Flyback Transformer - Failure

Failure

Flyback transformers are a frequent source of failure in CRT displays. Often, the CRT itself is blamed when the display has actually experienced a flyback transformer failure. The high voltage present in the many turns of wire, with the thin insulation required for the transformer to be of reasonable size, can result in leakage between the windings. As the leakage heats the insulation it carbonizes, increases conduction; in turn heat and carbonization continues a downward spiral until the leaked current is high enough for the high voltage to arc between the windings, and destroy the transformer (and sometimes other components in the display). As a result, replacement flyback transformers for almost every set on the market are available through dealers in electronic parts, typically for under $50. The problem is exacerbated by the tendency of the flyback to accumulate a coating of dust due to electrostatic attraction, which serves as a path to ground for leaks which might otherwise not be of sufficient magnitude to initiate the chain of events leading to destructive failure, as described.

As a result, occasional cleaning of the accumulated dust from the high voltage circuitry inside a television can be beneficial if proper precautions are taken -- however the small amount of additional life that is gained for the flyback transformer rarely justifies the time and effort necessary. It is debatable among technicians if displays installed in dirty, dusty locations experience more failures than those in cleaner locations, but many do say that dirty conditions contribute to malfunctions.

Another common failure mode is for one of the windings to develop a short circuit turn. This is often the high voltage winding with its thin wire with thin enamel insulation. Once a short circuit turn occurs, the transformer is prevented from ringing (magnetically speaking) as the short turn 'damps' the inductance. It is not possible to detect the short turn using conventional resistance measuring equipment.

A flyback transformer and its associated circuitry operate at very high voltages at low currents (<1mA-15mA), far beyond mains voltage. While most flybacks do not supply enough power to kill directly, the voltage they employ can cause violent muscle spasms if touched; and such spasms usually cause injury. A common injury that occurs when one is shocked is actually to be injured not as much by the shock itself, but when the victim's hand or arm is thrown back against other internal components in the display device. Therefore, only trained persons should touch or modify these devices, after first ensuring that the transformer is switched off and any stored energy has been safely discharged. The CRT attached to the flyback has an inherent capacitance which can hold a high voltage charge for more than a week after the power is switched off. Often, a high-resistance bleeder resistor is connected internally within the flyback transformer to ensure the charge is safely grounded when not in use, but many sets lack this, especially older models.

In many recent televisions, after replacing the flyback transformer, the control firmware must be recalibrated to account for slight differences in performance between transformers in order to maintain accurate picture reproduction. In older televisions and monitors, these needed adjustments were performed by turning potentiometers inside, or on the back of the set to achieve optimal picture quality. Also, when flyback transformers fail, they frequently will also damage the horizontal output transistor that drives the flyback transformer, and sometimes even blow fuses in the low voltage power supply circuits.

Unless the owner of the display device is savvy enough to repair it themselves, the failure of a flyback transformer frequently condemns the device as unrepairable, because the cost of repair can be higher than the replacement cost. Although the cost of the flyback transformer -- and other damaged parts -- is relatively inexpensive, the labor time needed to disassemble, replace the parts, and then re-adjust the display can make the repair prohibitively expensive.