Toshiba 32av733g1 power supply problem

[Hombre]

Hi

A friend of mine got me this TV. He was watching it during the storm, and lightning struck near so the set is dead now.
There wasn't any led's on so i opened it and found exploded Q908 so i replaced it with 2N3906 and stdby led came on. Turned set on and got green led, like everything is ok but there is no picture at all. Measured voltages - no 12V, 24V, there is just 5V on the output connector.
Noticed that resistor R957 is heating to much (brown PCB). Replaced it because it was burned.
When measuring primary side of transformer T950 there is 315v on pin 1, on pin 3 i can't measure anything because it's going out of range, or something like that. But if i remove ic950 A6069H than on pin 3 is 315V. I got another a6069 and tried it but same thing is hapening. I think that something around that is in mess but can't figure it out what

All fuses, transistors, diodes checked and everything looks ok, no bulged capacitors.

So if you have anything to say, please do

thx!

[budm]

Re: Toshiba 32av733g1 power supply problem

OK, check the DC voltage directly at the two legs of the main DC filter cap (the BROWN one with 450VDC) when the TV is plugged in but not turn on, and when it is turn on.
That section with blown transistor is the PFC Voltage booster, and it does not seem to be working because you are only getting 315VDC. You need to fix that section first.
By the way, can I get good clear picture of the top and bottom side of the board in 1/4 section or smaller so I can easily see the traces and the components?
The Q908 is for driving the Opto isoalator for the ACD (AC Detection) circuit, more likely the LED in the Opto is also damaged, something has to draw so much current to cause the burnt transistor like that, and since the transistor is connected in series with the LED Opto, you need to check that out. See page 50.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Voltage on main filter cap is 315VDc. That is when tv is plugged in. I can't turn it on because something went wrong again and don't have red and green led anymore, but when green led was still working (not stdby) voltage was also 315V.

[budm]

Re: Toshiba 32av733g1 power supply problem

OK, so the PFC voltage booster is not working, that can be due to the Main board is not sending out the PS_on signal to turn on the power supply. Are you still getting the 5V standby voltage?
It can also be locked out by the ACD signal. What DC voltage do you get at the connector for the ACD pin?

[Hombre]

Re: Toshiba 32av733g1 power supply problem

I still have 5V. I even *had* ~3V ps_on when led worked, but unfortunately not anymore - now is around 0.8V. On the other hand on ACD pin there is nothing at all, even when leds were on there was no voltage.
Interesting thing is that SPDIF port on mainboard is working - red light blinking (and when i first got tv it was on also, but when i managed to get those red and green leds, PS_ON to work the spdif wasn't blinking red).
Sry i maybe complicate to much, but that's the way i am, hope you will understand

[budm]

Re: Toshiba 32av733g1 power supply problem

OK, the ACD should be around 3VDC for normal condition. As you can see in the diagram, when B+_b from the power supply is present when you plug the TV into the out let, Q907, Q908 will turn on which will turn on the LED inside the Opto IC909, the photo transistor in the Opto will turn on which will supply the 5V to the resistor voltage divider R982, 983, os the out put voltage at ACD pin will be abot 3VDC.
I do not believe your Opto is working, like I had said, the transistor blew up, more likely the Opto is damaged also. You can short out pin3 and pi4 of IC909 Opto together to simulate ACD OK signal.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Just was on that part of the circuit when you typed this
Anyways, when transistor blew i replaced it but didn't notice that is something wrong with the optocoupler. Later i saw that it snaped on two halves. I replaced it and haven't noticed any changes. I think that i got the right part - PC123.
Don't know what happened because i havent touched anything, plugged it in and the leds are back on . So i have now 3V on PS_ON. On opto IC909 pin 4 there is 5V but it's not leting anything through, so no voltage on ADC . Other side of opto have around 11V, diode is working. I will try to replace it, but i put new one in and it looks like is broken. Don't know, we will see what happens next


EDIT:

OK, soldered another opto in (PC123) and nothing new happened, still no ACD signal. Hm.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Shorted optocoupler IC909 and i got 3V on ACD pin.
So i have now ACD, 5V, PS_ON and that's it. 12 and 24V still missing
Altough i would like to find out why opto won't work (don't like an idea of shorting it permanently )

Now when just plugged in on main filter cap there is 315V and when i turn the TV on there is 400V on the cap.

