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Chapter 7---Troubleshooting
Model 250 Service Manual 7-19
CRT Protection
Figure 7-15
CRT Protection path
The CRT Protection circuit is designed to protect the CRTs in case any of the
CRTs is not receiving a horizontal or vertical deflection waveform. If there are no
deflection waveforms, the electron gun inside the CRT will shoot an electron
beam at the center of the CRT, burning a spot in the CRT phosphor that will be
visible on the screen. The CRT Protection circuit also protects the CRTs from
excessive beam current that may damage the CRT.
Deflection Waveform Detection
The CRT Protection circuit protects the CRTs by monitoring the deflection
waveforms in two different ways. On the Scan Reversal PCB, there is a H_SENS
and V_SENS signal for each CRT. These sense lines detect the presence of the
horizontal and vertical deflection waveforms. The horizontal and vertical sense
lines for each CRT combine into a RED, GRN, or BLU_DEFL_OK signal (see
Figure 7-16). These RED, GRN, or BLU_DEFL_OK signals combine into one
DEFL_OK signal that goes back to the Horizontal Vertical Deflection PCB. On
the Horizontal Vertical Deflection PCB, the DEFL_OK (normally high and the
/H-ENA (normally low) go into an NOR logic gate with an inverted input for the
H-ENA signal. The /H_ENA signal shuts down the horizontal amplifier during
changes in source input. If DEFL_OK goes low or /H_ENA goes high, the output
/SWEEP_OK goes high. If the /SWEEP_OK signal, on the Video Amplifier PCB,
goes high, the G
1
supply and the G
2
Regulator circuit shut down. On the Video