Need schematic for Visio LC420

[jkb242]

Re: Need schematic for Visio LC420

So right now the PS-ON wire, BL-ON wire, and the Dim wire are cut and no longer have any connection to the main board for testing the power supply/inverter combo board, correct?
Checking sound will just have to find out because we do not know at this point what else are bad on the main board.

Please recall this set has a combined inverted power board and this is the second board with the TV same symptom. This board has only two wires leading from the inverter section of the power board, so the lamps are all in parallel.

Hope this helps.

[budm]

Re: Need schematic for Visio LC420

There 1 pair of high Voltage wires.
The lamps are driven out of phase that is why there are two transformers due to the length of the lamps, inside the panel there are ballast cap for each lamp so the lamps are not connected directly in parallel, unless the lamp are EEFL type then they they can be connected directly in parallel and not need for the ballast caps.
"Please recall this set has a combined inverted power board and this is the second board with the TV same symptom" bur we already know that it has problem with the bad main board, right now you are trying to verify if the power supply/inverter and lamps are OK or not.
What is the LCD panel model number?

CCFL, EEFL
https://cdn.badcaps-static.com/pdfs/...57143bb903.pdf

[jkb242]

Re: Need schematic for Visio LC420

Thanks for the reply and the link to the reference material.

The panel number is LC420.

By disconnecting the main board and applying 5vdc to the control pins on the power connector, have we not eliminated the main board for backlight testing?

So this forced on testing should be viable correct? If so, there must be a bad lamps in the panel. Are the LCD LAMP testers reliable. I was going to try that testing the individual lamps. Can you help me understand what the protection circuit sees if there is a bad lamp? Isn't the lamp gas filled so there is no means for shorting correct? Is the protection circuit on the power board? Again, this is the second board but it's possible that both power boards boards are not defective that I can prove for sure.

Thanks

[budm]

Re: Need schematic for Visio LC420

You have more than one lamp, so if the power supply board and the inverter circuit is good, the good lamps should flashes on for seconds BEFORE the protetion kicks in, so did it do that? If not then you have inverter circuit problem, the inverter circuit requires the boost Voltage to function.
Bottom line for the board to be good when using all 3 resistors to force on the board:
1) The Boosted Voltage (380 ~ 400VDC) is present and stay present, check?
2) Backlights come on and stay on, check?
3) If backlights flash on for seconds then you can have bad power supply (I.E. boosted Voltage drops down, faulty protection circuits, etc), bad inverter circuit, bad lamps. Good lamps will light up for seconds if inverter is good but the protection circuit will kick in, protection circuit will not kick in AFTER the inverter tries to fire up the lamps first to know if all lamps are good or not, thus flashing backlights.
You can have bad used board, it is not impossible to have bad boards )even more than 3) that were sold as tested which I ran into with other people that insisting on that the boards they bought are good because the sellers said so. I prove to them by doing the same test. It is up to you at this point.

[budm]

Re: Need schematic for Visio LC420

The panel number is LC420
LC420WUH-SCM1?

http://www.datasheetspdf.com/datashe...0WUH-SCM1.html
16 lamps total.

[jkb242]

Re: Need schematic for Visio LC420

Thanks I’ll check that the 390vdc remains constant upon power up with the 4 isolation resistors in place to force BL on. I certainly value your conclusions over what bad information I was likely provided by the seller of the used board. I’ll test the lamps individually on Monday which if verified leaves only the PS BOARD as you have concluded. I should have just purchased this board from SJ . I assume one can rely on the quality of their boards. I’ll report back.

Much thanks indeed!!

[budm]

Re: Need schematic for Visio LC420

The key point here is: If you have one or two bad lamps, the fact still remain that you should see the backlights flash on for seconds at least to show that the inverter did try to fire up the lamps.

[jkb242]

Re: Need schematic for Visio LC420

Yes, I think I fully understand your conclusions, the issue here is that the board I purchased is indeed bad as the one that was in the set originally. Perhaps the person who sold me the board did not actually know, but either way, I have indeed learned something from you in the process. I will attempt to find one at SJ and hope the lamps will pass the true test. I'll keep you posted. Thanks so very much.

[jkb242]

Re: Need schematic for Visio LC420

The key point here is: If you have one or two bad lamps, the fact still remain that you should see the backlights flash on for seconds at least to show that the inverter did try to fire up the lamps.

Received a new PS board from SJ same issue.

Tying four pins to 5vdc STB through 470 om resistor:

DIM, PSON, I-ON/0FF, LAMP

Nos sysem board attached or connected.

This set does not have a separate invertor board.

Continuity from each HV lead to lamps measures good.

Any suggestions please?

Thanks

[jkb242]

Re: Need schematic for Visio LC420

The key point here is: If you have one or two bad lamps, the fact still remain that you should see the backlights flash on for seconds at least to show that the inverter did try to fire up the lamps.

