Toshiba 58L4300U won't power on

[Nexus1966]

Hey everyone,

I just bought a Toshiba 58L4300U TV real cheap because it appears to be dead. It does not power on at all. I was hoping I would find some bad caps or something easy to fix, but no luck there. All the caps and other components look fine to me physically.

The power/standby LED for the TV never lights up at all.

Poking around with a voltmeter on the power supply board reveals the following:

• 5VCC output always has 5 volts
• ACD output always has 3.3 volts
• PWON output always has 3.2 volts
• 12 volt outputs provide 1.9 volts output briefly when attempting to turn on TV, but quickly drop back down to zero within a second.

Any suggestions on what to check next would be appreciated.

[budm]

Re: Toshiba 58L4300U won't power on

Are you getting about 400VDC between the two legs of either one of the main filter caps C1A, or C1B when PWON signal is present?

[Nexus1966]

Re: Toshiba 58L4300U won't power on

363 volts on each of the big caps as soon as I plug the power cord in. It fluctuates a little when I press the power button, it changed so fast it was hard to catch the number though.

[budm]

Re: Toshiba 58L4300U won't power on

So the PWON (PS-ON) does not change state between <1V and >2.5V when power switch is toggle?
363V for the PFC Boosted Voltage seems to be on the low side, the boosted Voltage will only present when POWON is present, when TV is off the DCV on the caps should then be around 160~170VDC.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

PWON has 3.2 volts as soon as I plug in the TV. If I press the power button enough times or hold it down long enough, it will sometimes momentarily drop down to almost 0 then it comes right back up to 3.2 again.

Should I try to test the power supply with the main board disconnected? If so, I can find some 12 volt bulbs to use as a load, but how do I trigger the power supply to turn on? Do I need to use a resistor to drop the 5 volt output down to 3 volts, then feed that to PWON? I'm more used to computer power supplies where you can short a lead to ground in order to trigger the power supply to be "on" manually.

[budm]

Re: Toshiba 58L4300U won't power on

You can disconnect the cable between the power supply and the main board first, then use 1K 1/4W or larger between the POWON pin and the 5VSTBY (5VCC) pin to force on the power supply and see if the 12VDC will stay on.
If not, try disconnecting the LED backlights connector to see if the 12V will stay on.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

OK, I disconnected the main board and use a 1K resistor to trigger the power supply. With or without the LEDs connected to the power supply, I get about 1.8 volts from the 12 volt output when I first trigger the power supply on, but it drops pretty quickly down until it gets close to zero.

[budm]

Re: Toshiba 58L4300U won't power on

OK, so you do have problem in the 12V power supply section.
Can you check the DCV between the two legs of either caps C210A, or c210B to see if the Voltage is present and then drops down too (those two caps are the filter cap for the LED backlights power supply section).
T1 power transformer is used to generated the 12Vs and the Voltage for the LED power supply.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

At those caps I get about 1.8 volts when first switched on, then it quickly drops to almost nothing.

[budm]

Re: Toshiba 58L4300U won't power on

At those caps I get about 1.8 volts when first switched on, then it quickly drops to almost nothing.

As expected because they are all generated by the same power transformer.
So now we have to dig deeper in that section of the circuit, not going to be easy without the diagram.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

I managed to find this post which looks like the same power supply and same type of problem:

https://www.badcaps.net/forum/showth...403#post615403

[Nexus1966]

Re: Toshiba 58L4300U won't power on

I would probably buy a new power supply board at this point if I found a reasonably priced one, but I can't seem to even find any replacements that are in stock. Seems like this must be common failure, I found a few places selling them and they are all out of stock.

Is it worth trying to repair this board?

[budm]

Re: Toshiba 58L4300U won't power on

You also have stuck on POWON also which I was not able to figure what the cause of the problem is in that thread. The POWON is generated by the main board, also his board has higher boosted Voltage (389V) compared to your power supply board.
But it looks like we have problems in both the power supply and the main board of your set.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

Thinking outside of the box a little here... would it be possible to hook up a computer power supply to run this TV? I know the main board only needs 12 volts and 5 volts, a computer power supply provides both. What voltage would the LEDs need?

Assuming I can wire up a computer power supply to send the right voltages to the right places, that would allow me to verify the status of the main board right?

[Nexus1966]

Re: Toshiba 58L4300U won't power on

Well, I did it.

I didn't know what voltage the LEDs need, so I didn't connect them yet. But I wired up a computer power supply, and confirmed the TV works, minus the back-light. The main board appears to be OK. I plugged in a Fire TV stick and if I shine a flashlight on the screen, I can see the menu from it and it looks good, aside from being really dim of course.

So... do I try to McGyver this with an external power supply, or do I fix the original power supply? I could just start randomly replacing components on it until it works I suppose. I lack the knowledge to go about testing the power supply components in a logical order.

[CapLeaker]

Re: Toshiba 58L4300U won't power on

I could just start randomly replacing components on it until it works I suppose. I lack the knowledge to go about testing the power supply components in a logical order.

Check the resistance on the secondary 12V pin to the GND pin. Is there any short? If so look at D201 if the center pin is shorted to any of the 2 outside pins...

[Nexus1966]

Re: Toshiba 58L4300U won't power on

I started just testing random components and found what looked to be a shorted diode.

D301A or D301B (they are in parallel) - a google search of the part number brought me to this: http://www.vishay.com/diodes/list/product-89144/

According to that page it is a schottky diode. I got an analog multimeter to test them with the instructions I found here: http://www.electronicrepairguide.com...tky-diode.html

One of them appears to test good, the other one is shorted both ways.

[vinceroger69]

Re: Toshiba 58L4300U won't power on

if you have not already remove the diode that tests faulty from the board and retest as some times they can read shorted in circuit.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

Yes I did remove them both from the board to test after my initial test showed them shorted both ways.

[Nexus1966]

Re: Toshiba 58L4300U won't power on

So knowing that one of those schottky diodes is bad, what else should I examine?