Testing Flyback Transformer - How To Test And When To Replace it

The question is what kind of problems can be found in a flyback transformer and how to test and when to replace it. Here is an explanation that will help you to identify many flyback transformer problems. There are nine common problems can be found in a flyback transformer.

a) A shorted turned in the primary winding.
b) An open or shorted internal capacitor in secondary section.
c) Flyback Transformer becomes bulged or cracked.
d) External arcing to ground.
e) Internal arcing between winding.
f) Shorted internal high voltage diode in secondary winding.
g) Breakdown in focus / screen voltage divider causing blur display.
h) Flyback Transformer breakdown at full operating voltage (breakdown when under load).
i) Short circuit between primary and secondary winding.

Testing flyback transformer will be base on (a) and (b) since problem (c) is visible while problem (d) and (e) can be detected by hearing the arcing sound generated by the flyback transformer. Problem (f) can be checked with multimeter set to the highest range measured from anode to ABL pin while (g) can be solved by adding a new monitor blur buster (For 14' & 15' monitor only.) Problem (h) can only be tested by substituting a known good similar Flyback Transformer. Different monitor have different type of flyback transformer design. Problem (i) can be checked using an ohm meter measuring between primary and secondary winding. A shorted turned or open in secondary winding is very uncommon.

What type of symptoms will appear if there is a shorted turned in primary winding?

a) No display (No high voltage).
b) Power blink.
c) B+ voltage drop.
d) Horizontal output transistor will get very hot and later become shorted.
e) Along B+ line components will spoilt. Example:- secondary diode UF5404 and B+ FET IRF630, K2141,        FS10KM & so on.
f) Sometimes it will cause the power section to blow.

What type of symptoms will appear if a capacitor is open or shorted in a flyback transformer?

Capacitor shorted

a. No display (No high voltage).
b. B+ voltage drop.
c. Secondary diode (UF5404) will burned or shorted.
d. Horizontal output transistor will get shorted.
e. Power blink.
f. Sometimes power section will blow, for example:
   Raffles 15 inch monitor.
g. Power section shut down for example: Compaq V55,
   Samtron 4bi monitor.
h. Sometimes the automatic brightness limiter (ABL) circuitry components will get burned. This circuit is
   usually located beside the flyback transformer. For example: LG520si

Capacitor open

a. High voltage shut down.
b. Monitor will have "tic - tic" sound. Sometimes the capacitor may measure O.K. but break down when under full operating voltage.
c. Horizontal output transistor will blow in a few hours or days after you have replaced it.
d. Sometimes it will cause intermittent "no display".
e. Distorted display i.e., the display will go in and out.
f. It will cause horizontal output transistor to become shorted and blow the power section.

How to check if a primary winding is good or bad in a Flyback Transformer?

a) By using a flyback/LOPT tester, this instrument identifies faults in primary winding by doing a "ring" test.
b) It can test the winding even with only one shorted turned.
c) This meter is handy and easy to use.
d) Just simply connect the probe to primary winding.
e) The readout is a clear "bar graph" display which show you if the flyback transformer primary winding is good or shorted.
f) The LOPT Tester also can be used to check the CRT YOKE coil, B+ coil and switch mode power transformer winding.

NOTE: Measuring the resistance winding of a flyback transformer, yoke coil, B+ coil and SMPS winding using a multimeter can MISLEAD a technician into believing that a shorted winding is good. This can waste his 
precious time and time is money.

How to diagnose if the internal capacitor is open or shorted?

By using a normal analog multimeter and a digital capacitance meter. A good capacitor have the range from 1.5 nanofarad to 3 nanofarad.*

1) First set your multimeter to X10K range.
2) Place your probe to anode and cold ground.
3) You must remove the anode cap in order to get a precise reading.
4) Cold ground means the monitor chassis ground.
5) If the needle of the multimeter shows a low ohms reading, this mean the internal capacitor is shorted.
6) If the needle does not move at all, this doesn't mean that the capacitor is O.K.
7) You have to confirm this by using a digital capacitance meter which you can easily get one from local
   distributor.
8) If the reading from the digital capacitance meter shows 2.7nf, this mean the capacitor is within range
   (O.K.).
9) And if the reading showed 0.3nf, this mean the capacitor is open.
10) You have three options if the capacitor is open or shorted.

       - Install a new flyback transformer or
       - Send the flyback transformer for refurbishing or
       - Send the monitor back to customers after spending many hours and much effort on it.

* However certain monitors may have the value of 4.5nf, 6nf and 7.2nf.

Note: Sometimes the internal capacitor pin is connected to circuits (feedback) instead of ground. TV rca flyback transformer circuits usually do not have a internal capacitor in it. If you have a flyback diagram and circuits which you can get it from the net, that would be an advantage to easily understand how to check them.

Author By : Jestine Yong, he is a electronic repairer and a writer. For more information on electronic repair please visit his website at : http://www.noahtec.com/electronic-repair-articles.htm