learnig

[Alberto Melo]

After I learned a bit about VCR's and CD-ROM's using the different FAQ's and
help from diferent people on this group, I am moving into the High Voltage
department.
Somebody got rid of a dead 15 inch PC colour monitor. After searching on the
net I found the caracteristics: max res 1280, with digital controls for
almoast everything and high refresh rate, so it WAS a decent set.

After reading the FAQ's I spent some time trying to identify some key
components. The green disk/cylinder close to the power Sup. might be a
fusable resistor, its blown apart and there are signs of lots of heat, or it
might just be a simple resistor, not sure. Some cheks on the HOT with a VOM
and its not obvious that it is bad, also the rectifying diods and main cap
dont seem to be bad. So I moved to the scary part, the flyback or LOPT.
After removing some pounds of solder I completely removed the thing from the
mainboard along with all the wires including the suction cup. I have done
this to confirm resistance readings I have done while the thing was in
place.
With the beast in front of me showing its bottom full of legs and focus and
screen knob pointed to my right side I started with the VOM on a 10* Ohm
scale. It reads 0 Ohms between all of the first 6 legs counting from the
left. The VOM's needle doesn't move at al if i measure between any of those
first 6 legs and any of the remainig 4 legs of the rigth side. Same result
between the suction cup springy wires and any of the 10 legs. The first leg
from the left was conected via the PCB to the Colector of the HOT. So let me
see if I learned something:
the first 6 leftmost legs belong to the primary
winding of the Flyback transformer. The remaining 4 legs of the beast belong
to the secondary winding. And of course the primary winding is completely
shorted out beyond redemption.
The only thing I need before I throw away the beast is somebody to
confirm this death certificate.
Any corrections to my assumptions would be welcomed.

A. Melo
Brussels

[Sam Goldwasser]

The primary resistance readings of a typical flyback will all be close to
0 - probably not detectable with an analog VOM if it doesn't have a X1 scale.
Your flyback may indeed be bad but this is not confirmation. See the flyback
testing FAQ as well!

--- sam : Note: Temporary problem with www.repairfaq.org, go to:

Repair | ITALY http://ftp.unina.it/pub/electronics/serfit1.html
+Lasers | USA http://www.pacwest.net/byron13/sam/faqsam.htm
| UK http://www.jasper.demon.co.uk/repairfaq/
| GERMANY http://repairfaq.physik.tu-berlin.de/repairfaq/

[Tony Duell]

Alberto Melo (albert...@tvd.be) wrote:
: With the beast in front of me showing its bottom full of legs and focus and

: screen knob pointed to my right side I started with the VOM on a 10* Ohm
: scale. It reads 0 Ohms between all of the first 6 legs counting from the
: left. The VOM's needle doesn't move at al if i measure between any of those
: first 6 legs and any of the remainig 4 legs of the rigth side. Same result
: between the suction cup springy wires and any of the 10 legs. The first leg
: from the left was conected via the PCB to the Colector of the HOT. So let me
: see if I learned something:
: the first 6 leftmost legs belong to the primary
: winding of the Flyback transformer. The remaining 4 legs of the beast belong
: to the secondary winding. And of course the primary winding is completely
: shorted out beyond redemption.
: The only thing I need before I throw away the beast is somebody to
: confirm this death certificate.

The resistance of the windings of most flyback transformers is less than
1 ohm. Unless you have a special meter, you're not going to find a
shorted winding by a DC resistance test, and you're not going to find a
shorted turn (by far the most common flyback problem) at all. It will
look 'shorted' on a normal meter.

I believe there's a comment on the faq site about doing a 'ringing' test
on a flyback transformer. In my experience this is just about the only
reliably way to test them.

Another hint. If the HOT is still good - especially if it's not shorted
all ways round, then the flyback is probably OK as well. Flyback problems
tend to kill the HOT instantly. But a defective HOT can be caused by a lot
of other things than a shorted flyback, so don't assume the worst if the
HOT is bad.

: Any corrections to my assumptions would be welcomed.

: A. Melo
: Brussels

-tony