68
16. RA-5A Power Supply Troubleshooting
Chapter 16 - RA-5A Power Supply Troubleshooting
5
6
1
2
3
4
8
7
6
5
AC
INPUT
IC6106
MM1476AF
GND
VIN
VOUT
GND
C6179
C6137
+
+
+
+
+
+
R6060
R6064
R6112
R6111
PH6004
TH6001
D6015
R6065
R6067
Q6010
FROM 6.5V PRIMARY
SUPPLY
D6017
C6023
R6059
R6062
C6065
C6072
D6016
UDZS-12B
R6057
4.7 1/4W
C6024
STANDBY
5V
FIGURE16-1 - RA-5A CHASSIS STANDBY SUPPLY
TO Q6105, Q6119
RELAY LATCH
16.1TVP14 1467
7/11/02
3
1
10
7
1.2Vp-p
2ms/DIV
Overview
The standby power supply has been changed from a switch-mode to a conventional, analog type. Although
simple to troubleshoot, it will be covered since it contains an OVP monitoring circuit that can cause the unit to
shut down.
The necessary operating voltages at power on have been divided between two switch-mode power supplies,
both located on the G board. They are very similar in design to the power supplies in the DX-1A chassis utilized
in the KV31/36XBR400 models. They are appropriately known as the main and secondary supplies. The main
supply must be operational for the secondary to function. It generates a 20VDC source known as VCC to
provide one of the necessary voltages for the secondary to operate.
Several voltage lines generated by these supplies are monitored for over-voltage and over-current conditions.
These will cause the unit to shut down without any indications from the diagnostics feature. Several photo-
optical devices are used to latch certain lines to turn off the main relay or stop the oscillators from running. These
will be covered in Chapter 17.
Standby Power Supply
As illustrated in Figure 16-1, the standby power supply is rather unique in that the incoming AC voltage is rectified
before being applied to the primary side of T6003. The DC pulses from the rectifier are used to fluctuate the
current in the primary winding. The secondary supplies unregulated B+ to 5V regulator IC6106. Zener diode
D6016 monitors the rectified AC for excessive voltage. If the voltage at the divider consisting of R6065 and
R6067 rises above 12V, Q6010 will turn on. The LED inside PH6004 will light and cause the phototransistor to
output a high to the main AC relay latch circuit to shut the unit down.
Primary Power Supply
This power supply consists of a self-running oscillator circuit and is not dependent on feedback to function. Once
the required voltages are input, square waves will be output by pins 12 and 16 of IC6002 as shown in Figure 16-
2. They will provide the necessary switching to the driver transistors Q6008 and Q6009. These square waves
will be 50/50 duty cycle and 180° out of phase so that one drive transistor is on while the other is off. The
frequency of the drive signals controls output voltages from the secondary lines.
