On Tue, 4 May 2010 20:15:46 -0700 (PDT), saber850
wrote:
http://www.ccl-la.com/blog/index.php...ng-214t-repair
I opened the case and analyzed all the caps--none are bulging or
leaking/oozing.
One small step... etc.
Look at the photos in:
http://www.ccl-la.com/blog/index.php/samsung-214t-repair
Do you see any bulging or leaks? Nope. Not all caps fail with
obvious physical evidence. That's where the ESR meter is used. You
can test the capacitors without removing them from the circuit.
Lacking that, just get some replacement caps, replace everything
listed in the article, and it should be back to working.
As for a better understanding of what's happening, it's difficult
enough to analyzer a circuit when all the parts are within normal
design limits. However, if you want to see what happens when you
insert a defective capacitor into the circuit, download a free copy of
LTSpice (formerly known as SwitcherCAD),
http://www.linear.com/designtools/software/
and buy a collection of SPICE switcher models:
http://pagesperso-orange.fr/cbasso/Spice.htm
http://pagesperso-orange.fr/cbasso/Downloads/LTSpice/PWM%20Switch%20LTSpice.zip
The author suggests you need the book to understand the models. I
agree.
Get the program working with a proper switching power supply. It
doesn't have to be exactly the same as the Samsung monitor. Then cram
in the equivalent of a bad capacitor, which would be a normal
capacitor, with the same capacitance value as on the label, but with
an added series resistance equal to about 2-5 times maximum ESR.
http://members.ozemail.com.au/~bobpar/2003esrchart.txt
http://members.ozemail.com.au/~bobpar/graph1.gif
http://members.ozemail.com.au/~bobpar/graph2.gif
http://members.ozemail.com.au/~bobpar/graph3.gif
A subtle hint is that with a switching power supply, the output
voltage almost never goes up when a component fails. It either goes
down, or massive amounts of high frequency ripple appears on the
output. How the rest of the LCD monitor responds to this is a bit of
a guess. Usually, it just shuts down, resulting in a black display.
Also, I forgot to caution you to *NOT* constantly power cycle the
monitor in an attempt to revive the circuit. It's bad enough having
the capacitors blow up. There's a chance that the associated
switching transistors will not appreciate the added load (which is
what's heating the capacitors) and blow up. For example, this loser
of a Dell monitor:
http://www.ccl-la.com/blog/index.php/dell-e173fpb-17lcd-repair/
usually takes out 6 xsistors, a diode, and anywhere between one and
eight electrolytics. I once tried to fix one, and didn't catch one of
the caps. Within a week, all 6 transistors were again blown.
Also, if you read the above instructions carefully, note that it
mentions resoldering the xformer leads. I do this whether it needs it
or not. What happens is that the xformers operate at about 40KHz(?).
Some of this 40KHz energy is transfered to the xformer leads,
resulting in cracked solder. It's very difficult to see without a
microscope. It was a problem with Tin-Lead solder but is really a
PITA with brittle ROHS Tin-Silver solder. The same thing can happen
at audio frequencies:
http://www.clarkeaudio.com/nfinfo.htm
Anyway, your next ordeal will be selecting the proper replacement
capacitors from the Digikey or Mouser online order pages. It's very
easy to pick the wrong part. Be careful, or ask for help. Also, buy
some spares. My prediction is that your matching monitor will soon
have the same problem.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558