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22. RA-6 Power Supply Troubleshooting
Dead Supply. Switching Transistors Shorted: A more cautious approach must be taken when this happens.
There is slight possibility that excessive current or a higher possibility that line transients caused them to fail and
the loss of a newly installed set is not only frustrating, but costly in parts and wasted time. Static resistance
readings of the secondary lines to ground are the first order of business. This supply has ± supply rails so do not
forget to read across them for low resistance readings. A quick visual check for overheated components is also
a good idea. Check for open fuse links on the secondary lines, as these are valuable clues. If the above items
check OK, the supply can now be started.
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.
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 800ma.
Current Rises Rapidly and Unit Shuts Down: Normal current draw is 400ma at turn-on with a rise to
approximately 800ma occurring three seconds later. This is due to the internal soft start circuit of IC6003.
An initial current surge of 2A or more indicates an over-current condition. Begin removing secondary
supply sources. This can be done by lifting diodes or fuse links. Start with the 135V line since this is the
likely source. Do not lift the diodes for the 135V. This will open the regulation loop and cause the supply
to output maximum power. Disconnect at CN504. If the over-current condition still exists, the problem
lies in T6001 or the driver circuits. Check the waveform at the output of Q6006 for symmetry. A defective
T601 will cause ringing distortion. If the waveform is not 50% duty cycle, IC6003 may be defective and
causing the driver transistors to overlap.
Little Or No Current Draw: This is likely to happen if the switching transistors failed due to line transients.
The high impedance FET gates are protected by 10 diodes (not shown) and seven are zener diodes.
Lifting of the diodes and checking each for shorts or leakage is going to be necessary. Compare drive
pulse shapes (if any) at both gates to be sure they are clean and of the same level. It is possible that
most or all of these diodes are defective. No drive pulses from pin 12 or 16 may indicate IC6003 was
damaged by the transient. Verify the start voltage is present at pin 18 (300~340VDC) and the V SENSE
line is at least 1.7VDC and less than 2.5VDC. The unit may be going into protect so watch closely for
diagnostic flashes of the timer LED.
Dead Supply. Switching Transistors Not Shorted: This is a condition that is most likely to be encountered if the
supply was not damaged by large transients. The important steps to take in this situation are to monitor AC
current and listen for relay clicks. This will verify whether the supply is not starting. The normal sequence of
events at turn-on is as follows:
1. The main relay RY6002 engages. The click of this relay verifies the standby supply is operating.
2. Inrush current relay RY6003 engages next. The VC1 voltage from D5006 of the main supply is applied
directly to this relay. The time-period from RY6002 engagement to RY6003 engagement is less than
500ms. It is very important to listen for this rapid event. It will verify whether the main supply has
attempted to start. If only a single click is heard, the main supply is not starting or RY6003 is not engaging.
There will be a second click at least one second later as this unit will go into shutdown if the main supply
or RY6003 is not operating.
