Sony KDP-57XBR2 Training Manual page 45

10. RA-4/4A Deflection Circuit Troubleshooting
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. As they get hot and run at high frequency, so it is common for solder connections to go bad. Solder
the connections well. You want the solder to flow up into the internal pin connections at the windings. Check the
pincushion drive Q5020. It may also be shorted and may or may not have been the cause of the original failure.
Either way, it will be the cause of losing the newly installed horizontal output if it is not checked. With that done,
it is time to apply power.
Place your scope probe somewhere near the horizontal output transistor. Most scope probes are not able to
handle the direct spike level present here so this is a safe method. Just turn up the gain on the scope a little and
you will easily see the horizontal retrace pulse. Apply full AC power with the variac and watch for the horizontal
pulses (assuming it appears) before the unit goes into protect. The H protect is muted for several seconds at
turn and since the tubes are not lit yet, the pulse can be viewed briefly. Monitor the AC current level closely while
doing this. Clean pulses should appear and be 11 to 13us wide. Any deviation from the normal pulse indicates
a tuning problem. Suspect T5002 or one of the deflection yokes. If the pulses are correct and excessive current
is being drawn (more than 2 amps), there is a problem in the driver circuit or the pincushion driver is in saturation.
The horizontal output transistor will also get hot rather quickly.
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 in the circuit.
NOTE: Many technicians prefer to "ring" suspected transformers. This is OK and is very good at 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.
Pincushion Circuit Troubleshooting
The pincushion correction circuit in this chassis is similar to those found in earlier model direct-view sets. As
seen in Figure 10-3, a DC coupled chain of transistors is utilized. What makes this type of circuit difficult to
troubleshoot is the DC coupling. A failure of any transistor in the line can turn Q5020 completely off or cause it
to go into saturation. It is simple in design but can be a headache to troubleshoot at times.
The vertical-rate parabolic correction signal from IC512 is amplified and inverted. It is then shaped and applied
to the pin driver. Notice that no horizontal gating pulses are used in this design. This is because the circuit is only
varying the amount of negative supply voltage to the horizontal output transistor to modulate the scan width
unlike the newer designs that modulate at the yoke return and/or the retrace damper diode.
Failures in this pincushion circuit usually involve the shorting of the pin driver, Q5020. Due to the DC coupling,
one or more of the other transistors down the line may be damaged. On numerous occasions, the technician will
replace a shorted pin driver only to still have problems. There is an old audio amplifier troubleshooting method
that works quite well for this situation.
Use your DVM and read across the B-E junctions of the transistors in the circuit. There are two things to look for:
Zero volts or more than 0.6V. This will assist in locating any transistors that are biased on or have a shorted or
open B-E junction. Voltage readings across the E-C junction are also useful in locating open/off or shorted/
saturated transistors. Since none of these devices run at saturation, so any voltage readings at 0V or supply
level indicate a problem. The object is to find a transistor that is fully on or off to determine if that transistor is
defective or being switched on or off by the previous one.
42