A faulty deflection yoke can affect the geometry (size and shape) of the
raster, result in insufficient high voltage and/or other auxiliary power
problems, and blow various components in the low voltage power supply or
|NotTaR of Television Sets : Deflection yoke testing
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- 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
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
- 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.
- 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.
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