I have a NEC Multisync LCD 1525M Monitor with a bad inverter (built by
Potrans model# DA00812600). It had a burnt 22 pF 3kv ceramic capacitor. It
is the cap just before wher the ccfl lamp hooks up. I replaced this cap with
the closest value I could find at the local surplus dealer, a 24pF 1kv cap.
I also replaced the .5a fuse with a regular fast blow and tried it out. It
worked for a short while. I powered it off and back on and it blew the .5a
fuse. I don't know if the original was a slow blow or not, but I can't get
anything out the side that had the burnt cap.

I can't find any shorts on the board and don't really know where to go from
here.. Any poiters before I replace every component on the board?

Here are some pictures.. I know the fuse i put on there looks really bad but
I didn't have a small resistor sized fuse to go on there.
Before
http://mikescomputers.homeip.net/LCD/DSC00357.JPG
http://mikescomputers.homeip.net/LCD/DSC00358.JPG
After
http://mikescomputers.homeip.net/LCD/DSC00365.JPG
Whole board
http://mikescomputers.homeip.net/LCD/DSC00361.JPG

"Michael Kennedy" wrote in message
...
I have a NEC Multisync LCD 1525M Monitor with a bad inverter (built by
Potrans model# DA00812600). It had a burnt 22 pF 3kv ceramic capacitor. It
is the cap just before wher the ccfl lamp hooks up. I replaced this cap
with the closest value I could find at the local surplus dealer, a 24pF 1kv
cap. I also replaced the .5a fuse with a regular fast blow and tried it
out. It worked for a short while. I powered it off and back on and it blew
the .5a fuse. I don't know if the original was a slow blow or not, but I
can't get anything out the side that had the burnt cap.


This is a bit vague to me. Are you saying there are two CCFL tubes and the
transformer has a seperate secondary and coupling cap for each lamp, and
that one lamp worked for a while? If only one lamp lights that should tell
you the inverter is working but there is a problem with either the relevent
secondary winding (check the DC resistance matches the other) the continuity
of the PCB (it looks very badly burnt) or the lamp itself. If the two lamps
share a secondary winding then that narrows it down even further. Connect
the other lamp to the 'working' output and see if it lights up.


The PCB is a mess, and carbonised PCB will have a significant amount of
conductivity to a HV high impedance source. Consider cutting away all the
offending bits of the PCB and if necessary soldering the capacitor straight
to the transformer leg. It won't look pretty bit it's better than the
ramifications of presenting HT across burned PCB! It's possible the heat
generated during the burning has damaged the transformer.

I can't find any shorts on the board and don't really know where to go
from here.. Any poiters before I replace every component on the board?


It's proably not a short on the primary side, more likely HV leakage on the
secondary side. The secondary of the inverter will knock out approaching
IIRC around 90V or so into the right load, and several thousand volts off
load. A relatively high resistance leakage on the secondary could easily
draw enough extra supply current draw to pop the 500mA fuse.


Here are some pictures.. I know the fuse i put on there looks really bad
but I didn't have a small resistor sized fuse to go on there.

Don't worry about that. Get it working then worry about aesthetics.

Dave

First off thanks for the detailed reply!

This is a bit vague to me. Are you saying there are two CCFL tubes and the
transformer has a seperate secondary and coupling cap for each lamp, and
that one lamp worked for a while?

Let me try to clear up the vagueness.. It has 2 transformers one on each end
of the pcb and each has a coupling cap. It also has 2 500ma fuses. The fuse
which goes to the non working side keeps blowing. I had both lamps powered
up and working for a short while, but they were a bit dim.(may be ccfl
tubes) Then I shut it off and powered it back on and the fuse blew on the
"bad" side. I havn't been able to get the "bad" side to light again. It just
blows the fuse imedately. I've ohmed from the fuse to the transformer's
output and it is within 6 ohms of the working side.

If only one lamp lights that should tell you the inverter is working but
there is a problem with either the relevent secondary winding (check the DC
resistance matches the other) the continuity of the PCB (it looks very
badly burnt) or the lamp itself. If the two lamps share a secondary winding
then that narrows it down even further. Connect the other lamp to the
'working' output and see if it lights up.

I've cut away all the burnt PCB and soldered the cap directly to the lamp
socket. I tried that and it still is blowing fuses so I guess thats not it.

