Deflection yoke testing

• A simple test to determine if the yoke is at fault for a major geometry problem (e.g., a keystone shaped picture) is to interchange the connections to the yoke for the axis that is not affected (i.e., the vertical coils if the width is varying from top to bottom). If the raster/picture flips (indicating that you swapped the proper connections) but the shape of the raster remains the same - the geometry is unchanged, the problem is almost certainly in the deflection yoke.

• Where high voltage (and other flyback derived voltages) are reduced and other problems have been ruled out, unplugging the deflection yoke (assuming no interlock) may reveal whether it is likely at fault. If this results in high voltage and a relatively clean deflection waveform or returns the power supply or deflection chip load to something reasonable, a defective yoke is quite possible.

  CAUTION: powering a TV or monitor with a disconnected yoke must be done with care for several reasons:

  • The CRT electron beam(s) will not be deflected. If it turns out that the yoke is the problem, this may result in a very bright spot in the center of the screen (which will turn into a very dark permanent spot quite quickly) :-(. Disconnecting only the winding that is suspect is better. Then, the other direction will still scan resulting in a very bright line instead of a super bright spot. In any case, make sure the brightness is turned all the way down (using the screen/G2 control on the flyback if necessary). Keep an eye on the front of the screen ready to kill power at the first sign of a spot or line. Disconnecting the CRT heater as an added precaution would be even better unless you need to determine if there is a beam.

  • Removing the yoke (which is effectively in parallel with the flyback) increases the inductance and the peak flyback voltage on the HOT. In the extreme, this may blow the HOT if run at full line voltage/normal B+. It is better to perform these tests using a Variac at reduced line voltage if possible.

  • The deflection system will be detuned since the yoke inductance plays a very significant role in setting the resonance point in most designs. Don't expect to see totally normal behavior with respect to high voltage. However, it should be much better than with the faulty yoke.

• If possible, compare all measurements with a known good identical deflection yoke. Of course, if you have one, swapping is the fastest surest test of all! In many cases, even a not quite identical yoke will be close enough to provide useful information for testing. However, it must be from a similar piece of equipment with similar specifications - size and scan range. Don't expect a color TV yoke to work in a high performance SVGA monitor!

  Note: the substitute yoke doesn't have to be mounted on the CRT which would disturb purity and convergence adjustments but see the caution above about drilling holes in the CRT face plate!

The deflection yoke consists of the horizontal coils and vertical coils (wound on a ferrite core), and mounting structure. Little magnets or rubber/ferrite strips may be glued in strategic locations. DO NOT disturb them! In rare instances, there may be additional coils or other components mounted on the same assembly. The following deals only with the actual deflection coils themselves - the other components (if any) can be tested in a similar manner.

Where the test procedure below requires removal of the yoke, see the section: Removing and replacing the deflection yoke first.

• Horizontal - the horizontal section consists of an even number of windings hooked up in parallel/interleaved with half of the windings on each of the two ferrite core pieces.

  The horizontal windings will be oriented with the coil's axis vertical and mounted on the inside of the yoke (against the CRT neck/funnel). It may be wound with thicker wire than that used for the vertical windings.

  • Resistance check - This may be possible without removing the yoke from the CRT if the terminal block is accessible. Disconnect the individual windings from each other and determine if the resistances are nearly equal. Check for shorts between windings and between the horizontal and vertical windings as well.

    Typical resistance of the intact windings (at the yoke connector assuming no other components): TV or NTSC/PAL monitor - a few ohms (3 ohms typical), SVGA monitor - less than an ohm (.5 ohms typical).

  • Inspection - Look for charring or other evidence of insulation breakdown due to arcing or overheating. For the horizontal windings, this will require removing the yoke from the CRT since little if any of the windings are visible from the outside. However, even then, most of the windings are hidden under layers of wire or behind the ferrite core.

  • Ring test. See the document "Testing of Flyback (LOPT) Transformers". This deals with flyback transformers but the principles are the same. Disconnecting the windings may help isolate the location of a fault. However, for windings wound on the same core, the inductive coupling will result in a short anywhere on that core reducing the Q.

• Vertical - The vertical section is usually manufactured as a pair of windings wired in parallel (or maybe in series) though for high vertical scan rate monitors, multiple parallel/interleaved windings are also possible.

  The vertical windings will be oriented with the coil's axis horizontal and wound on the outside of the yoke. The wire used for the vertical winding may be thinner than that used for the horizontal windings.

  • Resistance check - This may be possible without removing the yoke from the CRT if the terminal block is accessible. Disconnect the individual windings from each other and determine if the resistances are nearly equal. Check for shorts between windings and between the horizontal and vertical windings as well.

    Typical resistance of the intact windings (at the yoke connector assuming no other components): TV or NTSC/PAL monitor - more than 10 ohms (15 ohms typical), SVGA monitor - at least a few ohms (5 ohms typical).

  • Inspection - Look for charring or other evidence of insulation breakdown due to arcing or overheating. The accessible portions of the vertical windings are mostly visible without removing the yoke from the CRT. However, most of the windings are hidden under layers of wire or behind the ferrite core.

  • Ring test - Since the vertical windings have significant resistance and very low Q, a ring test may be of limited value.