POWER SUPPLY, TROUBLE SHOOTING TIPS.
SAFETY FIRST; Use only one hand when working on a powered up monitor to avoid electrical shock.
Always wear safety glasses.
Many of the failures that cause burnt components and boards are eliminated by the load sensitive switching mode power supply in the CERONIX monitor. This feature can cause problems with servicing the monitor if the proper trouble shooting approach is not used. The equipment setup, shown here, is necessary for efficient trouble shooting of the CERONIX monitors.
Problems that cause the power supply to chirp are:
1.Insufficient V+ line load.
2.Overloaded V+, +24V, or +16V lines.
3.Shorted V+, +24V, or +16V lines.
4.Power supply component failure.
5.Raw DC (V+ to
1. A quick check for the insufficient V+ load is to connect a 2K to 4K ohm 10 watt power resistor from GND to the V+ (cathode of diode 181 ). If the chirping stops, proceed to check the horizontal deflection circuit. First disconnect the board from the AC supply. Then connect 16 volts to the 16V line at the cathode of 169 . Also connect 24 volts to the 24V line at the cathode of diode 168 and to V+ at the cathode of diode 181 on the monitor. Now the complete horizontal and vertical circuits can be checked with the oscilloscope and DVM. The flyback waveform will be about
2. For the overloaded supply line problems, the monitor power supply can be made to run continuously by connecting the external power supply to the 17V line. To accomplish this, connect the external supply 0V clip lead to V- (resistor 137 lead by the power transformer) and +20V clip lead to the monitor power supply +17V line (cathode of diode 141 ). Sometimes the monitor will operate normally in this mode, in which case, watch for smoke and after a few minutes of operation disconnect the power connections and carefully feel around the conductor side of the board for hot spots. Overload conditions will not harm the power supply unless there is a problem in the power supply. If the power supply is suspect, read the
TEST section on this page. Next check the DC voltage of each of the power supply outputs. The overloaded line will have a lower than normal voltage reading. The defective component can be located by measuring the voltage drop along the trace of this line.
3. If the V+ crowbar zener 181 is shorted, a fault exists in the power supply which permitted the V+ line to exceed +160V. First replace the zener.
Never operate the monitor without the crowbar zener installed. Then read the
TEST section on this page. Shorts on the V+, 24V, and 16V lines other than the crowbar zener are not likely to be connected to the power supply even though the power supply chirps. By operating the power supply with the +20V external power supply many of these problems can be found using the same procedure as are used in trouble shooting monitors with linear power supplies.
4. The power supply may chirp if:
The transformer core is broken or a winding is shorted.
The .33 ohm current sensing resistor value is too high.
The +17V line is open. (goes away when external. PS is used)
5.There is a line voltage range of about 60% to 70% AC line voltage where a correctly operating monitor will chirp.
POWER SUPPLY TEST
To verify that the power supply is working correctly, connect the 17V line, as indicated in section 2 on this page. Also connect the oscilloscope GND to V- and the oscilloscope probe to
the FET drive (anode of diode 116 ). There should be a waveform at this point. If there is no FET
drive waveform, check the voltages and waveforms on the C5184 pins and compare them to the voltages and waveforms on the schematic. Connect the AC power cord with the AC voltage, from the variable transformer turned down to 0 volts. TAKE CARE NOT TO TOUCH THE OSCILLOSCOPE AND MONITOR CHASSIS DURING THIS TEST, SINCE THE VOLTAGE DIFFERENCE CAN BE AS HIGH AS 400 VOLTS. Connect the DVM to GND and V+. Slowly increase the AC line voltage and observe the power supply response. Do not exceed +145V on V+. If the monitor runs normally, a fault may still exist
in the power supply +17V line circuit. | Note; if | ||
the crowbar zener is shorted and the FET is | |||
internally shorted, the C5184 IC |
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| should also |
115 |
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be replaced. |
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