The 8560 power supply has many different kinds of fault 'Catastrophic Failures," earlier in this section.
• Your 8560 is configured for a 115 V line, but is con-nected to a 230 V line. See "Selecting the Proper Pri-mary Voltage" in Section 19 of this manual.
If the AC ON light is illuminated but the 8560 doesn't power up when you turn on the front panel DC switch, the about a second. This delay is entirely normal.
Procedure.
Use extreme caution once you have removed the power supply top shield. Dangerous voltages are present underneath the top shield.
WARNING
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Be careful to keep fingers and other objects out of the power supply fan blades. The fan housing has no protective covering from the inside of the supply.
Remove the four screws that secure the shield and lift the shield off.
2. If you can hear a faint chirping sound coming from the supply about once every second, proceed to step 3.
If you cannot hear any chirping, measure the voltage at the PONPS testpoint (TP7048) and at PON on the + 15 V testpoint (TP7076). The voltage should be correct within +/- 5 percent.)
b. If only PONPS is incorrect, check the shutdown testpoint (TP7046). If the testpoint voltage is above 11 V, check the yellow LED on the Regula-tor Board, which is visible through the fan. If the LED is lit, the overtemperature protection has tripped. To reset it, turn off the rear power switch and allow the instrument to cool down. Then apply power again. When the supply comes on, check the fan for proper operation. If the LED is not lit, proceed to step 3. The supply is probably chirping as previously described. You may not be able to hear this sound if you are in a noisy work area.
Power Supply-8560/8561/8562 Service
3. Turn off the 8560 at the rear panel power switch.
I
WARNINGI
Use insulated tools and probes at all times and ob-serve the supplementary cautions contained in the text. Stored charge in the large capacitors of the power supply can cause severe burns and/or elec-tric shock even when the supply is turned off.
4. Locate the fan housing at the rear of the instrument.
Remove the two screws (closest to the rear panel) on the fan housing's left side and the six screws that se-cure it to the rear panel. Disconnect the two wires con-nected to the fan and set the fan housing aside.
5. Notice the three plug-in circuit boards in the rear open-ing. From top to bottom these boards are: the Regula-tor Board, the Secondary Board, and the Inverter Board.
WARNING
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Dangerous voltages are present on the Secondary Board even when the front panel power switch is turned off. Line voltage is connected to the side of the large transformer (TtOll) that faces the Inter-connect Board.
6. Remove the Regulator Board and Secondary Board from their slots. Insert the Secondary Board in the top slot, and the Regulator Board in the second slot.
7. Connect the external trigger of an oscilloscope to the shutdown testpoint on the Regulator Board (TP7046).
The scope's sweep rate should be set to 10 ms/div
Troubleshooting Guide
initially. The triggering should be set to normal mode, dc coupling, external source, positive level, and nega-tive slope.
8. Turn on the 8560 with the rear panel power switch. If the front panel switch is also on, the supply should chirp faintly as before. A single trace should be dis-played on the scope with every chirp.
If you determine that the shutdown testpoint is not pro-viding a trigger signal but is staying above 11 V at all times, three possible causes exist:
• The undervoltage shutdown circuitry (U5040, U3040A) is faulty.
• The 8560 is configured for a 230 V line but is con-nected to a 115 V line. See "Selecting the Proper Primary Voltage" in Section 19 of this manual.
• If the 8560 was running for some time prior to failure, the supply's overtemperature protection may have tripped. See step 2(b) in this procedure.
9. Set the scope's vertical sensitivity to 5 V/cm. Connect a probe from the vertical input of the scope to TP7046.
The two traces shown in Fig. 12-6 illustrate the range of expected waveforms. If the waveform matches Fig.
12-6b, reduce the sweep speed until event Y (shown in Fig. 12-6a) is visible. Note the position of event Y on the screen. Event Y occurs when the supply shuts down.
WARNING
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Use extreme caution when servicing the Inverter Board. Dangerous voltages exist on the Inverter Board when the 8560 is turned on.
3900-65 Fig. 12-6. Waveforms at shutdown testpoint.
12-7
Troubleshooting Guide
10. Connect the scope probe to the 15K resistor R5084 on the Inverter Board on the opposite side of the resistor from the connection to TP5085. You should see a 10 V negative-going spike from + 15 V at the same position as event Y in Fig. 12-6a. You may have to increase the intensity of the display for the spike to become readily visible.
11. Connect the probe to TP5085.
• If the spike amplitude is the same as in step 10, proceed to step 11.
• If the spike amplitude is greater than in step 10, the primary current limit is being activated. A fault may exist in the main inverter control circuitry, or a secondary winding of power transformer T1011 on the Secondary Board may be shorted.
