Service manual

106 Replaces page 48
Transient Generator Troubleshooting (Figure 3-7)
The transient generator (U310-U316) allows the input power stages to switch between two load levels. It produces the
HIGH control signal which is sent to the DAC circuits to switch the transient DAC output.
Troubleshooting the transient circuit consists of checking the output of U314-7, the frequency and FSEL inputs to U311, the
trigger inputs to U311 and U314, and the outputs of U312 and U313.
To check the output of U314, turn the transient mode on from the front panel. With the transient mode on, check the
condition of the TRANS_EN signal at test point
according to Table 3-3. Now use a scope and check the output
frequency at U314-7. Because the frequency is determined by the programmed value, return the instrument to the factory
default settings. The factory default frequency should be 1KHz with a 50% duty cycle.
If the frequency at U314-7 is incorrect or nonexistent, check the inputs and output of U311. From the front panel, program
the transient frequencies according to the Frequency Selection Table. Check the output of U311-5. Check the FSEL inputs
at U311 pins 11, 10, and 9.
Also, check the outputs of U312 and U313 at pin 9. With transient mode on, they should output negative going pulses at
intervals equal to the inverse of the programmed frequency. For example, if 1KHz is programmed, U312-9 and U313-9
should generate negative pulses at 1ms intervals. You can use signature analysis to check the SD0-7 data lines into U312
and U313 (see Table 3-12).
To check that the transient generator is being triggered correctly, run the following program. This program continuously
generates 1.5
µs TRIG and TRIG signal pulses at U311-12 and U314-1. If the pulses are not generated, troubleshoot the
trigger circuits as described in the next section.
10 OUTPUT 705;"TRAN:MODE PULS;TWID 1"
20 OUTPUT 705;"TRIG:SOUR BUS''
30 OUTPUT 705;"TRAN ON"
40 OUTPUT 705;''*TRG"
50 ! PAUSE
60 GOTO 10
70 END
Note: With the PAUSE line included in the program, a trigger is generated each time you press CONTINUE.
Frequency Selection (FSEL) Table
Frequency @ FSEL
50% Duty Cycle 0 1 2 U311-5
1KHz LO LO LO 500KHz
500Hz HI LO LO 62.5KHz
100Hz LO HI LO 15.6KHz
10Hz HI HI LO 3.9KHz
5Hz LO LO HI 976Hz
1Hz HI LO HI 244Hz
0.25HZ LO HI HI 61Hz