coby 40" lcd TFT V4028 no sound

[cadillacman]

Picked this up tonight. Old dude and lady owned it. Bought it about 6 mos ago
( i have the $500 reciept and everything!!)

Supposedly worked fine untill a thunderstorm then supposdly would not power on. However she said everything else on the powerstrip still worked. She assumed it got hit by lightning. I have my doubts bieng everything else on the strip survived.

I look it over, nice. Not a scratch on it. Took it home. Plugged it in, it works!!!! However there is no sound with any input. It looks to me like mabye the old man had it apart, i see screwdriver marks on the screwheads.

My theory is that it was electronically locked up due to power variance:
( power here tends to get weak at times during storms- even my personal set sometimes gets locked up after a big nasty storm) and mabye thats why it wouldnt power up.

I think the old man took it apart and with some luck scratched a trace or left something unplugged. Obiously he didnt try plugging the set back in otherwise i wouldnt have ended up with it. It will get a good lookover while apart. Pics and more fun to come soon!

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

yeah baby!!! Its alive and working 100%.

I tore it down, caps are capxon( yeah i know) but none are even slightly bulged. As stated earlier this tv has only been used for approx 6 mos.

Solder on ps wasnt too bad. I touched up a few questionable joints but did not find one bad screaming obious joint. I found that when whoever had this thing apart, they left the power cable off of the power supply. I straightned the bent header pins, reconnected, reassembled the set and voila. I like fixes like this. Nice and simple. My investment? $30.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

this quit powering on today. A check of caps all still looked good. Replaced the 10uf startup cap(its next to the large filter cap), set is up and running again.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

k im at a loss here. it ran great ran all night tonight, Power down, back on fine all is happy. I unplug it for a couple seconds. Reconnect, WONT POWER ON AGAIN... ugh.... anybody else been here?

once its up it runs all day no problem. Mabye i should just shotgun all the 1000uf caps and whatevers associated with the +5... mabye i have too much ripple or something i dunno

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

And this morning my wife plugged it in. hit power, been running all morning powers on and off no prob. wth???? Im at a loss here i really am.

it was all happy. Unplugged it, replugged ok. Unplugged again, now back to not powering on.

Ive notived when this tv set is working and off, the led comes up bright but quickly goes dim(indicating standby mode?)

when i plug this in and the tv does not work, the led stays bright and then no respose. once i can get the set to show me a dim led it will power on and work forever untill it is unplugged again. I may just have to keep this set for myself.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

On my own on this one huh? Oh well. THe forum gives me a place to think out loud

THe more i think about this, its gotta either be a standby(5v) circuit issue or a mobo issue. I suppose i should check the header pins on the mobo as well while i have it apart again.

Going to poke into it tonight. Im hoping to either find weak +5 or ripple in it. WHats a good rule of thumb as far as how much ripple i should be seeing? I understand 0 is ideal but whats a real world figure?


If i cant find anything with the ps then i will assume its a mobo issue. This will become my personal tv then if thats the case and ill sell the old vizio plasma i have(the first set i ever fixed, still going strong heh) I could care less about the issue personally, i never unplug my set so once working it shouldnt be an issue hopefully.

[cadiman]

Re: coby 40" lcd TFT V4028 no sound

I have a couple of those too. I would replace all the crap caps and then you will have a good baseline to start from.

I really hate getting tvs that were messed with. Anytime you get a tv with more than one problem chances are it was taken apart and further harmed.

Keep checking connectors. They most likely unplugged everything.

Don`t unplug the power cord....just an idea.

After replacing all caps on the power supply I would start checking the larger caps on the mainboard.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

dont unplug, thats what im going to do. I cnt sell this with a honest straight face, so im going to sell the first tv i ever fixed, then put this one in its place. As long as my toddler doesent unplug it, i should be good.

I will run it untill it either fixes itself or dies completely than track it down. Untill then im gonna run it man.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

ran it till it died same symptoms as before.

Led light is on bright.(when working propetly this led is dim when set is in standby) no response. except now it stays broken.

5v ok at 5.05. 12v sucks 11.2v no ripple on either 12v or 5v lines found. caps still visual check ok but they are capxon so anything is possible.


If i unplug the connector off of the ps with the 12v terminals, and reconnect, the 12v will jump to 11.5, the led will go from bright to dim(standby) set will turn on and off like it should with onboard controls or remote, all is happy untill the set is unplugged. then the 12v goes back to 11.2 and the set will again have a bright blue led and no response.



SO my question to you guys that know way more than i do, will this low 12v be possibly causing the set from "booting up" and preventing standby when plugged in?

this set has a mainboard thats also av inputs video out and speaker amp in one unit. Im wondering if the board is seeing the 12 too low and stopping ther set from booting up sort of speak.

i believe im going to roll the dice and recap the whole damn ps, at least the 12v sections.


