VX 724 black screen green light flashing

[Jorge.ar]

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 pint the IC is probably damaged by now.

OK, found it, and also replaced before killing the SMPS ic.

[selldoor]

Re: VX 724 black screen green light flashing

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??

[budm]

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.

[Jorge.ar]

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.

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..

[Jorge.ar]

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.

Damn,.. so CET541 (original mosfet) is not IRF 541...

[budm]

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.

[Jorge.ar]

Re: VX 724 black screen green light flashing

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

[Jorge.ar]

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.

[stj]

Re: VX 724 black screen green light flashing

lol - simultanious post

[stj]

Re: VX 724 black screen green light flashing

if sg6841 has a small startup cap then replace it.

[budm]

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.

[Jorge.ar]

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!!!

[Nocta]

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!

[Nocta]

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.

[Nocta]

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.

[Jasgriff]

Re: VX 724 black screen green light flashing

Strange that they are different from the diagram.

Can you measure the resistors that are in series with the ADJ pin and tell us your findings.

[Nocta]

Re: VX 724 black screen green light flashing

The originals were TEAPO 22uF 25v. I still got them here.

[Jasgriff]

Re: VX 724 black screen green light flashing

Acording to the diagram those caps should be 10uf not 22uf what were the values of the ones you took out? I'm reading this on my phone so I apologize if I'm wrong.

[budm]

Re: VX 724 black screen green light flashing

"U1

Pin 3 (gnd): 4.96v
Pin 2 (tab): 3.94v
Pin 1 (Vin): 2.70v

U3

Pin 3 (gnd): 4.96v
Pin 2 (tab): 3.94v
Pin 1 (Vin): 2.25v"

Since the voltage regulators (xxx1117 series) have 1.25V ref built into the design of the regulator (fixed 1.25 between output pin and the Adj pin). You can tell buy what the output voltage will be by adding 1.25V to the Adjustable pin (pin1) then you will know what the out put will be. In your case, the output is correct to what you will get on U1.
U1 pin 1 is 2.70, so add 1.25 to it, then you will have 3.95V. But on U3, there is something not right with U3 setup.

So in this case, it is either the Adjutable pin network resistors are off in values or the voltages as hsown in the diagram is not applied to your monitor.

[Nocta]

Re: VX 724 black screen green light flashing

Hmm, those still look incorrect to me, but trying test the new regulators to see if they are shorted or not.

I'm testing the new regulators.

Hi Nocta,

Just a few bits...

To give you an idea of what the regulars are finding confusing I will show you what you are getting on 2 of the regulators compared with what you should be betting....

U1

Pin 1 (left): 4.96v Should be ( 5V so this is close enough )
Pin 2: 3.94v Should be ( 3V 25% out of spec )
Pin 3: 2.70v

U3

Pin 1: 4.96v Should be ( 5V so this is close enough )
Pin 2: 3.94v Should be ( 2.5v This is massively out )
Pin 3: 2.25v

Something is very strange about the fact that both of the output readings that you have given are exactly the same.

I have attached a diagram which shows you where each pin number is. Can you look at this diagram and use that as a reference when giving us readings from pin numbers as i'm sure they are the wrong way around.

I know you have said that you have replaced some caps and you removed some caps to get to the Voltage regulators. Can you you confirm if you have changed the following? if so can you tell us what you replaced them with? EC1, EC2, EC3, EC4, EC5 and EC6

Can you also check that EC5 is definitely in the the correct way around and all of the others too please. ( Its hard to tell from the pictures but it looks like I get can see more white under that EC5 cap on the + side of it ).

I fully recapped the logicboard so from EC1 to EC6 are all new. EC4 330uF 16v; the rest are 22uF 25v (I did not find 22uF 16v).

What I wrote is the other way around. I numbered them from left to right and the tab pointing at me. So, now, following your diagram pin 1 is pin 3 and pin 3 is pin 1.

U1

Pin 3 (gnd): 4.96v
Pin 2 (tab): 3.94v
Pin 1 (Vin): 2.70v

U3

Pin 3 (gnd): 4.96v
Pin 2 (tab): 3.94v
Pin 1 (Vin): 2.25v

I really appreciate your input. Thanks.