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8. RA-4/4A Power Supply Troubleshooting
Chapter 8 - RA-4/4A Power Supply Troubleshooting
Overview
The RA-4 chassis introduced a new, simpler, standby power supply. Major design changes occurred in the main
power supply. An oscillator circuit was now being employed that was independent of the output stages. This
oscillator provided the necessary drive signal to two, independent, driver circuits that shared the distribution of
power requirements for the set. The method of controlling the main power relay is different from past designs
and regulation no longer relied on the use of a control winding in what was known as the Power Regulating
Transformer (PRT). Oscillator frequency changes are controlled directly by an IC that monitors a secondary
voltage feedback. For a more detailed description of this circuit, refer to the TVP-08 training manual (P/N
TVP080299).
Standby Power Supply Troubleshooting
In order to understand and effectively troubleshoot this circuit, it is necessary to view the AC input circuit along
with the standby supply and these are shown in Figures 8-1 and 8-2. Along with a main power relay, RY6001,
there is also an inrush current relay, RY6002. It is important to know what this relay does to assist in protect circuit
troubleshooting which will be covered later.
The negative side of the bridge rectifier, D6002, is connected to hot ground via R6010. This 2.2
Ω
, 10 watt
resistor will limit the initial current surge at startup and will be shunted by RY6002 as soon as the +15V line from
the main secondary comes up. This happens very fast (less than a half second) after the main relay energizes.
RY6002 is protected against failure by monitoring the voltage at pin 4 of D6002. If the relay does not shunt
R6010 within five seconds, C6013 at the base of Q6004 will charge and cause the collector to go low and stop
the main power supply oscillator. The main relay will remain engaged.
The standby supply consists of T6003 and IC6001 as the main components to provide oscillation. Pin 5 of
IC6001 will provide the current return through the primary to ground. Unregulated B+ is provided by D6001 and
D6003 through fusible resistor R6012. The circuit is self-oscillating at this point. Voltage regulation will be
achieved by monitoring the feedback line from T6003/pin 4 through the rectifiers and charging C6014. This will
provide an average DC level to monitor and control the oscillator frequency. Notice the voltage tap between
D6012 and 6015. Approximately 15VDC occurs here to provide voltage for the main relay, RY6001 and the VCC
switch circuit. Pin 7 of T6003 will provide 7.5VDC for the 5V regulator IC6104 to power the main microprocessor
and other various circuits needed for standby operation.
Failures in the standby will usually cause a completely “dead” set. There will be no relay clicks heard and no
timer LED flashing on the front panel. It is simply a matter of checking for input voltage at IC6104 and, if present,
5VDC at the output. If input voltage to IC6104 is not there, verify the oscillator circuit is running. As mentioned
earlier, this oscillator is not dependent on feedback to run and does not require a start circuit. As long as B+ is
present at T6003 primary and pin 5 of IC6001, the likely defective components are IC6001 or the protect circuit
components consisting of D6005, D6006, C6015 and R6004.
