Acer 42" LCD tv no power or stdby light

[slowcap]

The model is at4220b and power supply board is a delta dps-291ap , it happened after we unplugged it from the mains and then replugged it in, there is no standby light and it won't turn on, the fuse is good in primary power supply and we have 240v coming in, no leaking or bulging caps and nothing looks shorted or dry soldered, no power on pson wire , no 5v 12v or 24v on wires from the power supply board, I had power supply board out and looked it over, when I fitted it back in , I turned it on, heard a crackle from power supply board area and it powered up , with standby led and also turned on, when I pulled the power it would not come back on, I can use a ddm and repair car computers but could use some help with this , thanks in advance

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

just found a blown resistor in the primary circuit of the power suppy, its resistor R841 can someone please tell me the ohms, board is a delta DPS-291AP

[budm]

Re: Acer 42" LCD tv no power or stdby light

Any pictures of the board?

[selldoor]

Re: Acer 42" LCD tv no power or stdby light

When you do a picture show where the resistor is/was.
Have you taken the resistor off the board - sometimes is blows off one side
but you can see the markings left on the other side?
If its burnt try cleaning with isopropyl alcohol.

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

I replaced the resistor, it was 0.1ohms, was a carbon resistor but could only get a 5w wire wound resistor, the board had been out all night and when I went to put the resistor in had a few big sparks as cap discharged, about 4 sparks in total. all in and still nothing,
one photo is of the board and I have circled in red where the resistor sits underneath. the second photo is the replacement resistor (red circle) and the FET that it connects to (black circle) there is a cap that jumps the same gap as resistor,

[budm]

Re: Acer 42" LCD tv no power or stdby light

Can you please use the attachment function to upload the pictures? Please do not post the pictures in line.
I would like to see the good pictures of the bottom side of the board, also in 1/4 section so I can trace the connections.
That resistor may be fusible resistor for circuit protection for the B+, cannot tell until I see the bottom side of the board.

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

hi, the burn mark around the resistor is from the first cap discharge

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

here theres might be better.

just found the fuse now blown. replaced fuse and it blew it straight away. maybe the FET Q301?

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

well I cant edit the last post anymore so will have to reply.

the mosfet is a n channel, number on it is 15n60c3, anyone know what it does? if it has shorted will this blow the fuse?


update: just unsoldered both FETS and both are shorted. will replace tomorrow.

Question. is there any point updating this thread? , just thought it might be useful for anyone else with problems of a similar nature. I hate it when people ask questions, get answers, then go away and not follow up with the solution or a reply, even "yes this fixed it" would be good enough.

[Alastair E]

Re: Acer 42" LCD tv no power or stdby light

Question. is there any point updating this thread? , just thought it might be useful for anyone else with problems of a similar nature. I hate it when people ask questions, get answers, then go away and not follow up with the solution or a reply, even "yes this fixed it" would be good enough.

Well--TBH, People DO have Lives--As well as posting on here, They post advice for free, and often dont even get a 'Thank-You'

The last post by another member was Less than a full day ago! If it was a week--Then I could understand your irritation, but a few hours!

Give 'em a chance!

[budm]

Re: Acer 42" LCD tv no power or stdby light

Well, I download the pictures, some are not in focus.
Do know how long it will take me to look at the traces on the board based on the pictures and tray to draw up the diagram without having the board in front of me?
If that is too long to wait, then I am sorry for asking for good clear pictures.

[twbranch]

Re: Acer 42" LCD tv no power or stdby light

Sick 'em!!!

Listen this is a good place for answers but you have to be patient. I have a few pieces needing repairs that have been on my bench for a week or so because I can't find manuals. It takes a while and these people aren't paying the bills by replying to this thread. Hell, I am at work to pay my bills. I am betting, because I haven't had time to actually go over the blurry pictures, that more than just a resistor is bad. That resistor is probably meant to blow because of another failure (0.1ohm is usually a fuse resistor). More information is needed, like a schematic, to find out what that resistor is protecting and what it is protecting that component from.

[budm]

Re: Acer 42" LCD tv no power or stdby light

This is what I get from looking at the poor quality pictures.
Q301, Q303, IC300 should be replaced. The component around the Gate drive circuits of Q301, Q302 also must be checked damage.
Q808?? (PFC Voltage booster) need to be checked also.
The 12V, 24V rectifiers in the 12v/24v section also need to be checked.
You should put a 75~100W lamp in series with your power cord so you will not keep blowing the new fuse if the shorts are not located, the lamp will light up brightly if short circuits still present.

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

Thanks for the reply have ordered the new mosfets from uk, will look at the other components tomorrow,

The post about replying was not directed at people who generously give up their time to help others, it was more intended for other people who ask questions and get help but never return to give the results, IMHO a lot of new threads could be avoided if a final post said "yes that fixed it" , I'm not talking about this awesome forum but from my hours of searching other forums and google results that don't have a conclusion, leaving me asking well did it work or did the person just give up and buy a new unit or bin it, sorry if I didn't make myself clear, I would be happy if a few days passed before I had a reply.

on a side note, ive been looking into why all 3 legs of the fets had shorted, and from my results heat is the killer, but its no too common for all 3 to be shorted, usually just 2 legs short. could a failed driver cause the fet to short this way?, I understand that the driver would now be receiving the full current as it backtracks up the gate, from the board that will be ic300 for q301 and q302.

[budm]

Re: Acer 42" LCD tv no power or stdby light

If the one of the IC Gate drive out put stuck HIGH that MOSFET will be fully on, so when the other MOSFET in this half-bridge drive circuit turn on, it will be like putting dead short on the B+ LINE.

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

That is exactly what seems to be happening, is there a way to test the gate drive circuit with the mosfets removed?, as I'm having to import them from uk to newzealand would rarer not blow up some new ones, I am looking to buy an oscilloscope in a couple of weeks, no help right now of course.

[ben7]

Re: Acer 42" LCD tv no power or stdby light

That is exactly what seems to be happening, is there a way to test the gate drive circuit with the mosfets removed?, as I'm having to import them from uk to newzealand would rarer not blow up some new ones, I am looking to buy an oscilloscope in a couple of weeks, no help right now of course.

It's far fetched, but we probably could figure out some similar mosfet that you could use as a replacement. That way you don't have to ship things across the world!

Be sure to check the bridge rectifier to see if it is shorted or not. Also check the fuse, if you haven't already.

Refer to post number 13, budm is right about replacing all the parts involved with the mosfets.

[budm]

Re: Acer 42" LCD tv no power or stdby light

You better replace the IC rather than taking a chance of blowing up your new MOSFETs. I ave a feeling the Gate drive components (didoe, resistors) are likely to be damaged when about 400V is dumped (shorted Drain to Gate) through those sections.

[ben7]

Re: Acer 42" LCD tv no power or stdby light

You better replace the IC rather than taking a chance of blowing up your new MOSFETs. I ave a feeling the Gate drive components (didoe, resistors) are likely to be damaged when about 400V is dumped (shorted Drain to Gate) through those sections.

Agreed!

[slowcap]

Re: Acer 42" LCD tv no power or stdby light

Ok , will replace the ic aswell, which are the gate drive diode and resistor that I should check/replace? ,
here are some much better pics, with same phone but with sunlight.