12. Test for an overvoltage problem on the supply outputs by connecting the scope probe to each of the following testpoints on the Regulator Board: +5 V Sense (TP7061), + 12 V Sense (TP7043), -12 V (TP7015),
+15 V (TP7076), and +24 V (TP7057).
a. If the voltage during event Y at any of these testpoints is greater than 105 percent of the desig-nated testpoint voltage, the supply is probably be-ing shut down by the overvoltage protection circuit (U2110A on the Secondary Board).
b. If the overvoltage is on the 5 V line, the reference voltage Vref (TP7066) may be set too high. Check Vref by measuring the voltage between TP7066
13. Check for a short or an overload on one of the supply's dc outputs if an overvoltage does not appear to be the problem. You can accomplish this by connecting the scope's vertical input differentially across the capac-itors listed in Table 12-1. The setup for this connection is accomplished as follows:
In order to differentially connect across the +5 V and + 12 V capacitors, connect a separate scope channel to each end of the capacitor you want to look at. Then, set the scope's controls so that the difference between the two channels is displayed. To connect across the
12-8
Power Supply-8560/8561/8562 Service
+ 12 V and -12 V capacitors, the same method can be used if your scope has a linear range of 130 cm at a vertical sensitivity of 100 mV/cm. Otherwise, you must use a differential probe, such as the TEKTRONIX
12-6a, the corresponding output is being overloaded.
DC Output
Troubleshooting the
+
12 V Secondary RegulatorThe output of the + 12 V secondary regulator is determined by a switching regulator that is slaved to the +5 V regula-tor. The regulating elements are the two paralleled switching transistors (02063 and 02073). These are driven by a com-parator (U2010) through a complementary emitter follower (01057, 02071). The comparator generates a switching sig-nal through comparison of Vref with the actual output volt-age. All of these components are on the Secondary Board, except for U2010 on the Regulator Board.
The two most common symptoms of failure in this circuit are overvoltage on the output (which causes the supply to shut down), and insufficient voltage (usually less than half the proper value, even with light loads).
Output overvoltage is usually due to a collector-emitter short in one of the switching transistors. This short effec-tively removes the regulator from the circuit. Insufficient voltage on the output is usually due to an open PNP driver transistor or a comparator that has failed. Refer to Fig.
12-5.
Power Supply-8560/8561/8562 Service
Procedure.
1. Make sure the 8560 is disconnected from the primary power source.
WARNING
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Use insulated tools and probes at all times and ob-serve the supplementary cautions contained in the text. Stored charge in the large capacitors of the power supply can cause severe burns and/or elec-tric shock even when the supply is turned off.
2. Remove the fan housing at the rear of the 8560 to gain access to the Secondary Board. Remove the two screws closest to the rear panel on the housing's left side and the six screws securing the housing to the rear panel. Disconnect the two wires connected to the fan and set the fan housing aside.
3. Notice the three plug-in circuit boards in the fan hous-ing openhous-ing. The Secondary Board is the middle board.
To remove the board, grasp firmly with a pair of pliers and pull the board out.
4. Check the forward base-collector and base-emitter voltages of 02063 and 02073 with a diode meter.
These voltages should be between 400 mV and 600 mV. (Since the transistors are paralleled, only two mea-surements need be made.)
5. Check for a collector-emitter short by connecting the positive lead of the diode meter to an emitter and the negative lead to the collectors. The reading should be more than 1999 mV for a good transistor.
6. If the switching transistor is good, check the two driver transistors in the same way. Reverse the meter leads when checking for a collector-emitter short in the NPN driver transistor.
7. If the driver transistors are good, check the comparator while the supply is running. A failure of this component usually shows up in one of two ways:
• The output may be incompatible with the inputs (for example, the positive input is above the nega-tive input, but the output is low). components are listed in descending order of probability. All listed components are located on the Secondary Board.
Troubleshooting Guide
Table 12-2
-12 V and +24 V Secondary Regulator Faults Suspect purpose of reducing conducted EMI. The following proce-dure is used to test the EMI filter amplifier.
To test the EMI filter, you need the following equipment:
Oscilloscope (465 or equivalent) Power supply (PS 503 or equivalent) Generator (FG 502 or equivalent)
Procedure
With tl:le Inverter Board out of the instrument, check the EMI filter amplifier as follows:
1. Set the PS 503 to 33 Vdc (+/- 5 percent). Connect the negative lead of the supply to pins 31 and 32 of the Inverter Board. Connect the positive lead to pin 36.
2. Set the FG 502 for a 50 mV 10KHz sinewave and connect it to pin 35 of the inverter edge connector.
3. Check for a 1.5 V (+/-200 mY) signal at the cathode of CR1058 on the Inverter Board. Increase the frequency of the generator to 500 KHz. Check that there are no major peaks or valleys in the waveform (.25 V). The 3 db point of the EMI filter is 500 KHz (-50 KHz, +300 KHz).