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16. RA-5A Power Supply Troubleshooting
Troubleshooting the Primary Supply
Since this supply is the starting point for the operation of all necessary power supply operating voltages, it can
generate erroneous symptoms that could lead a technician in the wrong direction. The secondary supply is
dependent on the primary supply running. Technicians usually check for 135V B+ at the start because a test
point is easy to locate at the horizontal output transistor. That might lead the technician to start troubleshooting
the secondary supply first. The unit will go into shutdown if the primary supply fails to run and can possibly
generate a diagnostics failure indication. The 6.5V source is monitored for OVP and OCP. The transistors that
perform this task are supplied by standby 5V. It is possible for the 6.5V OCP to engage because of the design.
Wasted time would be spent working in the shutdown circuits. It would be wise to always check the primary
supply for proper operation before proceeding with any unit that will not remain on.
Verifying Primary Supply Operation: A quick way to verify the primary supply is starting is to listen for two rapid
relay clicks at turn on. They should be less than a half second apart. The first click is the main relay, RY6001 and
the second the inrush current relay, RY6002. If they are distinguishable, the primary supply is starting since it
supplies the 6.5V source to RY6002. If two clicks are heard with one or more seconds in between, the unit is
shutting down because RY6002 is not engaging. Check RY602 for continuity and if OK, the primary supply
needs to be checked. It would also be wise to monitor AC current while performing this check. A current surge
of 3 amps or more will further verify a primary supply startup and signify the presence of an over-current problem.
Dead Primary Supply: If RY6002 does not engage and AC amperage is low or unreadable, the supply is not
turning on. Verify the presence of unregulated B+ at Q6008 drain. If not present, check for open R6099 and
shorted drivers. IC6002 will oscillate and send output pulses from pins 12 and 16 as long as certain conditions
are met: There must be a startup voltage at pin 18, which is derived from unregulated B+, and the V sense at pin
1 is higher than 1.7VDC. The V sense is used to monitor for low unregulated B+, so the AC input should be
higher than 70VAC. It is normally 2.5VDC at 120VAC. The oscillator cannot start below 70VAC.
The old method of jumping the main relay and bringing up the AC voltage with a variac will not work in this power
supply design. Even with the main relay jumped and an external DC source applied to the V Sense line, the
oscillator will not start until at least 70VAC is reached. At the 70VAC level, the oscillator will start and stop at one-
second intervals. The horizontal and vertical deflection circuits will start and stop.
The rise and collapse of the
deflection will burn the phosphors in the tubes if they are lit. Do not attempt to run the supply at less
than 80VAC. Always disconnect the CRT sockets when working this power supply.
As mentioned earlier, this power supply design is extremely durable and can survive a startup with a direct short
on any of the secondary lines. As long as measurements are made for static shorts on the secondary lines, the
supply can be started at full AC. It will stop oscillating on its own or the unit will go into protect mode if a problem
still exists. AC current draw will still need to be monitored at turn-on to provide an indication of what step to take
if there is still a problem. Normal AC current draw for this chassis is between 1.3A and 1.8A, depending on AC
line voltage and picture brightness. With 120VAC and color bars in the Vivid Mode, the unit draws 1.5A.
With a no-start condition verified, perform the following checks:
No Drive Signals Being Output: Unload the output pins. If oscillation is present, suspect loading from the
multiple protect diodes (not shown in Figure 16-2) present on these lines. This can happen if the drivers
(Q6008/6009) were originally shorted. If drive pulses do not appear, suspect faulty IC6002.Drive Signals
Present: Connect a scope to pin 2 of T6004 and observe for square drive pulses at roughly twice the
amplitude of the AC input voltage. Suspect open Q6008 or components driving the gate if not present.
Check the voltage at pin 9 of IC6002. If any voltage above .2VDC appears, R6083 and R6084 have
opened. If no voltage appears, suspect open C6017 or T6004 primary winding.
Dead Supply. Switching Transistors Shorted: A more cautious approach must be taken when this occurs.
There is slight possibility that excessive current caused this failure – and an even higher possibility that
line transients caused them to fail. The loss of a newly installed set is not only frustrating, but costly in
parts and time wasted. Static resistance readings of the secondary lines to ground are the first order of
