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Flyback Troubleshooting

 

 

Failure of the horizontal output stage to produce deflection, and the major byproduct high voltage usually results in a dead set because the supplies needed for signal processing are also absent. Often the failure results in a blown fuse. For example, there is little to limit current from the power-line operated supply if the H.O.T. is shorted.

If a primary-supply fuse has blown check the H.O.T./damper diode for shorts, and replace if necessary. If the fuse holds when power is reapplied, but there is no deflection or high voltage, check for the correct waveform at the base of the H.O.T. This might require a substitute bench power supply for the horizontal oscillator circuit to make this check, as the normal supply for this circuit is also produced from the flyback pulse, and a momentary starting voltage is used to get things going. If drive is present, check the yoke/flyback transformer for opens.

If the primary fuse blows again, the cause may be an additional load on the flyback transformer or failure of the transformer itself. First inspect the transformer and yoke for signs of arcing or char. If there are none look for loads that might pull the H output circuit down. Disconnect temporarily any hold-down circuit. Also lift aux. supplies, one by one that are driven from the flyback to see if any had been responsible for excessive load. Disconnect, or check the high-voltage multiplier to see if it is part of an excessive load. The idea is to eliminate all loads down to the flyback itself. The transformer represents a high replacement cost as well as the labor involved in replacing it, so it's a good idea to be sure the trouble has been definitely traced to the transformer before replacement is attempted.

Failure of the flyback transformer can be due to opens, leakage between windings or shorted turns. Opens can be found with ohmmeter checks. Inter winding leakage can be checked with a Hi-pot leakage tester. Shorted turns are the most difficult to find as they have little effect on total winding resistance, but a single shorted turn acts like a shorted transformer secondary and so acts like a heavy load.

Shorted turns can be identified by conducting a "ringing" test. This is done by establishing, and then interrupting a dc current in the primary, and then checking for the number of sine waves (rings) at the natural frequency of the inductance of the unloaded primary and the total shunt capacitance. If turns are shorted the rings die out very quickly. If a DSO (digital storage scope) is available this check can be set up quite easily using a low voltage dc supply or a battery, Set the scope to trigger when the supply is interrupted and check for the number of rings before their amplitude falls to some preset value, like 50%. Go/no-go criteria can be set by checking a known good flyback.

Ringing tests are also facilitated with Sencore instruments such as the Model TVA92 TV VIDEO ANALYZER and CM2125 COMPUTER MONITOR ANALYZER. These units employ a patented system that pulses the flyback at low level and counts the number of rings that exceed a reference amplitude.