Need schematic for Visio LC420

[jkb242]

Re: Need schematic for Visio LC420

Wow, you are really very helpful and I thank you for hanging in there with me.

Really, I do and I know that I am going to learn more about this issue than I already have with your help but I do have more work to do based on your suggestions.
To answer your question about the tester I have a cheap HV generator sold on Amazon and other places for checking CCFL's. No model number on the case attached is a picture.

This lights up the low wattage <14 watts you suggest using as test fixtures for checking inverter output and it performs nicely for that purpose. That is what I am using to test my CCFL bulb as a tester confirming that it will detect HV.

Given the two HV outputs on the PS which is connected to the TV lamps, when testing these output points would the test CCFL not need to reference the common point or ground point of the Primary side in order to make an accurate check? I did not do this when testing the CCFL test buld and it the strengt of the HV was sufficient to light the bulb at least medium brightness or possibly full brightness given its low wattage.

The results of the CCFL test bulb on each phase of the HV lamp drive is NO LIGHT and as far as I could see, no noticeable flash for protection circuit to check the load. I then attempted to check the lamps in the set however only injecting the HV from the tester into each leading to the lamps. Saw noe of the lamps light. But is that test truly valid since the lams need both phases of the signal to even flash, and I can only supply one phase at a time?

[jkb242]

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

Just wanted to add another bit of info:

In order to isolate any power supply issues from a lamp issue, when testing with CCFL test lamps, would I not have to use two test lamps, one for each phase connection from the PS? Otherwise, would I even see a blink on a single test light?

Testing the lamps in the set, again today with the HV test fixture in the picture from my last post, only one set of lamps on the set were seen to light through the holds in the back panel but no light on the screen. The other set of lamps in the set were no illuminated. It seems obvious that I have either no connection to those lamps in the panel or something more.

I think this needs to be corrected in order to go further, do you agree or am I drawing the wrong conclusion from these test today.

Thanks Happy Memorial Day!

[jkb242]

Re: Need schematic for Visio LC420

I removed the panel to expose the lamps and their connection to the two HV leads from the power supply HV transformers. All of the lamps on each end are connected in parallel as measured by DC resistance. The two HV leads connects to the lamp board at each common ends of all lamps. I suspect my HV tester does not provide enough constant voltage to fully ionize all lamps. None of the lamps show fully on illumination when connected in their current configuration. When removing and testing each lamp, (I did not check the all lamps) each one does fully ionize and displays a normal light. Removing the lamps is a tedious process to avoid breakage and damage to the plastic lamp supports. Even if one or two of them were bad, it would seem that upon power supply start, the lamps would flash as you stated until the protection circuit takes over. Right? Can you suggest any other tests to the panel? I was looking for the one common issue all power supply boards tested would have had in common in that none of the would produce BL, and only leaves the lamps.

[jkb242]

Re: Need schematic for Visio LC420

Tested each lamp with ccfl test inverter, all lamps are good??

Please assist, thanks

[jkb242]

Re: Need schematic for Visio LC420

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?

IC701 is a DDA007GN after searching all the sites I locating same the Alia Baba affiliates and links therefrom do not provide a datasheet as insicated so I have been unable to get the pin assignments and cannot verify VCC or the enable pin.

Thanks.

[budm]

Re: Need schematic for Visio LC420

I am still looking for pin out/application notes/schematic of that DDA007.

[budm]

Re: Need schematic for Visio LC420

With all the forced on resistors still connected, check the DC Voltage between cold side Gnd and each legs of R799, R760A, they should have about 12V DC on each legs.

[jkb242]

Re: Need schematic for Visio LC420

Thanks and please advise when find the pinouts, I’ll continue the search as well. I’ll report back on the voltages measured as suggested. Also have a little info on ringing of T703A/B probing at the inverter output using a 12v source current limited <1 amp. Not all that revealing but something..

[jkb242]

Re: Need schematic for Visio LC420

To avoid connecting the board to the AC mains I am applying current regulated 12V to the LV secondary at the drain side of the Q910 MOSFET. There is no voltage at R760A or R799. This maybe because it appears that both resistors may be sampling the primary voltage which won’t be there the way I am powering secondary side only. Going to check further and report back.

[budm]

Re: Need schematic for Visio LC420

Do not trying using any other power supply, just run the power supply board by itself with all the forced on resistors in place, the board does have 12V present when PS-ON is present. I have you check the those Voltages at the resistors is to find out if the I/ON/OFF (BL-ON) gets all the way to the switched transistors (Q729, 730)or not to give 12V to those resistors.
If you power up the board and still there is no 12V at those resistors then it means you did not make good connection with the forced on resistor to the I/ON/OFF (BL-ON) pin or the BOTH switched Transistors are bad which I doubt that both of them would have gone out.

[jkb242]

Re: Need schematic for Visio LC420

Sounds good will perform that check today.

Thanks

[jkb242]

Re: Need schematic for Visio LC420

With all the forced on resistors still connected, check the DC Voltage between cold side Gnd and each legs of R799, R760A, they should have about 12V DC on each legs.

DIM, PSON and Ion/off tied to 5v stby through 470 ohm resistors, measuring R760A 10.5V R799 11.4V both sides, virtually no drop reference to secondary ground.

Probing secondary of T7037A&B pins 8&9 45Khz measuring with freq detector on multimeter.

Q730 is a MOSFET?

Thanks

[budm]

Re: Need schematic for Visio LC420

OK, so the switched Transistors are turned on and supplying the Voltages, one end of those two resistors should go to the VCC pin of the PWM IC.
It sounds like the IC is working, you can disconnecting the the BL-ON force on resistor to see if the 45Hz will go away or not, I actually expecting higher frequency than that.
BTW, what is the LCD panel model number?

[jkb242]

Re: Need schematic for Visio LC420

Do not trying using any other power supply, just run the power supply board by itself with all the forced on resistors in place, the board does have 12V present when PS-ON is present. I have you check the those Voltages at the resistors is to find out if the I/ON/OFF (BL-ON) gets all the way to the switched transistors (Q729, 730)or not to give 12V to those resistors.
If you power up the board and still there is no 12V at those resistors then it means you did not make good connection with the forced on resistor to the I/ON/OFF (BL-ON) pin or the BOTH switched Transistors are bad which I doubt that both of them would have gone out.

Measured at T703A

[budm]

Re: Need schematic for Visio LC420

So does the signal go away when you remove the BL-ON resistor?

[jkb242]

Re: Need schematic for Visio LC420

Will check. The drive at T703 scope capture suggest things are normal to this part. You agree?

[budm]

Re: Need schematic for Visio LC420

If the Gate drive transformers are being driven then there should be action on the form those Power MOSFET's that drive the primary side of the inverter transformers. Are you sure that you do have 380 ~ 400V DC feeding that output drive circuit for the output to work? Some thing is over looked some where.

[jkb242]

Re: Need schematic for Visio LC420

Will chech the removal of BL-On feed and advise rests. Will also confirm the 390vdc is being delivered to the power HV switching stage.

Thanks

[jkb242]

Re: Need schematic for Visio LC420

Panel model is LC420

[jkb242]

Re: Need schematic for Visio LC420

So does the signal go away when you remove the BL-ON resistor?

There isn't a BL on pin, there is a Lamp pin but I thought that was not for turning on/off the BL but used to monitor the lamp? Is that incorrect?