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DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 27 of 80
Horizontal Stage
As in previous Sharp chassis a transformer-less driver circuit has been used, with the driver stage obtaining its
running voltage from the fly-back transformer, T601.
At switch on +8V from the power supply is applied to Q603 collector via R605 and Q602 collector via R628 and
R608. At the same time –16V is applied to Q602 emitter via R619. Horizontal drive from IC801 is sent to the
base of Q603, This will cause Q603 to switch, which in turn switches the horizontal output transistor Q601,
producing an e.m.f on pins 6 and 10 of the fly-back transformer T601. The e.m.f at pin 10 is rectified by D611 to
produce the positive run voltage and the e.m.f at pin 6 is rectified by D610 to produce the negative run voltage.
When the drive signal is high, the emitter of Q603 will also go high, turning on Q602. This results in Q602
collector going low and Q601 turning off. When the drive signal is low, Q603 emitter also goes low turning off
Q602 and allowing Q601 to turn on. As the current drawn from the +8V supply is minimal (via R628 and R608,
both 1kΩ), Q601 does not turn on fully, but does allow the transformer to energise sufficiently to produce
secondary voltages. D611 conducts on turns on Q601 harder, until the drive signal goes high on the next cycle.
Once the horizontal stage is running, D611 provides a low impedance/high current path to turn on Q601. D610
provides the same function to turn off Q601. These low impedance supplies are required to ensure that the
output transistor spends a minimal amount of time in the linear part of its characteristic. While Q601 draws
current and voltage is developed across it, power is generated which is dissipated as heat. This can lead to
premature failure of Q601.
With this type of driver circuit, it is essential that the circuit is started softly, otherwise transient pulses can
damage the horizontal output transistor. This is achieved by doubling the horizontal drive frequency; this
effectively reduces Q601 on time, until the run voltage has been established.
In addition to producing the EHT, focus, screen voltage and horizontal scan from the fly-back transformer T601,
the +/-13V rails for the field output are produced at pins 5 and 9. D609 rectifies the voltage at pin 9 producing
the +13V and D608 produces the –13v via pin 5 of the LOPT.
running voltage from the fly-back transformer, T601.
At switch on +8V from the power supply is applied to Q603 collector via R605 and Q602 collector via R628 and
R608. At the same time –16V is applied to Q602 emitter via R619. Horizontal drive from IC801 is sent to the
base of Q603, This will cause Q603 to switch, which in turn switches the horizontal output transistor Q601,
producing an e.m.f on pins 6 and 10 of the fly-back transformer T601. The e.m.f at pin 10 is rectified by D611 to
produce the positive run voltage and the e.m.f at pin 6 is rectified by D610 to produce the negative run voltage.
When the drive signal is high, the emitter of Q603 will also go high, turning on Q602. This results in Q602
collector going low and Q601 turning off. When the drive signal is low, Q603 emitter also goes low turning off
Q602 and allowing Q601 to turn on. As the current drawn from the +8V supply is minimal (via R628 and R608,
both 1kΩ), Q601 does not turn on fully, but does allow the transformer to energise sufficiently to produce
secondary voltages. D611 conducts on turns on Q601 harder, until the drive signal goes high on the next cycle.
Once the horizontal stage is running, D611 provides a low impedance/high current path to turn on Q601. D610
provides the same function to turn off Q601. These low impedance supplies are required to ensure that the
output transistor spends a minimal amount of time in the linear part of its characteristic. While Q601 draws
current and voltage is developed across it, power is generated which is dissipated as heat. This can lead to
premature failure of Q601.
With this type of driver circuit, it is essential that the circuit is started softly, otherwise transient pulses can
damage the horizontal output transistor. This is achieved by doubling the horizontal drive frequency; this
effectively reduces Q601 on time, until the run voltage has been established.
In addition to producing the EHT, focus, screen voltage and horizontal scan from the fly-back transformer T601,
the +/-13V rails for the field output are produced at pins 5 and 9. D609 rectifies the voltage at pin 9 producing
the +13V and D608 produces the –13v via pin 5 of the LOPT.
Figure 22: Horizontal Output Stage
Horizontal fly-back pulses (HFB) from pin 1 are fed to IC801 for both timing purposes and safety (prevents
excessive EHT), HFB is also rectified by D510 to produce +25V for vertical fly-back.
excessive EHT), HFB is also rectified by D510 to produce +25V for vertical fly-back.
Sharp Electronics (UK) Limited - May 2003
Revision 3
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