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Analyzing the RCA TX81/82 Horizontal Output Stage
The horizontal output stage found in the
RCA or GE TX81 or TX82 chassis differs
from conventional TV horizontal output
stages. While the TVA92 TV Video
Analyzer's Horizontal Out put Load and
Dynamic Tests can analyze the TX81/82
horizontal output stage, minor changes in
the lead connections and test
interpretations are required. This Tech Tip
briefly describes the operation of the
TX81/82 horizontal output stage and
explains how to use and interpret the
TVA92's Horizontal Output Load and
Dynamic Tests.
The TX81/82 Horizontal Output Stage
The horizontal output stage of most
televisions includes six basic components
(as shown in Fig. 1a): primary of the
flyback, damper diode, retrace capacitor,
output transistor, yoke, and yoke series
capacitor. A conventional output stage
applies a regulated B+ voltage from the Fig. 1b: The non-conventional horizontal output stage found in the RCA or GE
power supply to the flyback primary. The TX81/82 television chassis.
other side of the primary winding connects During normal operation, the horizontal
to the collector of the horizontal output output transistor is switched on by base
transistor. drive producing an increasing primary
current and magnetic field. When the
transistor is turned off, the flyback's
magnetic field collapses, charging the
retrace capacitor. When fully collapsed,
the retrace capacitor discharges,
producing current in the flyback primary
resulting in a magnetic field. With the
retrace capacitor discharged, the flyback's
magnetic field collapses and biases on
the damper diode to complete the hori-
zontal cycle. For a detailed explanation of
the operation of a conventional horizontal
output stage, See Sencore Tech Tip
#207, "Understanding The TV Horizontal
Output Stage."
Fig. 1a: Conventional horizontal output stage found in most television chassis.
The horizontal output stage found in the in the primary of LP03, voltage is current (increase deflection width) and
TX81 and TX82 chassis contains the same developed in the secondary which biases flyback secondary voltages, including the
components found in a conventional TV on DP11 applying a positive voltage to the CRT high voltage. A decrease in induced
output stage. See the simplified schematic flyback primary at pin 6. B+ voltage reduces deflection and high
of the TX81 and TX82 horizontal output voltage. By varying the conduction time of
stage shown in Fig. 1b. The primary of the The secondary voltage produced by LP03 the horizontal output transistor, the
flyback transformer (LP04) is located is determined by how much current flows deflection and high voltage can be
between pins 2 and 6. The flyback damper in the primary. The raw DC voltage from regulated.
is DP13, retrace capacitor is CP18, the power supply is applied to pin 12 of
horizontal output transistor is TP10, and LP03. The path for LP03 primary current is Understanding The TX81/82
the yoke series capacitor is CP40. through blocking diode DP14 and through Horizontal Output Stage
the horizontal output transistor. When the
In addition to these basic components, the horizontal output transistor is driven on by Let's take a look at the transformer action
TX81 and TX82 horizontal output stage base drive, current flows and produces an and currents during one horizontal cycle of
contains a B+ transformer (LP03) and its inductively rising current in the primary of the TX81/82 horizontal output stage. Figure
damping components (CP14, DP17). It LP03. 2a-d shows the action occurring at four
further includes blocking diodes and distinct times during one horizontal cycle
capacitors (DP14, CP60, DP10, CP53). The The design of the TX81 and TX82 chassis starting with the horizontal output
B+ transformer (LP03) produces B+ to is such that the base drive to the horizontal transistor turning on.
energize the flyback transformer. Its output transistor is varied in duty cycle or
damping components produce alternating conduction time for each horizontal cycle. When the H.O.T. turns on, current flows
currents in the primary, and damp A shortened conduction time permits a through DP14 and the primary of LP03
inductive voltages and energy returning lower level of current build-up in the (Fig. 2a). Voltage induced into the
into the primary. The blocking components primary of LP03 reducing the secondary secondary of LP03 produces a positive
isolate alternating currents in the primary B+ voltage applied to the flyback. A potential biasing on DP11 and applies the
of LP03 and the flyback primary. lengthened conduction time permits more positive potential to pin 6 of the flyback.
current buildup in the primary of LP03 The positive potential biases on DP10
The TX81 and TX82 chassis differs from increasing the secondary B+ voltage producing a path for flyback primary
the conventional output stage in the applied to the flyback primary. current. Current flows through the H.O.T. -
manner that B+ voltage is produced and DP10 through the flyback primary and
input to the flyback transformer. The B+ The level of secondary induced B+ voltage DP11. The buildup of current in the
voltage to the primary of the flyback at pin controls the flyback's primary current. By primary is determined by how long the
6 is produced inductively by the secondary increasing the B+ voltage, the flyback pulse horizontal output transistor conducts.
of transformer LP03. When current flows increases, resulting in an increase in yoke
HOT "ON" HOT "OFF"
(2nd part of trace) (1st part of retrace)
Raw B+ Raw B+
LP03
LP03
+ +
CP14
DP11 (Charges)