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3. RA-3/3A Power Supply Troubleshooting
job properly. Since they are part of a loop, one affects the other. This is where many technicians run into
difficulties. It is similar to working on vertical deflection circuits. They are one of the more simple circuits in a
television but seem to be one of the more difficult to diagnose because of the reliance on feedback. The first task
is to isolate the failure in either the feedback loop or the oscillator. This can be done in a surprisingly simple way.
The regulator loop will be opened. It can be done safely so long as you have the variac. In this situation, you will
want to remove the connection from Q602-C to Q601-G. Slowly apply AC voltage with the variac while monitoring
the input to IC655. Proper B+ on the secondary line should appear at low AC. If it does, the problem is in the
regulation loop. The nice part of this method is that it divides the number of suspected components in half. Here
are the two possible outcomes:
B+ is still low even with full AC input: Assuming proper unregulated B+ is getting to Q601, either the
oscillator feedback line (not to be confused with the regulation feedback) is impeded or the oscillator
circuit is noisy or out of frequency. Suspect dry capacitor C614, resistance increase in R616 or loading of
the feedback line by shorted or leaking D604, D607 or C613. Q601 could even be the cause.
B+ reaches full level at low AC input: The problem is in the regulation loop and is usually caused by noise
in the line and open filters. C618 is a good suspect. If it opens, noise spikes will overcome the zener
diode D605 and will be amplified by Q602. Watch out for filters C609 and C610 opening or increasing in
value. Their job is to help produce a nice ramp-shaped signal at the base of Q602. This is especially true
for C610. The rise time of this ramp is critical in the timing of turning off Q601. A leaky C618, or even
Q602 itself, is another possibility.
Excessive B+
This problem requires caution since damage to components is a possibility. By far, the most likely component is
going to be Q602. Based on previous experience with this type of power supply, the B-E junction is common to
open. Leaking or shorted C610 comes in at a close second. Check for an increase in value, or open, at R615,
and a decrease in value or open at C618
The Main Power Supply
The main power supply is still a somewhat conventional style that has been used in Sony televisions for many
years. It is only active when the unit is powered up by energizing the main relay RY601. It has earned a dubious
reputation amongst technicians due to its insatiable appetite for several sets of switching transistors before the
technician finally got it repaired, and, rightfully so. When there are two transistors working together and one is
supposed to be off while the other is on, extreme care must be taken when replacing them. You are going to
learn some nice tricks that will virtually eliminate the immediate destruction of your newly installed switching
transistors. The regulation principle, although appearing complicated, is quite simple to troubleshoot since it
relies on pure DC voltages to function rather than ramp-rise timing used in simpler switching supplies.
Operation
Refer to Figure 3-2. The switching transistors are now conveniently packaged into a single IC. Oscillation is
begun by the upper transistor in IC601. Its collector is connected to the unregulated B+. R610, R611 and R619
bias the transistors near conduction in a classic AB-type amplifier design. The upper transistor will start current
flowing into the primary of T604 at pin 2. Pin 1 will continue the current flow to pin 5 of T605 and exit at pin 6
where C602 will provide the path to hot ground. This will cause pin 3 of T604 (the feedback winding) to spike the
base of the upper transistor through C620 and R619, which will now begin conducting. Current will increase in
the primaries of T604 and T605 and continue to charge C602 to the ground return. The feedback output at T604/
pin 3 will continue to increase and turn the upper transistor on harder. Meanwhile, the lower transistor is being
kept off since its base is connected to another feedback winding via R617 and C621. This other feedback source
is 180 degrees out-of-phase with the other so it is a negative voltage at this time.
