59 Analog Troubleshooting

Analog circuit problems are evidenced by Error 8 or Error 9 in the display. These errors signify either that the Main Processor (A1U6) is not communicating with the in-guard circuitry or that the Analog Measurement Processor (A1U1) is not functioning correctly.

First, check the in-guard power supplies referenced to common:

Power Supply Testpoint Range

VDD A1TP10 4.95 to 5.45 V dc

Check the out-guard to in-guard communication for activity whenever a front panel button is pressed. If necessary, press a button repeatedly while looking for the following communication activity:

A1TP8 = GROUND to VCC pulses

A1TP4 = COMMON to VDD pulses

Check the in-guard to out-guard communication:

A1TP5 = VDD to 0.7 V above COMMON pulses

A1TP7 = GROUND to VCC pulses

Check the crystal oscillator referenced to COMMON:

A1TP3 = a 3.84 MHz sine wave (260 ns period)

Check the integrator waveform referenced to COMMON:

A1U1-45 should be a triangle wave when an input is being measured.

Check the 1.1 V reference:

In general, check that the relays are getting the proper drive signals and that they are in the correct position.

The 10 A current shunt (A1R3) or the mA current shunt (A1R2) can be used as a convenient COMMON test point.

5-10. Uart Test

This test checks both the optoisolators (A1U3 and A1U4) and the Analog Measurement Processor (A1U1) UART circuitry. The Main Processor (A1U6) first sends a break signal (5 V dc, logic high) to A1U1, then waits for a break signal in response from A1U1. Once this occurs, A1U6 commands A1U1 to remove its break signal response.

To initiate the UART test, hold the (RATE) button down for three seconds. If the test passes, "PASS" is shown in the secondary display. If the test fails, "FAIL" is shown in the secondary display. The test is run repeatedly, updating the "PASS" or "FAIL" display each time. An oscilloscope can thereby be used to trace the rectangular wave forms across optoisolators A1U3 and A1U4.

To exit the UART Test, turn the meter off, wait three seconds, then turn the meter back on.

5-11. DC Volts Troubleshooting

Put the meter in the 300 mV or 3 V range, and apply an input. Then trace this signal as described in Table 5-3.

5-12. AC Volts Troubleshooting

Apply a signal with the multimeter set for the 300 mV ac range. Then trace this signal as described in Table 5-4.

5-13. Ohms Troubleshooting

Use a meter with high input impedance to measure the open circuit voltage for each ohms range listed below. If a high input impedance meter is not available, the following checks can be made on the 30 kW and lower ranges only.


300 MW 3 V

With failures with these tests, suspect components are A1R5, A1RT1, A1K2, A1Q1, A1Z1, and A1U1.

Now check the signal path at the following points: W1, A1U1-23 (OVS), A1U1-58 (AFI), and A1U1-56 (AFO). Suspect components for these checks are A1R6, A1R7, A1K1, and A1U1.

Table 5-3. DC Volts Troubleshooting



Possible Fault

A1U1-23 (OVS) A1U1-58 (AFI) A1U1-56 (AFO) A/D low path

Input signal

Input signal, active filter input

Input signal, active filter output

Check continuity among A1U1-13, RRS, and



A1R9, A1K2

Table 5-4. AC Volts Troubleshooting



Possible Fault


Input signal

A1R5, A1R5, A1C1, A1K3

A1U1-2 (ACBO)

Amplified input (input x 2.5)

A1U1, A1R11-A1R14, A1Q2-A1Q8, A1Z2, A1AR1, A1C7, A1R16, A1R17

A1U1-68 (RMSI)

Amplified input



Amplified input



DC equivalent of amplified input


A1U1-61 (RMSF)

DC equivalent of original input

A1Z4, A1R19, A1C10

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