Subbing an LCD TV Power Supply

Before you get excited... :> think about *why* the supply died. Sure, it's possible that it just "gave up the ghost". But, it is also possible that some aspect of the *load* has changed in A Bad Way. Unless you fix the root cause, you stand a good chance of toasting your *new* supply... *or* something else in the set when it finds "enough" power available for it to cook itself without taking out the "semiconductor fuses" (otherwise known as transistors :> ) in the power supply.

Can someone link to a SMPS repair parts kit? I've not seen these.

Thanks.

For instance

UK e-bay, but I'm sure a bit of googling or e-bay research would turn up others in the U.S. or wherever

Arfa

Although

Interestingly, whole power supplies are available here, sometimes at ridiculously low prices. For instance, many sets use a Vestel chassis, and some of these PSUs are being sold brand new for as little as 25UKP, although you can also find the same PSU at the same vendor for 3 times that, but listed as being for a different make / model, so it's often advantageous to know what power supply is used in multiple different manufacturer's sets. Anyway, the point I was going to make is that a friend of mine whose shop specialises in repair of LCD TV sets, often buys in these power supplies at this price, and fits them into sets that they were never intended to go in, and apparently without any problem. As long as he can find the turn on signal from the system control micro, and there is physically room to fit a Vestel supply in the set in question, he just goes ahead and does it, if necessary, soldering the wires directly to the output connectors.

To take your point about cross-coupled regulation, I have not found this to be a problem. I repair a lot of Vestel supplies for the trade, and they are happy to regulate with any load from nothing to full, on any rail, all rails, or no rails at all. As for the rails coming up in any specific order, I think all of the ones that I've seen have all of the switched rails come up together. Certainly, my friend doesn't seem to have ever had an issue with the actual working of an internal supply substitute.

Arfa

I think we were failing to communicate properly here. I interpreted the plan as to buy three separate power supplies (5V, 12V, 24V) and use them to replace an integrated 5V, 12V, 24V supply. I do not think that will work well.

You appear to be discussing using a 5V, 12V, 24V supply designed for an XYZ brand of TV in a QED TV. I would have to agree, if dimensional and current ratings are compatible, that should work.

From a personal perspective, I 'grew up' with analog power supplies (often using vacuum tube rectifiers). These were inefficient, but easy to repair. The switch to SMPS supplies required more sophisticated techniques, but in the end was not that much more difficult, even for the multiple output supplies found in PCs.

At this point I am finding the supplies in LCD TVs to be extremely difficult to troubleshoot safely and effectively.

PlainBill

Agreed. The key to it seems to be obtaining schematics. I offer a trade repair service on a particular LCD power supply board, but I am only able to do that as a result of hours of painstaking research to find schematics to cover all the variants. I then spent a long time analysing just what made this supply tick (figuratively, not literally !! ) and then sat down and designed a test jig and a set of adaptor harnesses to allow the different variants to all be repaired using that jig. I can pretty much crack most problems on them in fairly short order now, but still get the occasional one that gets the better of me. Just yesterday, actually ... I have one at the moment that has a really odd fault. The PFC supply won't start up at full supply volts. Turn it down on a variac just 10 volts, and it will start every time. If you then turn back up, it keeps running, but the PFC supply starts to squegg, and the 390v rail drops to around 340v. At the same time, the 24v secondary side rail drops to around 23v, but curiously, all the other rails stay spot on. That might be the clue to what's going wrong, as the 24v rail is normally *very* tightly regulated. I would add that the problem occurs whether there is a load on any of the rails or not. It'll be an interesting one to eventually find when I have the time, but in the meantime to get my customer out of trouble with his, I've had to do him a swapout.

Arfa

I should have tested more thoroughly before posting. Turns out the power supply is good - all three output voltages are normal. The problem must be in the mainboard - the power light comes on but nothing happens. So it looks like a dead end for this set. What I might try to do is fit the power supply from this set into another one, rather than building an external supply....

Can I just make a little observation, Chris ? The common convention on newsgroups, is to post replies to the bottom of the existing text. This generally makes it easier to follow the chronology of the thread, and makes it easier to see exactly what points you are replying to. Some newsgroups are really pedantic about this, and you will get royally castigated by their regulars, if you top post ...

Replying to individual points within the body of the existing text is also generally accepted as being OK.

Arfa