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HYDRA
Service Manual
5A-16
Lack of outguard-to-inguard communication activity may be due to improper operation
of circuit elements other than A3U9. Using a high input impedance oscilloscope or
timer/counter, check for proper Analog Processor (A3U8) crystal oscillator operation. A
3.84-MHz sine wave (260 ns period) should be present at A3U8 pin 37 with respect to
A3TP9.
Check the A/D Converter voltage reference:
A3TP12 to A3TP11 (across A3C12) = +1.05V (+0.10V, -0.02V)
Setup the instrument to measure ohms on the 300Ω range. Monitor ohms on a channel
with an input of approximately 270Ω. Check that the Analog Processor IC (A3U8) is
making A/D conversions. The integrator output waveform at A3TP13 (referenced to
A3TP9) should resemble the waveform shown in Figure 5A-6.
A3TP13 TO A3TP9
0
1V/DIV 5 mS/DIV
s48f.eps
Figure 5A-6. Integrator Output (2635A)
Check for channel relay operation by setting up a channel and selecting and de-selecting
monitor measurement mode. One or more relays should click each time the monitor
button is pressed or channels are changed.
In general, check that the relays are getting the proper drive pulse signals for specific
functions and channels and that they are in the correct position.
5A-11. DC Volts Troubleshooting
Setup the instrument to measure a specific channel on the 300 mV or 3V range, and
apply an input to that channel. Then trace the HI signal (referenced to the input channel
LO terminal) as described in Table 5A-4.
If the input HI path traces out properly, remove the input from the channel and trace
continuity through the LO path. Check among A3L4-A3L24, A3K1-A3K14, A3R35,
A3R43, and A3U8 pin 11.