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Broken TV Switching power supply

Edugimeno

Jan 29, 2017
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Hi there!
I got a broken TV in my hands. It doesnt power on at all. It was clear from the first time the issue was in the power supply.
I checked secondary diodes first, all tested good
No AC power in the secondaries
Went back to the primary side. No AC getting to the primary. Transformer is good (~2ohms)
Mains rectifier and HV capacitor seem to be good (storing 320 volts of DC for a long time) (it runs on 220V)
So it all seems to be pointing at the electronics between the HV capacitor and the primary.
I can hear a super low, "twich...twich"...very very low. Aprox 0,5 -> 1 Hz
Checked switching Mosfet (2N70), tested good with the multimeter diode "trick" that charges the internal capacitor and starts conducting
Checked the 100Kohm parallel-series resistors that feed the mosfet's source and is good. Checked all diodes around and look good
I downloaded the datasheet for the controller (LD7575, attached) and after reading everything, I can't see what's causing all this switching.
I checked RT, CS, COMP and they are all swinging up and down at the same rate (once every 1-2 seconds)
Vcc for the LD7575 is swinging from 10 to 16volts. 0,5-1 Hz
HV (startup) is correctly fed to pin 8, 320 volts there...

Should I suspect bad LD7575?

Im attaching my PS schematic along with the controller IC:
https://www.elektrotanya.com/lien-chang_aivp-0026_for_vestel_20inch_lcd_tv.pdf/download.html Link to power supply (give it some time and a download link will show up below the preview image. It works...)
https://lib.chipdip.ru/923/DOC000923326.pdf Link to LD7575 (PS managing IC)

Thanks!!
 

73's de Edd

Aug 21, 2015
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Sir Edugimeno . . . . .

Initially check by ohmming across C224/6 12 V supply storage capacitors to see if an overloading on that supply output is causing the power supply to cycle off and on.
Then move across C214/5 to ohm out in the same manner to confirm that 5V line is not being overloaded at its output.
Finally check to see that ZD207 between the supplies is not at fault.
Check to see how high the derived run voltage from C103 back to LD7575 pin 6 is able to rise up to.

The PS units initial start up kick comes in from the 3 series 1 meg resistor from the raw DC supply, until an initial stream of pulses from the T1 bottom left corner winding , supplies the LD7575 pin 6 voltage thereafterwards, until set turnoff.

You suggest of all secondary supply electrolytics as being healthy, but it would not hurt to externally tack on a supplemental 1000 ufd to each of the supplies and then power up to see if the tic-tic-tic-tic period lengthens.

Its up in the air, on the LD7575 on its condition, if the 7N70 has crashed , usually that short HV input to the pin 5 crunches the driver stage in the unit.


73s de Edd
 
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Edugimeno

Jan 29, 2017
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Ok I followed your directions (first of all thanks a lot for taking the time to answer this in such a elaborated way), and I realized that my initial tests had missed something... I do have a shortcircuit under the 12V output. And it sucks I didn't realize before because I've been messing with all the components in the primary side and I've desoldered and soldered again a bunch of them and the tracks are not looking so good anymore.
Well, again, SC under 12V output. Capacitors were taken out (this I did before), checked good. Checked the schematic, it seems that the 12V out not only feeds the normal output but also the inverter for the backlighting. Is the issue in that side? Time to check.
As there's a coil between the normal 12V output and all the inverter part, I took it out of the board and now it powers up, I get power on 12V and 5V!!! It's a little off (15v and 6v), maybe due to lack of load in the outputs.
So at this point I need to check that U204 (278R12) and find out what it does and see if its faulty, and proceed according to this.
Thanks a million as without your message I would be still messing with the driver chip... BTW Vcc to pin 6 is now steady at 14.99V!! I like to see that :)
 

Edugimeno

Jan 29, 2017
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It seems the 278R12 is just a special 12V regulator, and when I took it out of the board, there's nothing shortcircuited in it. But still the board shows 0Ohm between pin 1 (in) and pin 3 (gnd) for this IC while it's out, and it doesn't show this issue while the coil is out. So there's got to be some shortcircuit across "the label 12v/inverter" and ground. I guess that the label "12v / inverter" in the second page is the same as "vin12vvdc" in the forst page, and it feeds some capacitors and some integrated mosfets for the inverter section, time to go for those, right?
Thanks
 

davenn

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Sep 5, 2009
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But still the board shows 0 Ohm between pin 1 (in) and pin 3 (gnd)

pin 3 is output .... did you typo ?

