JVC 250 Service Manual page 94

Chapter 5---Electronics
front/upright projection. This means the horizontal scan jumper is plugged into
J100, the vertical scan jumper is plugged into J53, and the convergence jumper is
plugged into J50.
NOTE: Whenever the horizontal scan jumper is switched from front projection to
rear projection, the convergence must also be switched from J50 to J50A.
The Scan Reversal PCB sends two signals (FRONT/REAR and
FLOOR/CEILING) back to the Horizontal Vertical Deflection PCB, which sends
the signals back to the System Controller PCB through the IIC data bus. This
allows the System Controller PCB to properly output convergence and shading
data, however the shading, convergence, geometry will need to be adjusted after
switching deflection modes.
The Scan Reversal PCB receives the deflection waveforms from the Horizontal
Vertical Deflection PCB. The horizontal deflection waveforms go through the
horizontal size adjustment inductors (L1, L2, and L3) and is output to the
Deflection Yokes on the CRT without any further signal processing. The vertical
deflection waveforms pass through the Scan Reversal PCB without any further
signal processing. The convergence data passes through the Scan Reversal PCB to
the Convergence Yokes on the CRT without any further signal processing.
The Scan Reversal PCB provides for the adjustment of horizontal scan width.
Inductor L1 adjusts the red horizontal width, and L5 adjusts the blue horizontal
width. L3 increases the horizontal size range of red and blue.
The Scan Reversal PCB contains two major components of the CRT Protection
circuit. The first component is the DEFL_OK signal that goes back to the
Horizontal Vertical Deflection PCB. The Scan Reversal PCB has a sensing
inductor (coil) that detects the presence of a horizontal and vertical deflection
waveform for each CRT. These sense lines combine to drive the DEFL_OK
signal. If any of the deflection waveforms are not present, the DEFL_OK signal
goes low. As the /SWEEP_OK signal it shuts down the G and G regulator
1 2
voltages on the Video Amplifier PCB. It also shuts down the high voltage section
of the High Voltage Power Supply (see CRT Protection section of Ch. 7--
Troubleshooting).
The other component of the CRT Protection circuit on the Scan Reversal PCB is
the H_LOCK+ and H-LOCK- signals. These signals are actually one voltage (+15
V) that forms a series circuit through each vertical and horizontal deflection yoke
connector on each CRT. If any of the deflection yokes are not properly or
securely installed, the H_LOCK- signal shuts down the horizontal amplifier on
the Horizontal Vertical Deflection PCB (see CRT Protection section of Ch. 7--
Troubleshooting).
Model 250 Service Manual 5-39