and have a few spares.

[Nocta]

Good point. I haven't looked for the transistor on eBay. I think I'll buy 5 of them so I can use 2 and sell the board. Plus, I have 3 spare transistors (just in case).
I ordered the used board, though. It was not expensive so .... :P

[Jasgriff]

I wish that was the case for me!

I have had a look at they are £2.49 each + £1.49 for postage. They are a real rip off.

I had a look and found that someone couldn't sell the same monitor smashed for 99p so I have messaged them to see if they still have it. No point spending 6.49 on 2 components that cost 1/3rd of that price.

[retiredcaps]

Originally Posted by Jasgriff

I have had a look at they are £2.49 each + £1.49 for postage. They are a real rip off.

Shows free shipping if I change country to UK

http://www.ebay.ca/itm//270889006243

http://www.ebay.ca/itm/390313925820

[Jasgriff]

Ahh so it does. I never thought of looking at a different country's ebay then changing it to uk. I have made an offer to this guy in the uk if that doesnt come off then I will order them from China Ta.

[NormanSaxon86]

Ebay is a good place to do business with China because of the pay-pal. See if you can get the seller to sweeten the deal.. I mean these parts are cheaper then what they sell for in America.

[Jasgriff]

Transistors arrived and fitted this has not cured the problem. I need to crack it open again and take a look at what's going on.

[retiredcaps]

1) Do the new mosfets test shorted?

2) Do the pins get any DC power?

3) Please post a picture of the top side of the power board.

[Jasgriff]

Right the new mosfets don't test shorted but the working old ones that I moved to replace the broken ones are now shorted. nooooooooo another couple of wasted mosfets!

There are 2 diodes connected to these mosfets and the diode on the broken ones appears to be shorted. I am guessing this is what is breaking the mosfets?

I will get some pics in a bit.

[retiredcaps]

Originally Posted by Jasgriff

There are 2 diodes connected to these mosfets and the diode on the broken ones appears to be shorted. I am guessing this is what is breaking the mosfets?

Probably. Desolder the diode and verify out of circuit that it is shorted.

[budm]

Did you check the tuning caps? there should be caps on the top side of the board for this Royer Oscillator circuits.

[Jasgriff]

Desoldered the Diodes and they tested ok outside of the circuit. I desoldered the mosfets too and 1 of them seems ok whilst the other is shorted.

are the tuning caps the ones that are connected directly to the mosfets?

[budm]

There should be a cap connected in parallel with the primary side of the inverter transformer. Do you have the picture of the top side of the board?

[Jasgriff]

Il get the pic now.

[budm]

The two blue caps in the top center of the picture. If they are off value then that will cause the Osc to run at the wrong frequency as designed and can cause the MOSFET to over heat or damaged.

[Jasgriff]

Right took them out to measure them.

1 measures out of range and the other measure 340nf.

7C224J is the model of the cap.

I am guessing this is what could be causing the short?

[budm]

They are 0.22uF 5% (224J), at least 250v rating.
One is reading 340nF (.34uF)? It does not have any resistance reading on this one?
If they are not within 5% (J) then you do have problem and that can damage your new MOSFETs