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Old 04-24-2012, 07:58 AM   #101
Nocta
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Default Re: VX 724 black screen green light flashing

DMM 2k Ohm

U1:
SMD 331: .325
SMD 308: .146

U3:
SMD 308: .197
SMD 308: .146

I can't read the numbers of the resistors but that's what I think it's written. I used a magnifying glass.

Thanks again and sorry for the delay. I've been a bit busy.
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Old 05-05-2012, 08:31 AM   #102
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Default Re: VX 724 black screen green light flashing

I won't give up with this monitor.

I've been looking closely at the diagram and you were right, some caps were 10uFx16. Exactly 8 caps (of 12). LOL.

Image is now even worse than it was before. Now I have some red and green vertical lines on the display. I checked again the VR and output voltage is 4v (which should be 3.3).

Should I order a couple of AMS1117-3.3 instead of AS1117-3.3 (the ones I used to replace the original VRs)?

Thanks.
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Old 07-19-2012, 10:52 PM   #103
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Default Re: VX 724 black screen green light flashing

Sorry for bumping this thread but I just wanted to update it.

I fixed it. It was the ribbon cable just like you mentioned once.
Best part is that I learnt my lesson. I was given a Viewsonic display with same symptoms (red lines, bad colour) and now it's fixed too

Thanks for your help and patience!
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Old 07-31-2014, 09:29 AM   #104
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Default Re: VX 724 black screen green light flashing

Hello!! Its been 4 years since last post in this thread...

Yesterday my VX724 died. Started with the power led blinking, and then died. After disassembling I noticed that the main cap had some fluid leakage.. changed the main cap, the 220uF caps and the 330uF caps. The others seems OK.

The main cap is charged but there is no 12v at the output. I disconnected the PSU and disconnected the inverter (opened the jumper 9 that connects +v from 220 caps to +v of 330uf cap)

I checked VIN of SG6841 and it's ok, but there is no PWM output and no signal in the gate of the mosfet (checked with oscilloscope, GND = negative of the main cap)

I don't know what else to check.. I will desolder the mosfet and check it.

Thanks!!!
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Old 07-31-2014, 10:00 AM   #105
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Default Re: VX 724 black screen green light flashing

What do you have on VDD pin, it can be that the startup cap on the VDD pin is bad.
From spec sheet, the Vdd must reach 16VDC for it to atsrt up:
The turn-on and turn-off threshold of SG6841 are
fixed internally at 16V/10V. During start-up, the hold-up
capacitor must be charged to 16V through the start-up
resistor so that SG6841 will be enabled. The hold-up
capacitor will continue to supply VDD before the energy
can be delivered from auxiliary winding of the main
transformer. VDD must not drop below 10V during this
start-up process. This UVLO hysteresis window insures
that hold-up capacitor is adequate to supply VDD during
start-up.
Attached Files
File Type: pdf SG6841V2.1.pdf (533.8 KB, 3 views)
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Old 07-31-2014, 10:00 AM   #106
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Default Re: VX 724 black screen green light flashing

if sg6841 has a small startup cap then replace it.
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Old 07-31-2014, 10:01 AM   #107
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Default Re: VX 724 black screen green light flashing

lol - simultanious post
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Old 07-31-2014, 10:57 AM   #108
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Default Re: VX 724 black screen green light flashing

Thanks to both of you

Bad news.. I replaced the mosfet (CER 541) with a IRF 540 and somethin exploded...

I remember that the SG6841 VIN pin was something like 3.2V.... I will try to fix the board and try again.
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Old 07-31-2014, 11:02 AM   #109
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Default Re: VX 724 black screen green light flashing

here a pic of the damaged part.. I think is R628

Attached Images
File Type: jpg IMG_20140731_135752.jpg (571.6 KB, 12 views)
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Old 07-31-2014, 11:06 AM   #110
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Default Re: VX 724 black screen green light flashing

IRF540 has Vds of 100V, your power supply if it is 230Vac, it will be exposed to about 330VDC. You may have also damaged the SMPS IC due to blown Gate drive network parts as shown in your picture.
http://www.vishay.com/docs/91021/91021.pdf

Last edited by budm; 07-31-2014 at 11:08 AM..
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Old 07-31-2014, 11:43 AM   #111
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by budm View Post
IRF540 has Vds of 100V, your power supply if it is 230Vac, it will be exposed to about 330VDC. You may have also damaged the SMPS IC due to blown Gate drive network parts as shown in your picture.
http://www.vishay.com/docs/91021/91021.pdf
Damn,.. so CET541 (original mosfet) is not IRF 541...
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Old 07-31-2014, 11:45 AM   #112
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by budm View Post
What do you have on VDD pin, it can be that the startup cap on the VDD pin is bad.
From spec sheet, the Vdd must reach 16VDC for it to atsrt up:
The turn-on and turn-off threshold of SG6841 are
fixed internally at 16V/10V. During start-up, the hold-up
capacitor must be charged to 16V through the start-up
resistor so that SG6841 will be enabled. The hold-up
capacitor will continue to supply VDD before the energy
can be delivered from auxiliary winding of the main
transformer. VDD must not drop below 10V during this
start-up process. This UVLO hysteresis window insures
that hold-up capacitor is adequate to supply VDD during
start-up.
I didn't find any capacitor in the startup circuit.. From +V of the main capacitor, there is four 390K resistors in series then to VIN.. and no cap to gnd.

Maybe the SMPS IC was damaged before the blown..
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Old 07-31-2014, 12:25 PM   #113
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Default Re: VX 724 black screen green light flashing

the cap will be on the VDD pin not at Vin pin. See the spec sheet of the IC and how it works, But at this point the IC is probably damaged by now.

Last edited by budm; 07-31-2014 at 12:39 PM..
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Old 07-31-2014, 12:28 PM   #114
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by Jorge.ar View Post
here a pic of the damaged part.. I think is R628

Id suggest R327 as a description R325 doesnt look too happy either

By the way where is this part CET541 - doesnt get a mention in the rest of the thread and doesnt google??

Last edited by selldoor; 07-31-2014 at 12:37 PM..
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Old 07-31-2014, 12:29 PM   #115
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by budm View Post
the cap will be on the VDD pin not at Vin pin. See the spec sheet of the IC and how it works, But at this pint the IC is probably damaged by now.
OK, found it, and also replaced before killing the SMPS ic.
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