Maintenance
7-9
7.9.3 Power supply checks
The Model 708A Switching System power supply will resist
problems affecting conventional power supplies. The power
supply automatically adjusts to a broad range of mains volt-
age (100-240VAC) and frequencies (50-60Hz), so the fuse
should not fail because of an incorrect mains voltage.
If the power supply fails, the instrument will be inoperative.
Power supply input and output voltages may be checked at
the connectors at each end of the board.
Table 7-2 lists the output voltages that should be checked.
NOTE
Programming the 99-IEEE switch set for
99 will turn all LEDs on.
Table 7-2
Power supply checks
Test point
Output voltage
Tolerance
Ripple/noise
J102-1
(Referenced to J102-2)
J102-4
(Referenced to J102-3, as adjustable)
+12.0VDC
+6.0VDC
±10%
±2%
120mV
50mV
Table 7-3
Microcomputer checks
Step
Item/
component
Signal
Comments
1
U101 pin 3
5VDC ±5% (0.5V)
The following mother board signals are referenced to digital common.
2
3
4
5
6
U104 pin 37
U104 pin 34
U103 pin 23
U104 pins 8-23
U104 pins 24-31
MPU Reset
E clock
SELECT VIA
Address bus (A0-A15)
Data bus (D0-D7)
Stays low (110-260ms on power-up), then goes high.
1.79MHz square wave for peripheral chips.
Low going pulses (1kHz).
Check for stuck bit.
Check for stuck bit.
7.9.4 Mother board checks
The procedures to check circuitry on the mother board,
including the microcomputer, relay control circuitry, display
interface, and digital I/O port, are located in Table 7-4
through Table 7-6.
Procedures for testing the light pen and master/slave inter-
face options are listed in Table 7-7 and Table 7-8.
The waveforms described in the troubleshooting tables are
present with power-up default conditions (idle state), unless
otherwise indicated. To view some of the waveforms, run a
program loop. Refer to the troubleshooting programs in Fig-
ure 7-4.
NOTE
If the ROM (U105) must be replaced for
any reason, restore the Model 708A to fac-
tory default conditions with the device-
dependent command R0 (paragraph
5.9.17).
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