Toshiba 58L1350UC short on Q301 Drain to source

[rddube]

Hello everyone,

Need some help in diagnosing all the ripple effects of a failing Mosfet Q301, which originally was a K13A25D which was shorted Drain to Source.

I replaced it with a P16NS25 which had about the same specs, and the TV powered on for a few seconds, but dropped dead again and Q301 was again short on Drain to Source.

So started looking at some other components (after removing the shorted Q301) and found diode D301A shorted (took it out of circuit because both D301A and D301B were shorted in circuit, but it turns out D301B is good.

On the other side (backside) tested the gate components, and found the D302 shorted in circuit.

So am not sure what else I could check, should I take D302 out of circuit to test it, and what do you think could be my problem? I checked all the led strips with a led tester and they are all fine. Thank you for your help and if you have any questions, don't hesitate to bud in!

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

yes, check it out of circuit, sometimes one bad diode makes the whole row test short, so I take them out one at a time and check them and also check the thru holes to see if the short is still there and you can see if the other diodes on the board still read short.

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Did you take out 301a and did you check the other shorted diodes while you had the first one out?

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

I usually don't put anything back in till I find and eliminate the short.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Did you take out 301a and did you check the other shorted diodes while you had the first one out?

Yes both 301a and 301b are out - 301b is good, but 301a is short

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

is 302 or any others still shorted?

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

If not, then probably just replace 301a and see if that solves it.

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

sometimes you will have other components like resistors on the same circuit as the diode that might make it read short in circuit, so checking out of circuit is the best way to know for sure.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

yes, check it out of circuit, sometimes one bad diode makes the whole row test short, so I take them out one at a time and check them and also check the thru holes to see if the short is still there and you can see if the other diodes on the board still read short.

D302 actually reads 20ohms, because the 2 resistors in parallell (R355 and R321) are 2R20 (R355) and 10R0 (R321), so I think this is ok. What do you think?

[nomoresonys]

Re: Toshiba 58L1350UC short on Q301 Drain to source

I think so, if no more shorts, then your probably ok to replace 301a and see if that cures it.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Ok, will give it a try and report back. Thank you for your help!

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Nope that didn't work. TV turned on for approx 4-5 seconds, then off. Q301 drain to source is short and replaced D301a is also back with a short.

What should I check now? I've removed the shorted Q301 and D301a.

D301B is fine??

Tks!

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Note that I replaced D301A which was a VSB3200 which I didn't have with a SB3B0 which seem to have the same specs. But maybe I fumbled there?

[diif]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Note that I replaced D301A which was a VSB3200 which I didn't have with a SB3B0 which seem to have the same specs. But maybe I fumbled there?

Double check the peak reverse voltage.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Double check the peak reverse voltage.

Both are 200V..but there is a problem elsewhere which I need to track down.

[diif]

Re: Toshiba 58L1350UC short on Q301 Drain to source

Both are 200V..but there is a problem elsewhere which I need to track down.

Ok, if you're sure, it doesn't look that way to me.

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

I don't think the SB3B0 is fast enough for the high frequency circuit, that's why it shorted.
Under Maximum rating:
Single phase, half wave, 60 Hz, resistive or inductive load.

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

So R_J, you think if I get another VSB3200 (they are scarce, haven't found any yet or an equivalent, best would be on Digikey for me as I am in Canada) and replace it with Q301 again, that might do the trick?

[rddube]

Re: Toshiba 58L1350UC short on Q301 Drain to source

I don't think the SB3B0 is fast enough for the high frequency circuit, that's why it shorted.
Under Maximum rating:
Single phase, half wave, 60 Hz, resistive or inductive load.

Do you think this one would be ok ? SB5200TA datasheet attached

[R_J]

Re: Toshiba 58L1350UC short on Q301 Drain to source

If the drive for Q301 is ok from the drive circuit, and the backlight leds are not bad, It should be ok.
Digikey says SR5200-TP is an alternative.
If only the one diode shorted when they are in parallel, I suspect the diode itself was the problem. unless the other one is totally open.