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5. RA-3/3A Deflection Circuits
Once the new transistor has been installed, it is always a good idea to re-solder the connections on the horizontal
drive transformer. These transformers handle a lot of current to forward bias the B-E junction of the output
transistor. They get hot and run at high frequency, so it is common for solder connections to go bad. Solder the
connections very well. You want the solder to flow up into the internal pin connections at the windings. With that
done, it is time to apply power.
Jump the main power relay. Place your scope probe somewhere near the horizontal output transistor. Most
scope probes are not able to handle the direct spike level from the flyback so this is a safe method. Turn up the
gain a little on the scope and you will easily see the horizontal retrace pulse. Slowly apply AC power with the
variac and watch for the horizontal pulse (assuming it appears). If the flyback is defective, one of two waveforms
is likely to appear. The first may appear as a sinusoidal waveform with a distinct ringing appearance. This
definitely indicates flyback problems. The second phenomenon will be a horizontal retrace pulse with a second,
lower level, “phantom” pulse. It is sometimes normal for this “phantom” pulse to appear at low B+ input levels but
it should disappear quickly as you increase AC voltage. Monitor the AC current level closely while doing this. If
the phantom pulse remains and current starts to rise, it is time to replace the flyback.
One last situation that might occur is that the retrace pulse looks great but as you approach higher AC input, it
begins to jitter and dance on the scope screen. The current will also be higher than normal. Back off the AC
power quickly as this usually indicates excessive loading on the flyback secondary. Take resistance readings to
ground looking for shorts. Do not forget to read resistance across any + and – supply rails. The vertical output
IC is notorious for shorting the +15V and –15V rails together which would be missed by static resistance
measurements to ground
NOTE: Many technicians prefer to “ring” suspected transformers. This is OK and is a good method for detecting
shorts in the windings. However, it is not 100% accurate. Problems in transformers sometimes appear under full
voltage and current load so the above procedure is very effective, although it requires caution and close current
monitoring. For those of you fortunate enough to own the expensive test equipment to drive and test the flyback,
use the equipment in lieu of the above procedures.
High Voltage Shutdown: The only protection for the circuit in this chassis is excessive high voltage. This is
monitored by IC502 as illustrated in Figure 5-1B. Pin 7 is maintained at a constant 8.2V for reference. Pin 6 is
supplied with a divided-down sample of heater voltage. This sample voltage should remain slightly below the
reference at pin 7. If pin 6 rises above 8.2V, pin 1 will go low and ground pin 43 of the Y/C Jungle IC206. The
input at pin 43 has two purposes: one is to provide an H pulse sample from the horizontal output stage for
horizontal AFC centering, and the other is to monitor for an external pull-down which tells the IC206 to stop the
horizontal oscillator. The unit will not go into full shutdown. The horizontal and high voltage will cease, but the
rest of the unit will still be powered up and the timer LED will begin blinking six times.
Since the unit has protection for over-voltage on the 135V line, a failure there is unlikely although it would be a
good idea to check it anyway. The problem is likely caused by one of the tuning capacitors in the horizontal
output stage. Their job is to provide a current path to ground for the flyback transformer while the horizontal
output transistor is off. The impedance is critical to provide enough current to keep the kickback pulse voltage
level at the proper potential. Otherwise, the kickback pulse will rise dramatically and cause excessive high
voltage. The key to diagnosing this is to observe the H pulse width. It will be much narrower than the normal 11
to 12us duration and, although you will not be able to read it directly, the pulse amplitude level will be much higher
than normal.
Pincushion Correction
As seen in Figures 5-1A and 5-1B, this circuit is relatively simple in its operation (as there are a small number of
components). It still manages to give technicians a tough time based on the number of requests for technical
assistance. A lack of understanding is possibly the main reason. Here are some proven techniques to quickly
locate the possible cause:
The circuit requires two basic waveforms to operate. The E/W signal supplied by the Y/C Jungle IC301/pin37
provides the necessary modulation reference to gradually increase the scan width towards the center of the
screen edges. This waveform is applied to IC502/pin 9. This will be inverted and amplified by Q503 and applied