***NEW DEVELOPMENTS..

sharing a trace with the 12v pins is r122. It is marked orange, orange, brown, gold. that should be 33.1 ohms correct? because im reading like 325 ohms.

[PlainBill]

Re: coby 40" lcd TFT V4028 no sound

ran it till it died same symptoms as before.

Led light is on bright.(when working propetly this led is dim when set is in standby) no response. except now it stays broken.

5v ok at 5.05. 12v sucks 11.2v no ripple on either 12v or 5v lines found. caps still visual check ok but they are capxon so anything is possible.


If i unplug the connector off of the ps with the 12v terminals, and reconnect, the 12v will jump to 11.5, the led will go from bright to dim(standby) set will turn on and off like it should with onboard controls or remote, all is happy untill the set is unplugged. then the 12v goes back to 11.2 and the set will again have a bright blue led and no response.



SO my question to you guys that know way more than i do, will this low 12v be possibly causing the set from "booting up" and preventing standby when plugged in?

this set has a mainboard thats also av inputs video out and speaker amp in one unit. Im wondering if the board is seeing the 12 too low and stopping ther set from booting up sort of speak.

i believe im going to roll the dice and recap the whole damn ps, at least the 12v sections.


***NEW DEVELOPMENTS..

sharing a trace with the 12v pins is r122. It is marked orange, orange, brown, gold. that should be 33.1 ohms correct? because im reading like 325 ohms.

Do you have a tattoo artist near you? Perhaps you should have one tattoo the resistor the color code on your arm. The first two bands are the significant figures, the third is the multiplier, the fourth is the tolerance. So that resistor is 330 ohms, 5%.

PlainBill

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

D'OH!! well i guess the hunt will continue.

I have a resistor color chart pulled from google but obiously i read it wrong or it lied to me.

because it was coming up as 331 x 0.1

[Tvbob]

This was taught 30yrs ago

oops

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

not to me it wasnt, self taught, and obiously i taught myself wrong.

its ok though thata why i have you guys to smack me upside the head and say " thats wrong dummy!!"

[PlainBill]

Re: coby 40" lcd TFT V4028 no sound

D'OH!! well i guess the hunt will continue.

I have a resistor color chart pulled from google but obiously i read it wrong or it lied to me.

because it was coming up as 331 x 0.1

Whew, it's worse than I thought!!! If a resistor has four color bands, it is 1st digit, 2nd digit, multiplier, tolerance. If it had 5 bands, you would have been correct.

PlainBill

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

wow. The sad part is that at one point in time, i was pretty good about reading resistors but as time went on and i got away from the electronics hobby(used to fix car amps back when they were worth fixing) but i guess time away has really messed me up. Capping the power supply now. Wish me luck.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

Well anything to do witht he 12v was recapped. same..

heres what ive noticed. No matter what working or not powered on or not my ps on is 4.3v

with the tv on and working i have 11.2 v on my 12 line( i think new caps made it worse lol). WIth the tv working and switched off, i have 0vdc on the 12v line.
My 24v also switches has voltage on and none off...

Now if i unplug the tv and reconnect to "break" the set, somehow i have 11.2v on thr 12v line and 24v on the 24v line with the set off/not working

so its getting more interesting. Im having a hard time where to go from here. I dont know how to track down wether the ps is the issue (12v running when it shouldnt) or if the mobo is the issue, 12 is just not high enough,(perhaps its supposed to come up for a second then shutdown when plugged in as a test- voltage too low, tv sits locked up) or what i just dunno man.

[jsog]

Re: coby 40" lcd TFT V4028 no sound

Why not just replace all the low-quality capacitors? If that doesn't fix it, at least it eliminates them as a cause.

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

Ive capped everything but the 24v inverter supply there is no ripple, no bulging, no reason to suspect that and im almost out of 1000 uf caps.

[PlainBill]

Re: coby 40" lcd TFT V4028 no sound

I'm trying to get my mind around this. For some reason the control that turns the main supply on / off is not working. That MUST be a power supply problem. But I can't figure out how that would affect the sound. Or has that been fixed?

PlainBill

[cadillacman]

Re: coby 40" lcd TFT V4028 no sound

The sound is fine. There was a connector unplugged from the power supply marked a24 or something like that.... apparently the audio runs on 24v because thats what it meters out at and thats the connector that was off..

Im having a hard time wrapping my mind around how the power supply "knows" when to turn the 12 and 24v on and off. frickin weired.

i will take and post some better pics of the power supply and connector designations...