The transformer might be the problem. It has a resistance of 668 ohms vs 931
ohms on the good one. I'm not sure which is the primary and secondary but
I'm guessing the primary is the one which ohms close to zero. Check my
picture to make sure I'm ohming them in the correct place.
http://mikescomputers.homeip.net/DSC00367.JPG


- Mike

I've tried ohming from the fuse to just before the bad capacitor and it ohms
1347ohms on the bad side and 1341 ohms on the good side.. So it doesn't
look like I have a short of any kind that I can see. Would this be a good
assumption..

Sorry for posting so much but I really want to learn about fixing
electronics. This inverter seems like a simple enough project. I just need
some help.

I have an old 200khz oscillosocope that needs calibrating but it works.
Would this be usefull at all for working on this? I wouldn't immagine it
being usefull at all without some sort of wave generator.

- Mike

"Michael Kennedy" wrote in message
...
I've tried ohming from the fuse to just before the bad capacitor and it
ohms 1347ohms on the bad side and 1341 ohms on the good side.. So it
doesn't look like I have a short of any kind that I can see. Would this be
a good assumption..


The secondary of the transformer should be isolated from the primary side
and therefore the fuse, so such measurements are irrelevent. Try checking
the switching transistors for shorts, as the fuse is blowing I'd start
there. I think you'll also find the transformer has a shorted winding. Take
both transformers out of circuit and measure the windings, they should have
very similar readings between pins. If not, you likely have a bad one.

Sorry for posting so much but I really want to learn about fixing
electronics. This inverter seems like a simple enough project. I just need
some help.

I have an old 200khz oscillosocope that needs calibrating but it works.
Would this be usefull at all for working on this?

Yes, it will tell you if there is drive to the transformer and therefore
confirm whether or not the primary side of the circuit is working.

I wouldn't immagine it being usefull at all without some sort of wave
generator.


The inverter is an oscillator and generates its own waveform to drive the
transformer, you don't need a 'wave generator'.

Dave

"Michael Kennedy" wrote in message
...
I have a NEC Multisync LCD 1525M Monitor with a bad inverter (built by
Potrans model# DA00812600). It had a burnt 22 pF 3kv ceramic capacitor. It
is the cap just before wher the ccfl lamp hooks up. I replaced this cap
with the closest value I could find at the local surplus dealer, a 24pF 1kv
cap. I also replaced the .5a fuse with a regular fast blow and tried it
out. It worked for a short while. I powered it off and back on and it blew
the .5a fuse. I don't know if the original was a slow blow or not, but I
can't get anything out the side that had the burnt cap.

I can't find any shorts on the board and don't really know where to go
from here.. Any poiters before I replace every component on the board?

Here are some pictures.. I know the fuse i put on there looks really bad
but I didn't have a small resistor sized fuse to go on there.
Before
http://mikescomputers.homeip.net/LCD/DSC00357.JPG
http://mikescomputers.homeip.net/LCD/DSC00358.JPG
After
http://mikescomputers.homeip.net/LCD/DSC00365.JPG
Whole board
http://mikescomputers.homeip.net/LCD/DSC00361.JPG


The original was probably a slow blow at probably 1 or 1.25 amp (but I'm
guessing). The ccfl's are probably 4 watt bulbs, 2 of them, total draw 8
watts, inverter efficiency is around 75% which means the whole thing is
drawing 10 watts when running correctly. With a 12 volt supply and you get
just under an amp draw.

If you have a scope, see what kind of a trace you get on either side of C9.
If the transformer is open on the secondary, you'll get a clean 1/2 wave on
either side of C9. If it's working, you'll get the 1/2 wave but it will be
misshapen and may have some harmonics on it. The voltage on C9 side is low
since you are on the primary. The inverter will usually work for about 2 -
4 seconds with the bulbs unplugged. The square chip in the center is the
controller and will shut the inverter down if it doesn't sense the bulbs
light up. Here's a file that should give you a close approximation of your
inverter: go to http://www.anachip.com/eng/supports/tech/appnote.php and
take a look at ANP005

The other thing you can do is to check continuity on the transformer. The
primary windings and the feedback winding are very low but the secondary
winding should be 100 to 400 ohms if I recall correctly.

That charred board may end up being your biggest liability since the voltage
on that side of things normally runs around 600vac @ 60Khz or so. It might
just be easier to find a cheap monitor with a cracked LCD on eBay and take
the boards out of it.

Dave

I beleive I have found a shorted zener diode at ZD1 it ohms 119 both ways.
ZD2 on the opposite side of the board ohms 343 foward and over 700 reverse.
I guess that is why I'm blowing fuses. Now to get a chart on how to read
zener diodes.