[budm]

Re: Toshiba 32av733g1 power supply problem

As I have indicated, the transistor that blew up drives the Opto's LED, so the Opto and resistor around that section should be checked or replaced, you only replace the transistor that drives the Opto. How about the transistor and Diode that drive the blown up transistor? Not much in that circuits, just transistors, Diode, resistors, Opto. I already explain how that section works, so it is just the matter of checking out the orperation of each components in that circuits.
Since PS_ON is present, then next place is to find out if the PS_ON receiving circuits are working or not, as you can see, there is Opto, transistors, etc that should turn on when the PS_ON is received. Do you know how to read and understand circuit diagram. I can spend long hour explaining how it works to you but that will consume too much time.
Just have to troubleshoot block by block and understand what the circuits need to function.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

OK, thx for your time. I will inspect better that part where the transistor blew up.
I am not pro obviously, still learning and have a lot to learn
Thx

[budm]

Re: Toshiba 32av733g1 power supply problem

OK, in that case, I will have to mark up to diagram for you to show you what voltage you should have in the circuits.

[budm]

Re: Toshiba 32av733g1 power supply problem

OK, TV unplugged, take the resistance readings on these two ICs between its Ground Pin against all other pin,
IC 950 PIN3 Ground its all other pins.
IC901 PIN 6 against its all other pins.
The resistor R957 (4.7 Ohms) is the protection resistor that supply the voltage to run IC950 (5V STBY SMPS IC)
IC901 (PFC Voltage booster IC)
IC970 (12V/24V SMPS IC)
So something was drawing too much current that cause the resistor to burn up. When that resistor burn up, the 5V STBY and the rest of the ICs as listed and the ACD sensing will not function. See if that resistor is open again.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Ok

First thing i measured is IC950 resistance between pin3 (GND) and 1 - it is 3ohms and that is according to schematics ok. No shorts on the other pins.
Then measured IC901 - pin6 (GND) towards pin8 was around 80ohms. I removed it, short still present. So i unsoldered J927 (jumper wire) to isolate VCC_1 line from IC901 and IC970 and short between pins 6 and 8 on IC901 was gone.
So pins 10 (GND) and 12 (VCC_1) on IC970 are measuring around 100 ohms. Removed C976 and C977 to eliminate their fault but 100ohms is still there betwen pins 10 and 12.
So it seems that IC970 (L6599) is gone.

Resistor R957 is OK, measuring 4,7ohms.

[budm]

Re: Toshiba 32av733g1 power supply problem

So if the IC is removed, the 100 Ohms reading between VCC1 and Ground is no longer 100 Ohms? That 4.7 Ohms looks like it got hot. Hopefully, that is all it needs, the IC.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Yes, it was the L6599 that was shorted. I removed it and the short was gone. Tested all resistors, they all seam fine.
Soldered IC and the the PS is working, 12 and 24V present
But the ACD is still not working, opto not letting 5V thru so i will have to inspect that part of circuit a bit more.
Backlight not turning on altough 24V is coming on the inverter board, no shorts. But can't measure anything on ON/OFF pins.
I will try to debug it tomorrow, have to go to sleep now

Thanks budm, aprecciate your help!

[budm]

Re: Toshiba 32av733g1 power supply problem

Check the voltage of the Base and Emitter of those two transistors in the ACD circuit, the reading should be around 0.6~.07V for those transistors to be on.

[Hombre]

Re: Toshiba 32av733g1 power supply problem

Thx to the budm tv is fixed and working

Replaced Q907 (i have put 2N3904 instead of that small smd pack - there is a plenty of room there so no problem with that ) and ACD and PS_ON voltages are now ok

PS: I noticed in top of the screen some bad pannel issues, but i hope it won't get worse soon

Thx budm once more

[tom66]

Re: Toshiba 32av733g1 power supply problem

Those top marks are due to TV being stored for some time. They'll typically go away after a day or two of normal use. Screen has just got cold, and needs to warm back up. Oddly I have only ever seen that with LG-Philips / LG.Display LCD panels which I believe this set uses. Must be something LG does different to everyone else.

Glad to hear it's working!

[budm]

Re: Toshiba 32av733g1 power supply problem

Yeah!

[Hombre]

Re: Toshiba 32av733g1 power supply problem

I forgot to see who's panel this is, but i also asume it is the LG because i remember the inverter board is made by LG

Once again, thx!