North-South Output; Dynamic Focus; Degaussing Coil - Zenith ZCM-1490 Service Manual

North-South Output
The N-S output circuit provides the current to modu-
late the vertical ramp at the horizontal rate. Refer to
Figure 5-8. The output of IC7501 is coupled to opera-
tional amplifier IC7502. The positive polarity output
(pin 6) is applied to the noninverting input of IC7502
and the negative polarity output (pin 12) is applied to
the inverting input. The signal from the N-S paral-
lelogram control (R7431). is also added to the nonin-
verting input through R7543. IC7502 provides gain to
drive op-amp IC7503. Power amplifier IC7503 in turn
drives step-up transformer T7501.
·"VlIC
L--+----llll
'----fh-~-_I[!J
.?--Mlr----; ~.-75'-' ~I!l
PIIDV
.,...
PIIDV
Figure 5-8. N-S Output
The secondary of T7501 is in effect in parallel with
the vertical yoke and the N-S pincushion correction
waveform is thus imposed onto the vertical output.
Although the secondary of T7501 is in series with the
vertical yoke, the output of the vertical scan is at AC
ground compared to the horizontal rate signal, and
the yoke coupling capacitor and resistor present a
low impedance.] Thus, T7501 is in effect in parallel
with the vertical yoke.
Page 5·14
Dynamic Focus
The dynamic focus circuitry varies the focus voltage
at the horizontal rate so that the voltage at the raster
edges is increased and the voltage at the center is
decreased. The horizontal rate waveform is applied
to the base of Q7701. R7703 controls the amplitude
of the signal applied to the dynamic focus circuit.
07701 and 07703 amplify the incoming waveform.
T7701 steps up the resulting waveform to generate
the 500V pop focus voltage.
Degaussing Coil
. Degaussing refers to demagnetizing the iron and
steel parts of the picture tube, in particular the steel
shadow mask and frame within the CRT. This is nec-
essary because a steady magnetic field magnetizes
these parts and affects the beam register on the
color phosphors, resulting in poor purity.
A degaussing coil is wrapped around the CRT and
cont~olled
by a. positive temperature coefficient ther-
mistor inthe power supply. When the monitor is first
turned on, current flows through the thermistor and
activates the degaussing coil. As the thermistor heats
up, its resistance becomes very high and the
degaussing coil is deactivated. The thermister can
require up to 30 minutes to cool and reset before the
degaussing coil can be reactivated.
Circuit Descriptions