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18. RA-5A Deflection Circuit Troubleshooting
Chapter 18 - RA-5A Deflection Circuit Troubleshooting
Overview
The deflection circuits in the RA-5A chassis are similar to the RA-4/4A chassis in that horizontal sweep and HV
are generated in separate circuits. Pincushion correction is performed in a different manner and HV protection
is not monitored as part of the self-diagnostics feature. HV protection is accomplished with fewer components
unlike the redundant circuits found in the RA-4/4A chassis.
Horizontal Deflection Troubleshooting
Referring to Figure 18-1, horizontal sweep is generated in the conventional way using an H pulse from pin 40 of
IC3006 on the A board. It is amplified and transformer coupled to the horizontal output transistor Q8024. The
deflection yokes and T8003 are driven. T8003 generates 200VDC for the dynamic focus correction circuit and a
± 100V source to provide offset of the red and blue deflection yokes for centering to the green CRT. Horizontal
centering control is also provided by IC8012 and Q8039.
IC 8005 is a PWM control device that serves to modulate the horizontal output B+ level to T8003 for pincushion
correction. The H pulse at pin 14 provides the timing for the sawtooth reference while the E – W signal supplies
the necessary vertical parabola signal for scan width modulation.
Q8031 and Q8101 buffers and shapes the H pulse for use in OSD timing and, more importantly, tells the H
protect circuit that horizontal sweep is occurring.
Since there is no flyback transformer incorporated into this circuit, current demands on the horizontal output
transistor are lower and damage from CRT and flyback arcs are nonexistent. This also makes troubleshooting
easier. Loss of horizontal deflection is likely to occur because of Q8024 failing. It is wise to check Q8027 when
this happens since it is easily damaged from the large current surge when Q8024 shorts and it also may have
been the reason for the failure.
Apply full AC power with the variac and watch for the horizontal pulse (assuming it appears). Monitor the AC
current level closely while doing this. If the unit is still going into protect mode, this will have to be observed
during the period from startup to shutdown. Clean pulses should appear and be 11 to 13us wide. Any deviation
from the normal pulse indicates a tuning problem. Suspect T8003 or one of the deflection yokes. If the pulses
are correct and excessive current is being drawn (more than 3 amps), there is a problem in the driver circuit or
the pincushion driver Q8027 is in saturation. The horizontal output transistor will also get hot rather quickly.
Another situation that might occur is that the retrace pulse looks great but begins to jitter and dance on the scope
screen. The AC current will also be higher than normal. This usually indicates excessive loading in the circuit.
Look for shorts in the secondary lines of T8003.
Pincushion Troubleshooting
The simplicity of the pin correction circuitry in this chassis should provide easy troubleshooting. The key symptom
to observe is whether the picture is stretched or of insufficient width with pincushion distortion.
Picture Too Wide: Q8027 is not modulating the scan width accordingly and is either shorted or in saturation.
There should be approximately a 15V difference between the source and the drain. Shorting the gate of Q8027
will aid in determining whether IC8005 is overdriving it or the transistor is at fault. If the picture width is substantially
reduced, check IC8005 for missing H pulse at pin 14 before replacing.
Insufficient Width: Q8027 is open or not receiving pin correction signals. Check for a parabolic signal at the
gate. If not present, verify the E –W signal is at pin 6. If it is, check the associated components around IC8005
before replacing it. Pay close attention to the components supplying feedback to the operational amplifiers in
this IC. Beware that the NVM IC storing the pincushion correction data has caused problems in some previous
models.