and it doesn't show this issue while the coil is out. So there's got to be some short circuit across "the label 12v/inverter" and ground.

if you did typo and really mean pin 1 in and pin 2 ground ... and when you took the inductor, L203, out
then it infers a fault with one of the earlier components .....
possibly one of the following .... C223, 224 or 226
 
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Edugimeno

Jan 29, 2017
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pin 3 is output .... did you typo ?
No no I was meaning the 278R12 regulator, not the driver IC.
1 and 3 in the regulator and INPUT and GND, so the 12V rail was still shorted to GND.


if you did typo and really mean pin 1 in and pin 2 ground ... and when you took the inductor, L203, out
then it infers a fault with one of the earlier components .....
possibly one of the following .... C223, 224 or 226
No, the issue was after the inductor, and I traced the whole diagram until I found it. It was one of the integrated dual mosfet IC's that drive the HV coils for the backlighting. Specifically A304, it had S2 grouded. I removed it from the board, and placed the board back into the TV, now I have standby LED on, and kinda turns on, I actually see a program name for a while and then goes away, I guess it detects the faulty voltage in that side and shuts off, but I still see the +5v and 12V outputs OK so it's all good.
I just ordered the IC AO4606 on ebay on a local provider so this TV might be up&running next week.
I bought this TV set for 9€ "as is" so not bad at all

Thanks everyone who helped here!!
 

davenn

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Sep 5, 2009
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It seems the 278R12 is just a special 12V regulator, and when I took it out of the board, there's nothing shortcircuited in it. But still the board shows 0Ohm between pin 1 (in) and pin 3 (gnd) for this IC while it's out, and it doesn't show this issue while the coil is out

so please clarify .... WHAT coil and WHAT IC .... use the correct labels so we know what ones you are talking about
 

Edugimeno

Jan 29, 2017
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so please clarify .... WHAT coil and WHAT IC .... use the correct labels so we know what ones you are talking about
I was referring to L203 that feeds U204. But like I said I already found that the culprit was A304, an integrated dual mosfet to drive the backlight coils.
Thanks!
 

73's de Edd

Aug 21, 2015
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Sr Edugimeno . . . . .

Heyyyyyyyyyy . . . . .hombreeeeeeee . . . . ¿ Qué pasóóóóóóóóóó ?

Ahhhhh yes . . . . just shed the loads and then pinpoint the problem loading down the unit.

Certainly was not expecting that low drop out threshold 12VDC / 2 amp 278 R12 regulator to be at fault.
BUT on its output supply line there ARE being 4 matched AO4606 s, complementary set of FETS, that drive the backlight power transformers.

If you just sequentially lifted each ones Source 2 pin, until the load line short abated, and were lucky to just find only one of the 4 bad . . .great !
Otherwise, you will then be investigating the gate drive presences, as are being supplied by their adjunct controller the BIT3193.

Here is its Da-Da sheet . . . . . in case you are not familiar with it.

http://lib.chipdip.ru/149/DOC001149297.pdf


73's de Edd
 
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Edugimeno

Jan 29, 2017
14
Joined
Jan 29, 2017
Messages
14
Sr Edugimeno . . . . .

Heyyyyyyyyyy . . . . .hombreeeeeeee . . . . ¿ Qué pasóóóóóóóóóó ?

Ahhhhh yes . . . . just shed the loads and then pinpoint the problem loading down the unit.

Certainly was not expecting that low drop out threshold 12VDC / 2 amp 278 R12 regulator to be at fault.
BUT on its output supply line there ARE being 4 matched AO4606 s, complementary set of FETS, that drive the backlight power transformers.

If you just sequentially lifted each ones Source 2 pin, until the load line short abated, and were lucky to just find only one of the 4 bad . . .great !
Otherwise, you will then be investigating the gate drive presences, as are being supplied by their adjunct controller the BIT3193.

Here is its Da-Da sheet . . . . . in case you are not familiar with it.

http://lib.chipdip.ru/149/DOC001149297.pdf



73's de Edd

Hola Amigooooo! Como estas??
No worries now...I actually found it. It was one of the AO4606...read above. One of them had one S2 pin shorted to ground. And a big black stain under it...I lifted it and the TV fired up...I have already ordered one (well, 3, minimun order, just 2.5€/3) online, so hopefully this thing is working again soon.

Gracias por tu ayuda!!
 
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Edugimeno

Jan 29, 2017
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Oh for those following this thread...yes, replacing the mosfet IC (AO4604) fixed this TV, it's working again!! Thanks everyone!
 
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