Monitor & TV Repairer - The Secrets on How To Testing Flyback Transformer (FBT) or LOPT 


Testing Flyback Transformer (FBT) or LOPT

1. Nowadays, more and more monitors comes in with flyback transformer problems. In

    many cases, the flyback transformer can become short circuited within one and a

    half years after being serviced.The question is what kind of problems can be found 

    in a flyback transformer and how to test it. Here is an explanation that will help you to

    identify many flyback transformer problems.


2. There are nine common problems to be found in a flyback transformer.


    a) A shorted turned (C) in the primary winding.

    b) An open or shorted capacitor (C1) in secondary section.

    c) Flyback Transformer becomes buldged or cracked.

    d) External arcing to ground.

    e) Internal arcing between windings.

    f) Shorted internal high voltage diode (D1) in secondary winding.

    g) Breakdown in focus / screen voltage divider causing blur display.

    h) Flyback Transformer breakdown at full operating voltage (breakdown when under


    i) Short circuit between primary and secondary winding.


3. Explanation will be base on (a) and (b) since problem (c) can be seen with the

    naked eyes 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. Problem (i) can

    be checked using an ohm meter measuring between primary and secondary



4. A shorted turned or open in secondary winding is very uncommon.


5. What type of symptom 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.

   f) Sometimes it will cause the power section to blow.


6. What type of symptom will appear if a capacitor is open or shorted in a flyback




HV Capacitor Shorted

HV Capacitor Open

a. High voltage shut down.

a. High voltage shut down.

b. B+ voltage drop.                                      

b. Monitor will have "tic - tic" sound.   

Sometimes the capacitor may meassure o.k  

but break down when  under full operating



c. Secondary diode (UF5404) will burned  

    or shorted.          

c.  Horizontal output transistor will blow in a 

     few hours or days after you have replaced  


d. Horizontal output transistor will get shorted.

d.  Sometimes it will cause intermittent

      "no display".


e. Power blink.                                              

e.  Distorted display ie, the display will go in

     and out.


f. Sometimes power section will blow, for

   example: Raffles 15 inch monitor.               

f.   It will cause horizontal output transistor to

     become shorted and blow the power   


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 transfomer. For example:     







7. 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).

   c) This meter is handy and easy to use.

   d) Just simply connect the probe to point (A) and (B) refer to the diagram.

   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.


8. 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.*


   a) First set your multimeter to X10K range.

   b) Place your probe to point (D) and (E) refer to diagram.

   c) Point (D) is the anode red cap connected to the CRT. You must remove it in order

        to get a precise reading.

   d) Point (E) is any grounding of the monitor.

   e) If the needle of the multimeter shows a low ohms reading, this mean the internal

       capacitor is shorted.

    f) If the needle does not move at all, this doesnt mean that the capacitor is O.K.

   g) You have to confirm this by using a digital capacitance meter which you can easily

        get one from local distributor.

   h) If the reading from the digital capacitance meter shows 2.7nf, this mean the

       capacitor is within range (O.K).

   i) And if the reading showed 0.3nf, this mean the capacitor is open.

   j) 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.


Author By : Jestine Yong



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1 Feb 2007

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thanks again.
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