Cata~trophic failures generally occur while the instrument is up and running. Failures of this type cause the instrument lights to go out and the line fuse to blow. These symptoms are sometimes accompanied by a burning odor or popping noises.
Catastrophic failures most often originate in the high-voltage switching circuit of the main inverter. This cir-cuit is located on the Inverter Board, which is one of three plug-in circuit boards in the 8560 power supply.
In the following inspection and repair procedure, the most likely causes of failure are investigated first.
1. Make sure the 8560 is disconnected from the primary power source.
WARNING
I
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.
Troubleshooting Guide
2. To gain access to the Inverter Board, remove the fan housing at the rear of the 8560. Remove the two screws closest to the rear panel on the housing's left side, and the six screws that secure the housing to the rear panel. Disconnect the two wires connected to the fan and set the fan housing aside.
3. Three plug-in circuit boards are now visible through the opening that the fan housing normally covers. The In-verter Board is the lowest of these three boards. To remove the Inverter Board, grasp the left side of the board firmly with a pair of insulated needle-nose pliers (see Fig. 12-3), and pull the board out.
I
WARNINGI
Do not touch the metal on the pliers; high voltage may be present on the board while it is installed.
Keep away from grounded objects while pulling the board out. After removing the board, allow five seconds for the capaCitors on the board to dis-charge before handling the board.
I
WARNINGI
Whenever the 8560 is connected to
a
power source and the unit's power switch is on, use caution in the area of the Inverter Board. A 200 Vdc potential exists on the screw in the left-hand corner of the board. Refer to Fig. 12-4 for the location of this screw. The same 200 Vdc potential may exist on the shield below the board if the insulating coating on the shield is damaged. Damaged shields should be replaced immediately.12-3
Troubleshooting Guide Power Supply-8560/8561/8562 Service
L 2071
f---,
D
T 3051~
~
@
T 4051
- D 4)"---J} C~~ 101
-c=>-~---~---~
Pull here
4759-29
Fig. 12-3. The Inverter Board.
Fig. 12-4. The Inverter Board-200 Vdc.
4. Notice the finned heatsink on the left side of the In-verter Board. The two transistors (03010 and 02010) on the finned heatsink are the main inverter's high-voltage switching components. Look for obvious signs of failure, such as cracked transistor cases or charred circuit board.
12-4
NOTE
While performing the rest of the procedure, refer to the power supply Inverter Board schematics at the rear of this manual.
5. If the failure appears to be in the main inverter, the following components (listed in descending order of probability) may have failed:
• High-voltage switching transistors, 03010 and 02010
• Turn-off capacitors, C4019 and C1019
• Turn-off transistors, 04011 and 01010
• Drive transistors, 05074 and 05075
• Bleeder resistors, R4014 and R1016
If either 03010 or 02010 is obviously damaged, re-move the bad transistor(s). Be careful when removing these transistors. The two screws holding down the collector tabs of these transistors are the only secure mechanical connection between the heatsink assembly and the circuit board.
Even if 03010 or 0201
o
appear undamaged, check the components in-circuit with any diode meter having a 1 mA measuring current. Forward base-collector and base-emitter voltages for good transistors are between 400 mV and 600 mV. See Fig. 12-5 for pinouts. Re-move any transistor that shows a bad junction voltage.Power Supply-8560/8561/8562 Service
After 03010 and 02010 have been removed, all of the other components listed at the beginning of step 5 can be checked with the diode meter. The capacitors should read between 1300 mV and 1600 mV in either direction, although a resistor failure or turn-off transis-tor failure could affect these readings. The turn-off tran-sistors should show forward junction voltages between 400 mV and 600 mV. The bleeder resistors should read between 900 mV and 1100 mV in either direction. The drive transistors should eventually read between 400 mV and 600 mV when the positive lead of the meter is connected to the source and the negative lead is
con--15V +Input -Input -15V
o
+15VOutput Bypass Hysteresis 156-1126-01 U2010
(on Regulator Board)
~ 0 (
BeE
151-0679-00 03010 02010 151-0625-00 04011 01010 (on Inverter Board) 151-0625-00 02063
02073 (on Secondary Board)
Troubleshooting Guide
nected to the drain. With the leads reversed, the read-ing should eventually rise to more than 1999 mV.
("Eventually" means that the meter may take a few sec-onds to charge up the capacitors in parallel with the component being measured.)
Replace any of the components that seem to be faulty.
Rebuild the heatsink assembly. (Make sure all parts are present, including the plastic inserts and rectangular aluminum washers.) Make sure there is heatsink com-pound under the transistors.
6. Proceed to the bring-up procedure, later in this section.
0
1 r
E B C 151-0439-00 151-0364-00
03102 02071 (on Secondary Board)
G 0 S 151-1127-00 151-1128'-00
05074 05075 05076 (on Inverter Board)
(3900-64)4759-31
Fig. 12-5. Semiconductor component pinouts.
12-5
Troubleshooting Guide