Toshiba 58L1350UC short on Q301 Drain to source

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

The backlight leds are all good, tested. Not sure what to look for or what to test for the drive for Q301 from the drive circuit? I can upload closer photos if you need R_J - can you point me in the right direction? Tks!

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

You would need to scope the signal from the drive ic, to be sure it is ok and not just supplying dc to turn the mosfet on hard and shorting it. For the diode to short it either had excessive current flowing through it, or it could not switch fast enough and just shorted on its own.
It should be possible to replace Q301 and without the backlights connected, and only the one good diode in place, see what the voltage is across C320
The raw DC voltage supplied by T1/D401ab will be across C410A/B, If the boost circuit is working the voltage across C320 will be higher, but then likely drop back down due to the led protection circuit.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

You would need to scope the signal from the drive ic, to be sure it is ok and not just supplying dc to turn the mosfet on hard and shorting it. For the diode to short it either had excessive current flowing through it, or it could not switch fast enough and just shorted on its own.
It should be possible to replace Q301 and without the backlights connected, and only the one good diode in place, see what the voltage is across C320
The raw DC voltage supplied by T1/D401ab will be across C410A/B, If the boost circuit is working the voltage across C320 will be higher, but then likely drop back down due to the led protection circuit.

Thanks R_J for the counselling. Where is the drive IC ? I got this article on a guy that had the same problem and only changed Q301, but I don't know if it is the exact same one as mine? I'll measure voltage C320 and let you know.

https://techhometravel.com/toshiba-58l1350uc-no-power/

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

The drive ic would be located on that sub board, I would think it is ok.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Ok, so changed Q301, plugged it in without the backlights connected, powered up voltage on C320 started at 127 and went up to 180 and the TV turned off. Waited approx 30 seconds, powered on and it powered on, but after 4-5 seconds shutdown (with a click) and now Q301 is shorted again and the other Diode D301B is also shorted.

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

That seems odd unless the other diode was damaged, without the backlights connected there should not reallly be any current flowing through the diode, the fact the voltage went up to 180v would mean the boost circuit is indeed working. C317 or C318 are not shorted are they?

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Ok, R_J, what do you think I should do now? Do you think I should order some SB5200TA diodes, change both and Q301 and retry without the leds connected?

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

That should fix it, but maybe there is something else that shorts when the boosted voltage comes up? there is not much in that circuit to go wrong. The drive for the mosfet must be there or you would not have had the boosted voltage, the mosfet would have just shorted.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Ok, so will order some SB5200TA diodes and will report back when I get them...probably in a few days. If you think of anything else that I should test, don't hesitate! Tks!

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

maybe also look the bottom of board over closely, make sure you don't have any bad solder joints.

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

some of the joints look cracked unless it's just the way the picture looks.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

maybe also look the bottom of board over closely, make sure you don't have any bad solder joints.

Ok will do, I think you are quite right, so will retouch the solder joints. Tks!

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Ok so solder joints retouched and am waiting for the SB5200TA diodes (delivery will be 2022-03-08) and will report back.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Update: As I was ordering diodes, decided to get STP17NF25 to replace the original K13A25D and replaced the 2 VSB3200 diodes with SB5200TA diodes and it was a slam dunk. TV working perfectly!

Thanks to R_J and Nomoresonys and to all others that helped me out with this. Thanks again!

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Glad to hear it, another nice save.

[ja3robin]

Re: Toshiba 58L1350UC short on Q301 Drain to source

I wanted to write in to share my experience, and say thank-you for this post.

My symptoms:

- Toshiba 58L1350UC TV suddenly turned off/went dark while I was playing Legend of Zelda
- Would not power on or show any signs of life
- Opened it, and confirmed 5CCV being generated, but not 12VCC at the power supply output connector

Identified shorts in-circuit (see photo):

• Q301 - 0.7Ω between Drain and Source
• D301A and D301B - 0Ω across both

Verified out-of-circuit by removing components.

• Q301 shorted between Drain and Source
• One of the diodes shorted
• the other diode good

Followed the advice above and replaced with slightly up-rated components:

• Q301: Originally K13A25D, instead used STP17NF25 (Digikey 497-7511-5-ND) - rated 17A instead of 13A
• D301A and B: Originally VSB3200, instead used SR5200-TP (Digikey: SR5200-TPMSCT-ND) - rated 5A instead of 3A

Reassembled, powered-up, works again no problem!

Total repair cost:

• MOSFET = $2.41
• Diodes = $0.83 each
• Shipping to Canada = $8.00
• Total: $12.07 👍

Thank you for the advice in this thread!