snip


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