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Topic: AOC N27XA22-2 PSU fault  (Read 3708 times)
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ProDave
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« on: November 18, 2009,07:49:05 »

I've got an AOC Envision N27XA22-2 27" LCD tv with a faulty PSU.

It's a set I got faulty with a job lot, so I don't want to spend a lot on fixing it as I don't know what state the rest of the tv / panel etc is in.

The obvious fault is the PSU has blown.  The soldered in on board mains fuse has blown and the incoming bridge rectifier is short circuit. Having replaced both. it's still dead.

Further investigation revealed a blown track. That feeds a TOP426W switching regulator.

So my interpretation of what happened is the TOP246Y went SC, taking the bridge and fuse with it and the PCB track.  The TOP246Y is now O/C and apparently dead.

Before I spend money replacing the TOP246Y I'm keen to find a data sheet for this set, or at least a circuit diagram for the PSU, but I can find precious little about this particular model anywhere on the net.  I did however find someone on ebay selling an identical looking PSU but listed as being for an AOC L27W551T television, but again can't find a service manual for that one either.

Can anyone help with what appears to be a pretty obscure unpopular and probably old tv.

Further information. The part number of the PSU board is 715T1180 and googling that reveals that it is also used in another AOC set model number N32W551T and also a Norcent LT2722

EDIT:

Funny how collating all the information to post it helped solve my own problem.  I googled "Norcent LT2722 Service Manual" and found a free service manual download.  So although that's not an identical set (it's a newer version with more AV inputs) I now have the full service info for the power supply including schematics and a fault finding flowchart.
« Last Edit: November 18, 2009,08:53:50 by ProDave » Logged
sparky
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« Reply #1 on: November 18, 2009,20:46:51 »

hi
it might be a good idea to post pic of psu
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ProDave
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« Reply #2 on: November 18, 2009,23:04:13 »

I haven't got time to take a picture of my own PSU at the moment, but here's an identical one for sale on ebay at the moment. http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350278507377&ssPageName=STRK:MEWAX:IT

Now all I need is some time to sit down at the workbench and fault find the PSU.

What I have determined from looking at the service manual is my suspect device is the "standby power" regulator.
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ProDave
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« Reply #3 on: November 25, 2009,23:32:00 »

Just an update if anyone's interested.

Having checked through the PSU (now I have the circuit), there was nothing wrong with any of the power components.

A simple S/C 1n4148 diode was preventing the main PSU starting up.  That got the set powering up, but no backlights.  That's a seperate section of the PSU.  The cause of that not working was the PSU Ic had failed.  

It took a while as the only place I could find the device was Farnell, and I wasn't going to pay 5 postage for a 1.38 part, so I had to wait until I had other things I needed to make up a sensible order.

Now it's all working fine.

I guess what happened is the PSU controller went faulty turning the power mosfet on permanently, and for once the fuse blew and not the transistor.


Now all I need is to find the Sky remote code for this set. Anyone know it?  I've tried all the AOC codes listed by sky and they don't work.  These sets also go under the brands of Envision and Norcent, but I've not seen any codes listed for those brands at all.
« Last Edit: November 26, 2009,09:48:47 by ProDave » Logged
Turnip
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« Reply #4 on: November 26, 2009,02:25:13 »

Hi ProDave,

  Thought to attach Shematic from site you suggested. What circuit ref was the IN1418 - could be useful

  Chris.
« Last Edit: November 26, 2009,05:42:39 by Turnip » Logged
ProDave
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« Reply #5 on: November 26, 2009,09:15:28 »

See page 74 of the service manual, Power board.

The initial fault (after fixing the blown input bridge and fuse) was that IC981 was not working. Initially I thought the device itself was at fault. But looking up the data sheet for that device and working out how it should work, I soon found the voltage at pin 3 was insufficient to start the device.

The s/c 1n4148 is not shown on the schematic.  There is another connection from pin 3 not shown that connects to this diode.  So the board I have must be a different revision (probably earlier) than that in this service manual. Unfortunately it's all back together now so I can't give you the circuit board reference.

That got the 12V power working and the set turned on, but no backlights. IC901 was not operating.  Again looking up the data sheet for that device to work out how it should work revealed that VCC needs to be a minimum of 8V for the device to start, but it was measuring only 4V.

It didn't take long to find out it was the device itself pulling VCC down, so replacing IC901 got the backlights working.  So the set is now working once more.


If nobody can help me find a sky remote code for this set, does anyone know where I can buy a remote control that will work with this set?
« Last Edit: November 26, 2009,09:47:56 by ProDave » Logged
Turnip
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« Reply #6 on: November 27, 2009,01:47:53 »

Hi ProDave, and thanks for all that.

Guess you have tried Wiltsgrove for the RCU, they are quite helpful esp on the phone - Chris.
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ProDave
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« Reply #7 on: November 28, 2009,22:03:47 »

I've found the Sky remote setup code for this tv, it's 1130 if anyone is interested.

Now that I have the sky remote working the tv, I won't bother trying to buy an original one.
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