Horizontal circuit drive analyzer and method of analyzing the horizontal circuit drive of a video display

A method and apparatus for testing the horizontal drive stage of a cathode ray tube display where the horizontal drive stage has a drive output which is directed to the base of a horizontal output transistor. In the method and apparatus, the output from the horizontal drive circuit is disconnected from the base of the horizontal output transistor and is connected to the apparatus of the present invention. The apparatus includes a load which simulates the load normally provided by the base of the horizontal output transistor. A meter differential amplifier is connected across the load and readings therefrom are projected on a display. The horizontal drive stage is then energized and readings from the display are compared to known operating requirements of the base of the horizontal output transistor.

The present invention relates to a method and instrument for analyzing the 
horizontal sweep circuit drive of a video display of the type used for 
television receivers and other CRT monitors, and it more particularly 
relates to a method for detecting an abnormal drive current to the 
horizontal sweep circuit of multifrequency video displays. 
BACKGROUND OF THE INVENTION 
When the horizontal output stage of a video display fails, the defective 
components are often replaced without first testing for defects in the 
horizontal drive stage. If the horizontal drive stage is in fact the cause 
of the failure of horizontal output stage components, the substituted 
components may operate for a short interval of time, allowing the display 
to be placed back in service, only to fail again. Multifrequency video 
displays are adapted to operate through a range of horizontal frequencies, 
and the horizontal drive stage must produce a base current to the 
horizontal output transistor which is suitable for the current required 
for each frequency at which the display operates. Displays with a wide 
horizontal operating range frequently have horizontal drive stages which 
are capable of switching between two or more levels of base current to the 
horizontal output transistor (H.O.T.) so that it is lower at lower 
frequencies and higher at high frequencies. A lower base current is needed 
for lower frequencies because the H.O.T. conducts for longer periods of 
time at the lower frequencies thereby allowing a significantly higher 
collector current than would occur if the same collector current were 
applied at higher frequencies. Two methods are commonly employed in the 
driver stage to switch the current level to the base of the H.O.T. One 
method is to change the level of the primary current in the driver 
transformer by changing the primary resistor which is in series with the 
windings of the transformer. The second method is to switch a resistor in 
series with the output for the driver circuit and the base of the H.O.T. 
If the base current to the H.O.T. drops below its designed operating range, 
the H.O.T. will not provide the collector current which is normally 
required for proper operation of the display. The symptoms may not be 
detectable if the current is reduced only slightly, but more significant 
reductions will cause the collector current to become limited, thereby 
limiting the horizontal deflection. As the base current is reduced, the 
efficiency of the H.O.T. is correspondingly reduced, and this will cause 
the H.O.T. to overheat and prematurely fail. 
Intermittents, or fluctuations in the driver stage output current, can also 
cause failure of the H.O.T. This is because intermittents cause the H.O.T. 
to be turned on and off at inappropriate times thereby reducing the 
efficiency of the H.O.T. and causing it to heat excessively. 
If service personnel merely replace a defective H.O.T. and return the video 
display to service without first testing the driver stage output to 
determine if it is providing the current required for each frequency of 
the display and that it is not subject to intermittence, the repaired 
video display may again fail shortly after it is returned to service. 
Presently, there is no method or apparatus for testing the output of the 
horizontal driver stage, and it would be desirable to provide a method and 
apparatus for undertaking such a test. Since the horizontal output stage 
operates at high voltages, it would also be desirable that the instrument 
analyzes the equipment under test without energizing the horizontal output 
stage of the display to reduce the danger of shock to service personnel or 
damage to test equipment. 
SUMMARY OF THE INVENTION 
Briefly, the present invention is employed to test the horizontal drive 
stage of a cathode ray tube display where the horizontal drive stage has a 
drive output which is directed to the base of a H.O.T. To test the 
horizontal drive circuit, the output from the horizontal drive circuit is 
disconnected from the base of the H.O.T. and is connected to the apparatus 
of the present invention such that the apparatus of the present invention 
will replace the load seen by the horizontal drive circuit. 
In accordance with the present invention, a load is connected between the 
output of the horizontal drive circuit and ground which simulates the load 
normally provided by the base of the H.O.T. A metering means, such as a 
conventional voltage measuring circuit, is connected across the load to 
measure the output of the horizontal drive circuit, and the readings from 
the metering means are thereafter projected on a display. The H.O.T. is 
typically a conventional bipolar or MOSFET type transistor which operates 
as an on and off switch. To accommodate the requirements of such 
transistors, the horizontal drive stage output provides a square wave 
having positive and negative phases which alternately switch the H.O.T. 
between the on and off condition. In the preferred embodiment, the load 
which is attached to the output of the horizontal drive stage includes a 
resistor in series with a Schottky diode which permits the drive current 
output produced by the driver stage to flow through the resistor during 
only the positive cycle, thereby simulating the positive drive cycle 
provided to the base of the H.O.T. The voltage across the resistor is 
proportional to the positive current from the output of the horizontal 
drive circuit. In the preferred embodiment, the voltage across the 
resistor is amplified by a differential amplifier and converted to an 
average DC voltage by a low pass filter and then measured by a 
conventional voltage measuring circuit. The voltage measurement relative 
to the drive current is then shown on any suitable display. 
