Timeline
In my last post I described that my broken monitor motivated me revive
my electronics hobby. I planned to do an in depth analysis on the monitor.
But as luck would have it my replacement monitor started getting a little
twitchy as well. So I did a quick repair attempt and it worked out well.
Most of the fault analysis I have already done a while ago. But being very
happy with my working monitor I decided to write it all down.
The Breakdown
Basically my monitor, a Fujitsu-Siemens LSL 3260W, just broke down during
normal operation. There was a small amount of smoke and a distinct electronic
smell. I tried to turn it on again but it would only turn on a bit dimmed for
a few seconds and then the backlight would turn off. The image was however
still visible when you looked on it at an angle.
Fault Analysis
Since the image was still visible at an angle and just the backlight was off I
had hopes that it was nothing too major. A first look inside the monitor
revealed this little culprit right at the connection to the backlight:
You can clearly see where the magic smoke escaped.
After a bit of searching and comparing it to the capacitor on the
mirrored output section for the other two CCFL tubes I found that it is a "12J
3KV JNC" ceramic capacitor. Which is as 12pF capacitor with 3kV voltage
rating.
Searching for more Information
My knowledge in this area is pretty limited so I did a lot of research online.
However what you can find on the whole power and backlight board, an "ILPI-082", is close to
nothing. Some people have run into similar problems which has been a bit
helpful to get an overview. I thought it might also be helpful to look for
more information on the IC in the CCFL driver section. Maybe there would be
some information in the datasheet about CCFL driver circuits to give some
pointers. I did however find absolutely nothing on the "SS7160". Narrowing the
Search down to CCFL and DIP-8 did also not help.
Testing the Backlight
Without digging too much deeper I first though maybe just one backlight CCFL tube
might be bad. I got myself a little ready to go CCFL inverter. Here some
pictures.
This inverter produces high voltage. So be careful if you use one.
The socket did not fit exactly but I was able to test all 4 backlight tubes of
the monitor. Oddly enough they just lit up 42cm even though they are longer.
The inverter probably has the wrong specifications. Maybe I look into the
reason for this odd behaviour in a project later on. But the backlight tubes
all showed the same behaviour from which I reasoned that they are probably OK.
Understanding the Basics of CCFL
Since my curiosity was now turned on I first read up on CCFL on Wikipedia.
My basic understanding is now that the CCFL tube needs a very high voltage to
fire up and then still several hundred volts during normal operation. On the ILPI-082 board
you can clearly see mirrored sections for the four backlight outputs. There
are two transformers, large gaps around the outputs and the little caps
next to the outputs.
The photo already contains the replacement cap. I photographed it after the
repair.
I would have loved to test and record the behaviour described above for all four outputs on a
scope but I do not have the appropriate equipment yet.
You can also get a short overview about CCFL backlight circuitry and the
purpose of the small capacitor in this
video
from mikeselectricstuff
(electricstuff.co.uk). An invaluable
resource as always.
Other Oddities
On the back of the ILPI-082 board I saw a bit of gunk around a resistor. Have
a look:
This resistor surely has seen better times but I do not have proper SMD
soldering tools yet so I quickly measured it with my trusty Brymen multimeter.
It seemed OK so I left it just the way it was.
The Repair
At this point the repair is pretty straightforward. I compared the two mirrored
output sections for the CCFL backlight and tried to work out the value of the
defective capacitor. It was a bit burned and as such difficult to read. At the
time I figured it was 3pF. Later on it turned out to in fact be "12J" which is
12pF. Nevertheless it will also work with with 3pF. Just watch out for the
voltage rating of 3kV.
The capacitor I bought as a replacement was this one from a Taiwanese supplier
on eBay (eBay offer).
After a bit of soldering it is in place.
Not very elegant, but it works. Hooray!
Concluding remarks
After all this was a quite simple repair and my monitor stood around broken
for way too long. But I did nevertheless learn quite a bit. When I come across
a CCFL tube from a broken monitor or so I think I will mess around with it a
bit. Maybe I can figure out why it is that CCFL tubes can partially light up
and even capture the firing voltage burst on the scope. Stay tuned for more.