Toshiba 20W300PG No backlight

[playman]

Hi there
I have a Toshiba LCD TV that has black screen (no backlight)
but shows image when directed a light onto the screen.

C347 & C338 (both are 470uf 25v chang) were bulged, I replaced them with 470uf 25v su'scon I had here.
Both fuses are ok, I only found 2.
I noticed no change in that repair.
What else could be the culprit?

I hope the pictures are good enough, I took them with my galaxy s3 phone.

[playman]

Re: Toshiba 20W300PG No backlight

No one that could know what's wrong?

[flocko]

Re: Toshiba 20W300PG No backlight

have you checked white smd fuse f300 on back of board?

[playman]

Re: Toshiba 20W300PG No backlight

have you checked white smd fuse f300 on back of board?

I checked it in circuit got no reading @ 200Ohm, switched to 200kOhm and got a reading of 53.1 then gradually
lowering down to 39.7 which I stopped measuring, checked it again 3-4 Min later and got a reading of 1.7 @ 200kOhm
and it gradually raised to 7.1 which is when I stopped measuring.
Is that normal?

I also noticed another fuse on the board F103 which measured 1.3Ohm @ 200Ohm

I can always measure SMD fuses in circuit right?

[flocko]

Re: Toshiba 20W300PG No backlight

the fuse should read the same as when you have the red and black leads together (short circuit)but must be measured across the fuse not to ground Flocko

[playman]

Re: Toshiba 20W300PG No backlight

yea that's what I did, I measured across the fuse, the meter gives me 0.8Ohm @ 200Ohm reading when I connect my leads together.

Do you believe that the F300 is broken?

[flocko]

Re: Toshiba 20W300PG No backlight

looks that way,if you have no new fuse to replace itwith use a fine wire across the fuse for testing only, if that works and the backlight comes on then you can get the correct fuse. Flocko

[playman]

Re: Toshiba 20W300PG No backlight

Ok I de-soldered the fuse and tested it and it was broken, it says 3 on it, that means 3A fuse right?

I soldered a wire where the fuse was, plugged the TV in and saw a standby light, tried to turn it on and nothing
happened, standby light changed into fast blinking and then few seconds
later it disappeared and now it's like the TV is completely dead, No lights visible.

Wire seems still be intact.
No visible damage can be seen, no sound was heard or smoke/smell.
All 4 the fuses (main, F102, F103, F300 wich is a wire) are okay, I just know of those 4.

[stj]

Re: Toshiba 20W300PG No backlight

that was predictable.

something is probably shorted that made the fuse fail.

[playman]

Re: Toshiba 20W300PG No backlight

What can I do now?

[Caleb]

Re: Toshiba 20W300PG No backlight

remove the wire you put in and see if it returns to its previous behavior, if so you havent fouled anything else up yet, then work down the line and see if you can find shorted components in that section. I would look at that quartet of mosfets as likely suspects.

[budm]

Re: Toshiba 20W300PG No backlight

Need to check those 4 SMD Power MOSFETS Q301~304 to see if it has S to D shorted out or have very low resistance.

[playman]

Re: Toshiba 20W300PG No backlight

remove the wire you put in and see if it returns to its previous behavior, if so you havent fouled anything else up yet, then work down the line and see if you can find shorted components in that section. I would look at that quartet of mosfets as likely suspects.

After I removed the wire it seems to return to previous state, show's standby light and
turns on and I can see the menu with a flash light.

Need to check those 4 SMD Power MOSFETS Q301~304 to see if it has S to D shorted out or have very low resistance.

Then I would guess that D is pin 2 and S is pin 3 right?
Q301=Red probe on D(2) Black on S(3) = Open Cirquit (no reading)
Q302=Red probe on D(2) Black on S(3) = Open Cirquit (no reading)
Q303=Red probe on D(2) Black on S(3) = 188.4 @200Ohm
Q304=Red probe on D(2) Black on S(3) = 10.4 @200Ohm

Q301=Black probe on D(2) Red on S(3) = 1638 @ 2000kOhm then open circuit (no reading)
Q302=Black probe on D(2) Red on S(3) = 1677 @ 2000kOhm then open circuit (no reading)
Q303=Black probe on D(2) Red on S(3) = 53 @ 200kOhm and degreasing rapidly down
Q304=Black probe on D(2) Red on S(3) = 10.5 @ 200Ohm

Is it right to guess that all FET's are broken except Q304?

[tom66]

Re: Toshiba 20W300PG No backlight

Looks like Q304 is definitely bad, Q303 maybe.

[playman]

Re: Toshiba 20W300PG No backlight

I did forget to mention that I measured them in circuit, sorry for that.

So I might as just replace them all, right?
Or are the Q301 & Q302 just fine enough to use.
Is there anything else that might have gone?
would be nice to know that before I try to order as there is no part shop in my town

[tom66]

Re: Toshiba 20W300PG No backlight

Given the cost of four FETs is likely to be minimally more compared to just two, I would replace them all.

[playman]

Re: Toshiba 20W300PG No backlight

I was about to order the parts when I noticed that the FET's have two different
type numbers 2x STD30PF03L & 2x STD17NF03L, I hope that does not make the measuring bad.
The problem is the STD30PF03L has not been available in the store here in Iceland since 2009, and
Farnell their supplier don't have it either.

Is there any other FET I can use instead of the STD30PF03L?

http://pdf1.alldatasheet.com/datashe...TD30PF03L.html
http://pdf1.alldatasheet.com/datashe...TD17NF03L.html