The invention further includes a protector switch circuit which monitors 
the collector of the H.O.T. to prevent damage to the horizontal output 
stage in the event the service technician did not disconnect the base of 
the H.O.T. from the output stage. The protector switch circuit also 
provides protection to the diode and the associated circuitry of the test 
instrument from spurious currents. 
A technician can monitor the voltage output from the drive stage to 
determine if the correct current capacity is available to the horizontal 
output circuit. If the current to the base of the H.O.T. is inadequate, 
the technician can then troubleshoot the defective drive circuit. 
Similarly, if the output shows variations in voltage to the drive stage, 
he can troubleshoot the driver circuit for intermittent driver problems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, the horizontal output stage 10 of a 
multifrequency cathode ray tube video display (the remaining portions of 
the display are not shown) includes a transformer 14, commonly referred to 
as a flyback transformer, one winding 15 of which is connected to a B+ 
power source 16. A yoke winding 18 is connected between ground and the 
opposite terminal of winding 15 of the transformer 14, and a retracing 
timing capacitor 20 and a dampener diode 22 are connected across the yoke 
18. Connected to the junction between the yoke 18 and the winding 15 is 
the collector terminal 24 of a horizontal output transistor (H.O.T.) 26, 
the emitter terminal 28 of which is connected to ground 30. 
The horizontal drive circuit 32 of the device under test has a horizontal 
drive output 34 which is connected to the base 36 of the H.O.T (before 
testing performance). To test the voltage at the horizontal drive output 
34, a test apparatus 40 in accordance with the present invention is 
provided. The test instrument 40 includes a first connector 42 for 
connection to the emitter terminal 28 of the H.O.T., a second connector 44 
for connection to the horizontal driver output 34 and a third connector 46 
for connection to the collector terminal 24 of the H.O.T. The test 
instrument 40 further includes a on/off actuator button 48 which is 
adapted to close a switch 50 thereby placing a current sensing resistor 52 
in series with a diode 54, such as a Schottky diode, between the second 
connector 44 and the first connector 42. A protective circuit disable 
switch 56 is connected between the third connector 46 and the second 
connector 44. The protector current disable switch 56 is also connected to 
the switch 50 such that any spurious voltage detected between the 
collector 24 of the H.O.T. 26 and the drive stage output 34 will cause the 
protector circuit disable switch 56 to open switch 50 thereby 
disconnecting the resistor 52 and the diode 54 from the circuit. 
A differential amplifier 58 is connected across the current sensing 
resistor 52 to amplify the voltage across the resistor, and the current 
from the differential amplifier 58 is measured by a conventional voltage 
measuring circuit 60 after it is converted to DC voltage by a low pass 
filter 62. The voltage as measured by the circuit 60 is then shown on a 
suitable display 64 such as a liquid crystal display or the like. 
OPERATION OF THE INVENTION 
To carry out the method of the invention, power is first disconnected from 
the display 10 under test. Then the base 36 of the H.O.T. 26 of a display 
is disconnected from the horizontal drive output 34 as shown. Thereafter, 
the first connector 42 of the device 40 is attached to the emitter 
terminal 28, the second connector 44 is connected to the horizontal drive 
output 34 and the third connector 46 is connected to the collector 
terminal 24. 
The method of the present invention is an active test and, therefore, power 
is then applied to the horizontal drive circuit after which the technician 
depresses the button 48 to close the switch 50 and holds the button 48 
down until meaningful test readings are obtained. If there are no spurious 
currents detected between the connectors 42 and 46, the protection circuit 
will not disable the switch 50, and the resistor 52 and diode 54 will be 
connected between the horizontal drive output 34 and ground, thereby 
simulating the load imposed on the horizontal drive circuit by the base 36 
of the H.O.T. The positive swing of the voltage from the horizontal drive 
output 34 is applied across the resistor 52 and this voltage is amplified 
by the differential amplifier 58. The output of the amplifier 58 is 
measured by the voltage measuring circuit 60 and shown on display 64. A 
technician can read the display 64 to determine the voltage relative to 
the drive current which the horizontal drive circuit applies to the base 
36 of the H.O.T. and compare this voltage to specifications for the H.O.T. 
in a suitable reference manual. If the voltage differs from the proper 
operating requirements of the H.O.T. so as to be inadequate for the proper 
operation of the H.O.T., the technician can then troubleshoot the 
horizontal drive circuit 32. 
After the test is completed, power should again be removed from the display 
being tested and connectors 42, 44, 46 should be detached from the 
horizontal output circuit. Thereafter, the horizontal drive output 34 
should be reconnected to the base 36. 
While one embodiment of the present invention has been shown and described, 
it will be apparent to those skilled in the art that many changes and 
modifications may be made without departing from the true spirit and scope 
of the present invention. It is the intent of the appended claims to cover 
all such changes and modifications which fall within the true spirit and 
scope of the invention.