As mentioned in my last post, I tried a replacement board from Sj and had the same issue. No backlight flash no backlight. The system board has been completely eliminated but no backlight. Any suggestions?

Thanks

[budm]

Re: Need schematic for Visio LC420

So the 390VDC or so is present between the two legs of the main filter caps, all the command (DIM, PSON, I-ON/0FF) Voltages are all > 2VDC, so I cannot see why you are not getting any backlights action at all, I have no idea at this point.

[jkb242]

Re: Need schematic for Visio LC420

Actually, I had 4 leads, each through a 470Ω resistor. The additional lead I had connected was the lamp lead. Should this lead be included in the connection to the 5vdc stby rail? The boost voltage at the main cap is 390vdc. Should it also appear across the primary of the two inverter coils? Have you ever seen the two caps at the inverter coils fail? I haven't measured them, with set off of course, for capacitance but figured that was a very long shot. Possibly these are thyristors for protection, but without a schematic not sure.

Thanks!!

[budm]

Re: Need schematic for Visio LC420

The Lamp pin is the error signal out that is used to indicate if the backlights circuit has problem or not. It the transformer driver is not switching then the Voltage on the coil will be steady 390V but that depend on how the winding is connected to the driver output circuit, I.E. if it has DC Blocker/coupling cap on the high side.

[jkb242]

Re: Need schematic for Visio LC420

So, should I try the test again without the the lamp pin being held high or is that likely to make any difference. Is there something special about this Visio series that will not allow the backlights to be turned on unless the system board is attached? I guess this is likely a distant possibility since it seems that if the HF frequency OSC/chopper is working there must be HV AC at the output terminals on the investor. To determine this, I need to couple my scope to the invertor output with the ground lead connected to the low voltage side of the power supply since I do not have an isolation transformer as the - side of the boost circuit is 50 Vac above the low voltage side. Do you concure?

[budm]

Re: Need schematic for Visio LC420

You have to look at the PWM IC for the backlights circuit to see if has VCC Voltage to run or not then also look to see the EN pin is also active (usually goes high for the IC to run).
I never run into problem forcing on the power supply/backlights circuit.
BTW, do not try to measure the Voltage on the High Voltage output side of the Inverter transformer, it can have starting Voltage of 2000V!
I use CFL as High Voltage probe to see if there is high Voltage at the output of the Inverter or not:
https://www.badcaps.net/forum/showth...x+dx-40l261a12

[jkb242]

Re: Need schematic for Visio LC420

Thanks, I feel the PWM is a good check, in three different boards?? That seems doubtful if the PWM is on the power board. Not certain I remember seeing it, unless it’s discrete components or it’s located on the system board. Thanks very much. I’ll check you link to see how you sample the HV. Thanks for the warning too, I fully understand but do many may not. Appreciate your mentioning this.

[budm]

Re: Need schematic for Visio LC420

Thanks, I feel the PWM is a good check, in three different boards?? That seems doubtful if the PWM is on the power board. Not certain I remember seeing it, unless it's discrete components or it's located on the system board. Thanks very much. I'll check you link to see how you sample the HV. Thanks for the warning too, I fully understand but do many may not. Appreciate your mentioning this.

Why do you think it is doubtful the PWM is not in the power board?
What is the P/N of the IC701? IC701 and associate circuits drive the Primary side of the MOSFET Gate drive transformer T703A, the secondary is for drIving all 4 MOSFETs' Q701, Q706, Q715, and Q720 which is setup as a full-bridge drive circuit to drive the primary windings of the inverter transformer T701, T702, the primary windings of each transformer is connected in series with each other, you can easily verify what I see by looking at the bottom side of the board.
https://www.badcaps.net/forum/attach...2&d=1524080896
BTW, R805 shows <1 Ohms?

[jkb242]

Re: Need schematic for Visio LC420

I meant to say that it isn’t likely the inverter circuit would be bab on three different boards one of which was a SJ board which tested good. This analysis help me understand the PWM is on the power board. I made another test this afternoon using your idea of the CCFL bulb from a 120VAC lamp bulb as a test light. First I connected the CCFL tester between the CCFL leads to confirm the test lamp was good. The I connected the tester directly to the lamps in the set. One lead was connected to the tester, with the tester ground to chassis ground, power board not connnected and removed from set. Neither lead when tested showed any backlight on the screen. One lead showed BL looking through the holes in the chassis but no light on screen. The other lead showed no BL at all. Connecting one lead to the HV probe on the tester and the ground lead connected to the other BL lead also produced no BL. It appears that the problem with the set is the BL connection leads leading to the panel or a component connected to the BL feed at the lamps inside the display panel. Not sure exactly how to locate this. Should I test each lamp individually, or can I not test the entire array at one time to the HV TESTER? Thanks!!

[jkb242]

Re: Need schematic for Visio LC420

I’ll confirm R805 and advise.

[budm]

Re: Need schematic for Visio LC420

OK, please refresh my memory since the thread is getting really long, you have CCFL tester for testing CCFL lamp, correct?