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3. RA-3/3A Power Supply Troubleshooting
Once C620 has charged, the upper transistor will lose its large B-E current and turn off. The impedance of the
primary in T604 will determine how fast C620 is charged and this will vary depending on the amount of current in
the control winding at pin 7 and 8 of T604. This is how this supply will regulate. The field will not collapse and all
induced voltages will reverse polarity. The feedback voltage at T604/pin 3 will now go negative and pin 4 will go
positive and turn on the lower transistor in IC601. IC601’s job is to pull pin 2 of T604 primary to ground potential.
The voltage stored in C602 will now have a path in which to discharge so that it is ready for the next cycle when
the upper transistor conducts again. The oscillator can continue this cycle indefinitely as long the components
remain stable and external power is available. All that needs to be done now is to alter the on-off time of the
switching transistors in order to regulate the secondary outputs.
The winding at pins 7 and 8 of T604 is what is known as a “cross winding”. The more current that flows through
this winding, the lower the net inductance of the windings in T604. It is similar to the effect that occurs when
adjusting the ferrite core in an adjustable coil or transformer. By lowering the inductance, the primary of T604 will
provide less resistance to current changes and will allow C602 to charge faster. If it charges faster, the field
collapse will also be shorter in duration and you now have a shorter charge/discharge rate and, hence, higher
oscillation frequency.
The current in the control winding is handled by IC654. It monitors the 135V B+ line. If the 135V line were to rise,
IC654 would act as an inverter. Pin 7 of T604 is fed by 18 volts from the secondary of T605. Since IC654 inverts,
it will pull down pin 8 of T604. Current through the control winding will increase, the impedance of T604 will
decrease and the oscillator frequency will rise. This will move the frequency further above the fixed resonant
point of T605, which will increase its impedance. The net effect will be a drop in secondary output voltages. The
opposite will occur should the 135V line decrease.
One more item to cover: The “soft start” circuit. This circuit is important to understand since its failure can cause
major problems for a technician. When the main power supply is first energized, a large current surge will occur
since the oscillator is starting from zero frequency and rising (which will cause it to cross the resonant point of
T605) and all of the filter capacitors on the secondary lines are waiting to be charged up. Consequently, it is
important that the oscillator be “forced” into high frequency as soon as possible. Q653 is allocated to this task.
There is a constant 12V source provided to the control winding by Q651. If it were not there, the control winding
would have no voltage at initial turn-on. As mentioned earlier, this would cause low frequency oscillation and,
hence, maximum output. This condition is undesirable while all of the filter capacitors are charging as it could
cause IC601 to fail.
Q653 is connected to the return side of the control winding through R669. Its base has a 1mf capacitor connected
to it. This capacitor provides approximately three seconds of charge time to keep Q653 on, which will provide
pull-down to the return side of the control winding. Extra current is now provided through the control winding to
keep the oscillator frequency high during the initial turn-on period. Once C673 has charged, the base of Q653
will go high and it will turn off and hand over control to IC654. Problems in this circuit can cause instant failure of
the switching transistors or may cause them to fail weeks later after the repair. A five second test can be done to
check this circuit and this will be covered in the troubleshooting section.
