repairing LCD

Thread Starter

Bear_2759

Joined May 23, 2008
120
Hi Guys,

so a while back (2yrs) we had a power surge at work, 42inch LCD killed. I thought I'd take it home and try and fix it rather than chuck it in the bin. over the last 2 years I've gradually tried to fix it and I'm sooooo close now. after opening the back of the unit there was a whole lot of popped caps on the power supply. first idea was to replace the popped caps. this didn't make a difference. unit still had no power. months later I searched on good ol fleebay for the power supply unit (by model number). found stacks of them so just bought one. now when I put it in, I noticed that even though the 2 power supplies had the same model# they were different !??!!?!?!?!. the new one I bought were missing 3 pins for the output plug. 2x +5v and a +12v. but with this one the unit at least had power but wouldn't turn the screen on.

so today, I checked the front/back of both boards. on the old one, the extra pins are directly connected to the others (of the same value). no other components inbetween. so I thought I'd try connecting a few wires between them on the back side of the plug. hey presto it worked!!! tv powers on, display on screen only problem now is there's a constant buzzzzzzzzzzz from the built in speakers. now I dont know if this is caused by my "fix" or if it's because of the power surge back when the TV was first killed. I had thought I'd just disconnect them and run speakers through the aux output. but what do you know there's no aux out. so if I can't get rid of the buzzzz it means I cant use the built in HD tuner.

so far though I've got a working full HD 42" LCD that I can use any of the other inputs on, just not the built in HD tuner :D

can anyone suggest ways that I can test/isolate the issue that's causing the buzz through the speakers? the buzz doesnt increase/decrease with the volume control if that helps...
 

Thread Starter

Bear_2759

Joined May 23, 2008
120
ok so here's some pics.


the back of the unit. (everything)


the new PSU


the board with all the outputs. dont know what it should be called


assuming HD tuner. but that could also be on the other board


the old PSU.
 

R!f@@

Joined Apr 2, 2009
9,734
U missed a cap in the supply.

Try renewing all the electrolytic in the secondary stage.

There will be a 33V regulator. Basically a Zener regulated used as the VT of the tuner. Try to find that area and replace any caps there. Mostly would be between 10u to 100uf ones
 

R!f@@

Joined Apr 2, 2009
9,734
Depends on from circuit to circuit. To be exact I need to trace the Tuner Supply pins. The Vcc, or Vdd. and VT.

Depending on the tuner it might use a 12V line and 5V line. But the VT will always be there. As far as I have seen it is there.
 

R!f@@

Joined Apr 2, 2009
9,734
Tuning Voltage. labelled VT..which is 33V max

Any ripple in the tuner supply or VT will be injected into the video...

My first assumption regarding ur problem.
 

Thread Starter

Bear_2759

Joined May 23, 2008
120
but this is the new replacement PSU. not the one that was fried so there shouldnt (touch wood) be an issue with power, and couldn't I just disconnect the power from the tuner card to isolate it and see if that is the issue?
 
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