Coil testing is very easy compared to testing three-wire components such as SCR, FET, etc. In general, a coil consists of many turns or wire wound around a common core. The core could be made of iron or even air. When an electric current is passed through the coil, a magnetic field is produced. A coil in some aspect s acts right in front of a capacitor. A capacitor blocks DC while allowing AC to flow through it; a coil allows DC to flow through it while restricting the flow of AC current. Another name for a coil is an inductor.
The coil or inductor can be tested with an analog, inductance, or coil meter such as Dick Smith’s pullback tester. A small size coil I would usually just test with an analog meter and could check it on board as well. Set your analog meter to X1 ohm and place the probes along the small coil. The meter should show some reading (or continuity) and this showed that the coil winding is fine. Small coils rarely go bad because they have less winding compared to large coils where they could have many turns of winding and the chances of shorting are very high.
When testing larger coils or inductors, such as the B+ coil on your computer monitor, you need an inductance meter to find out the exact value of the inductance, which is in units of henry (h). Experienced use of an inductance meter to check coils for good or bad is not recommended because a shorted coil (shorted between windings) could have a good inductance value and you would lose the opportunity to check a bad coil. Unless you want to use the inductance meter to calculate the reading and do rewind, loop, etc. in that coil. I would only test a large coil with a Dick Smith return meter. This meter could easily detect any shorted winding.
Now it’s time to share my real case example: A computer dealer sent me a monitor for repair with flickering power symptom. I don’t usually fix the monitor right away, but I would first use the flyback tester to scan all the main coils (smps, flyback, b+ coil, and horizontal deflection coil) before using my digital or analog meter to test. When measuring the B+ coil, the LED lights went out and it is supposed to show at least 5 LED bars or more. When soldering the coil I didn’t see any burn marks on the winding or loop and it does indeed look shiny. Because I trust the meter, I opened the winding and to my surprise the inner winding had been burned to a crisp, but the outer winding looks really good! A new B+ coil brought the monitor back to life.
Using a flyback tester to test the coil has helped me locate many shorted coils on the primary winding of the switched mode power transformer, the B+ coil, the primary winding of the flyback transformer, and the horizontal deflection coil. The flyback meter can also be used to check the condition of the ballast on a fluorescent lamp!