LOGIK LCX27WN2, AC ADAPTER ADP-150BB B fault

[timtheenchanter]

Im trying to repair the above tv/PSU

The psu is at fault, symptoms, TV runs fine for 30-45 mins, after which shuts PSU shuts down.

having opened the psu up, there is nothing immediately obvious as wrong, there is a very slight bulge on the top of the main cap , but have seen much worse on power amps (my usual partakings with electronic repair)
would this bulge be the culprit?
I could buy a new psu, but am lothe to splash out if its a couple of quids worth of caps.

anything else to look to on this psu??

 

[ChrisFrost]

If it's failing after running a while then have you thought about heat? Caps normally work better warm than cold, so it would rule out a cap problem. But transistors usually work better cold than warm.

Try a can of freeze spray and work through the silicon components methodically.

 

[timtheenchanter]

cheers, will have a look, must say, its not the easiest board to work on, the entire underside has earthed copper sheets over it (RF sheilding??) going to pull those off this eve, and hope they are not stuck down to see if there are any dry joints under there.

If that fails, Ill break out the freeze spray.

 

[briancbb]

Realise there is a challange in not getting beaten by a power supply. To me, this was well in the old days of transformers, rectifiers and capacitors, simple stuff - but switched mode leaves me standing.

How about one of these for a tenner post paid, just bought on for running led strip lighting, delivered 36h after ordering.
http://www.amazon.co.uk/12V-Monitor...SYGQ/ref=sr_1_1?ie=UTF8&qid=1305705712&sr=8-1

Centre positive connector, and short mains lead supplied.

 

[timtheenchanter]

I'll be having a look online again later, but this TV seems to have a bit of a thirst, its a 12V 12.5A psu. most newer LCDs seem to run 24V PSUs (so less current req)

bizzarly, the psu is a cased unit, but then goes into a compartment on the back of the TV.

 

[briancbb]

Ooops.
Sorry about that, misread the 12.5A in the details about the PS. Somehow I read 5A and missed the 12. in front.

 

[ericmark]

Likely the PSU is of the switched mode type. What it does is convert the AC input to DC and charge a capacitor. It then turns it back to AC at a high frequency, this means the transformer required is very much smaller then with old type PSU. It then transforms it back to DC and samples the voltage. This is then fed back to the high voltage side to alter the mark/space ratio and so correct the voltage. Should the voltage get out of speck it will also auto shut down. It also likely has a temperature sensor which will also auto shut down the unit. The loop system makes them very hard to fault find and the normal method is to renew parts and see if it works rather than true fault finding. Likely the thermistor is faulty and is closing it down premature. However it also could be the TV drawing too much power.

I had same problem with a lap top and I got a replacement from Maplin but at £70 for what you require I can see why you want to repair the old one. The metal casing would be required for a switch mode unit because of the frequency it works on. You will note the one I have linked to works from 100 volt to 240 volt this is typical of a switch mode unit. The old types were much more limited with input voltage requirements. Maybe you could use a PC power supply but I am unsure of links required to get them to power up.