3. RA-3/3A Power Supply Troubleshooting
Chapter 3 - RA-3/3A Power Supply Troubleshooting
he Standby Power Supply
The standby supply is a simple switched-mode type utilizing a single transistor, transformer and feedback circuitry
to generate approximately 7VDC provided to a 5-volt regulator IC. Regulation will be provided by changing the
oscillation frequency based on current demand. Notice that the regulated 5 volts not only provides standby
power for the microprocessor, but also powers the main relay. This is a deviation from previous designs. It is
important to remember this when the need arises to replace the relay and you are obtaining one from a scrap
board that came out of an earlier chassis. The relays in previous projection and direct-view sets were rated at 12
volts and will not function on this chassis.
Referring to Figure 3-1, the supply works by supplying unregulated B+ through the primary of T602 to the drain
of Q601, which will provide the needed ground path via R604 to provide current for the transformer. Q601 G will
need a kick-start and that will be provided by R609. The FET will now begin conducting and current will begin to
flow through the primary. The feedback winding at pin 4 of T602 will now start generating a positive voltage,
thereby charging C614, which will drive the FET harder. This increases current in the primary, which further
increases the feedback voltage, which further increases the conduction of the FET. This event will continue until
two things occur: Q601 reaches saturation and C614 charges. This is how the on time of Q601 is controlled to
change the frequency.
C616 will route the feedback voltage via R615 to Q602B. This voltage will be smoothed out by C610 and C609
to provide a rising sawtooth to control the on time of Q602. Q602 will now become part of the RC network of
C614, R608 and R616. If current demand on the secondary of T602 decreases, voltage levels in the transformer
will rise accordingly. This will cause a rise of the feedback voltage and will cause Q602 to conduct harder and
alter the RC time constant. C614 will now charge more rapidly and Q601 will turn off earlier. By shutting the FET
off earlier, the on time is decreased along with the duration of the field collapse in T602. Shorter conduction time
and field collapse equals faster switching cycles and, hence, higher frequency. The higher the frequency, the
lower the output voltages at the secondary.
Q602 will also provide protection against over-voltage or over-current situations. If the current rises dramatically,
a voltage drop will occur across R604. When it rises to 0.6V or more, Q602 will conduct to raise the frequency,
or, in severe cases, momentarily stop oscillation. This would cause the familiar "ticking" noise as the supply
keeps trying to re-start.
Should the regulator loop fail, zener diode D605 will monitor the voltage stored by C618, which is charged by
D610 during the field collapse periods. If this exceeds 6.2 volts, D605 will fire into Q602B and produce the same
reaction as an over-current condition.