- Mike

"Dave" wrote in message ...
"Michael Kennedy" wrote in message
...
I have a NEC Multisync LCD 1525M Monitor with a bad inverter (built by
Potrans model# DA00812600). It had a burnt 22 pF 3kv ceramic capacitor. It
is the cap just before wher the ccfl lamp hooks up. I replaced this cap
with the closest value I could find at the local surplus dealer, a 24pF
1kv cap. I also replaced the .5a fuse with a regular fast blow and tried
it out. It worked for a short while. I powered it off and back on and it
blew the .5a fuse. I don't know if the original was a slow blow or not,
but I can't get anything out the side that had the burnt cap.

I can't find any shorts on the board and don't really know where to go
from here.. Any poiters before I replace every component on the board?

Here are some pictures.. I know the fuse i put on there looks really bad
but I didn't have a small resistor sized fuse to go on there.
Before
http://mikescomputers.homeip.net/LCD/DSC00357.JPG
http://mikescomputers.homeip.net/LCD/DSC00358.JPG
After
http://mikescomputers.homeip.net/LCD/DSC00365.JPG
Whole board
http://mikescomputers.homeip.net/LCD/DSC00361.JPG


The original was probably a slow blow at probably 1 or 1.25 amp (but I'm
guessing). The ccfl's are probably 4 watt bulbs, 2 of them, total draw 8
watts, inverter efficiency is around 75% which means the whole thing is
drawing 10 watts when running correctly. With a 12 volt supply and you get
just under an amp draw.

If you have a scope, see what kind of a trace you get on either side of
C9. If the transformer is open on the secondary, you'll get a clean 1/2
wave on either side of C9. If it's working, you'll get the 1/2 wave but
it will be misshapen and may have some harmonics on it. The voltage on C9
side is low since you are on the primary. The inverter will usually work
for about 2 - 4 seconds with the bulbs unplugged. The square chip in the
center is the controller and will shut the inverter down if it doesn't
sense the bulbs light up. Here's a file that should give you a close
approximation of your inverter: go to
http://www.anachip.com/eng/supports/tech/appnote.php and take a look at
ANP005

The other thing you can do is to check continuity on the transformer. The
primary windings and the feedback winding are very low but the secondary
winding should be 100 to 400 ohms if I recall correctly.

That charred board may end up being your biggest liability since the
voltage on that side of things normally runs around 600vac @ 60Khz or so.
It might just be easier to find a cheap monitor with a cracked LCD on eBay
and take the boards out of it.

Dave

Okay... The color charts I found versus the colors on this diode don't make
sense.. It is orange, red, black and green.. so that makes it a 1N320E ?? I
can't find any diodes with that part number.

Maybe I'm just color blind.. Can someone tell me if I got the colors right?
http://mikescomputer.homeip.net/LCD/DSC00382.JPG

Color Codes I found
http://www.oldradios.co.nz/data/index5.htm

- Mike

I took the transformers off and they both ohm very similarly... I think they
are both good, but it is late and I'll give it a more careful check
tomorrow.

- Mike

"Michael Kennedy" wrote in message
...
Okay... The color charts I found versus the colors on this diode don't
make sense.. It is orange, red, black and green.. so that makes it a
1N320E ?? I can't find any diodes with that part number.

Maybe I'm just color blind.. Can someone tell me if I got the colors
right?
http://mikescomputer.homeip.net/LCD/DSC00382.JPG

Color Codes I found
http://www.oldradios.co.nz/data/index5.htm

- Mike

Okay.. I don't know what is going on.. I replaced the shorted zener and the
light lights for a split second and then blows the fuse.. Must be a short
somewhere..

Could it be that I used a 24pF 1kv capacitor on the output instead of a 22pF
3kv?

- Mike

"Michael Kennedy" wrote in message
...
I took the transformers off and they both ohm very similarly... I think
they are both good, but it is late and I'll give it a more careful check
tomorrow.

- Mike

"Michael Kennedy" wrote in message
...
Okay... The color charts I found versus the colors on this diode don't
make sense.. It is orange, red, black and green.. so that makes it a
1N320E ?? I can't find any diodes with that part number.

Maybe I'm just color blind.. Can someone tell me if I got the colors
right?
http://mikescomputer.homeip.net/LCD/DSC00382.JPG

Color Codes I found
http://www.oldradios.co.nz/data/index5.htm

- Mike