Fluke 5720A 3801 A8 8255 Port A Fault, 3802 A8 8255 Port B Fault, 3803 A8 8255 Port C Fault

5700A/5720A Series II Calibrator

Service Manual

3801 A8: 8255 Port A Fault

The diagnostics software writes and reads to port A of the 8255 IC on the Switch Matrix assembly. A failure indicates the Switch Matrix assembly (A8) is probably at fault.

3802 A8: 8255 Port B Fault

The diagnostics software reads port B of the 8255 IC on the Switch Matrix assembly. A failure indicates the Switch Matrix assembly (A8) is probably at fault.

3803 A8: 8255 Port C Fault

The diagnostics software reads port C of the 8255 IC on the Switch Matrix assembly. A failure indicates the Switch Matrix assembly (A8) is probably at fault.

3804 A8: Zero Amp Lo Noise Fault

The Internal Cal Zero Amplifier circuit on the Switch Matrix assembly is tested for noise in the low gain configuration. This is done by connecting the circuit input to RCOM, by turning on Q7, and connecting the output to the RCL line where it is measured by the adc circuit on the DAC assembly. The adc circuit takes ten readings and this failure occurs if the difference between the lowest value and highest value of the ten readings is over 0.0001V. A failure indicates the Switch Matrix assembly (A8) is probably at fault.

3805 A8: Zero Amp Lo Offset

The Internal Cal Zero Amplifier circuit on the Switch Matrix assembly is tested for excessive offset in the low gain configuration. This is done by connecting the circuit input to RCOM, by turning on Q7, and connecting the output to the RCL line where it is measured by the adc circuit on the DAC assembly. The adc circuit takes ten readings and this failure occurs if the average value is over 0.035V. A failure indicates the Switch Matrix (A8) assembly is probably at fault.

3806 A8: Zero Amp Lo Gain Fault

The gain of the Internal Cal Zero Amplifier circuit in the low gain (10) configuration is tested. The circuit input is connected to the output of the DAC assembly and the circuit output is connected to the RCL line. The RCL line is connected to the +input of the adc circuit and the -input is connected to the 6.5V reference. The gain of the Internal Cal Zero Amplifier is determined by setting the DAC output to the nominal 0.65V and then adjusted it until a null is achieved. The gain is tested to be 10 ±6%. A failure indicates the Switch Matrix (A8) assembly is probably at fault.

3807 A8: Zero Amp Hi Noise Fault

The Internal Cal Zero Amplifier circuit on the Switch Matrix assembly is tested for noise in the high gain configuration. This is done by connecting the circuit input to RCOM, by turning on Q7, and connecting the output to the RCL line where it is measured by the adc circuit on the DAC assembly. The adc circuit takes ten readings and this failure occurs if the difference between the lowest value and highest value of the ten readings is over 0.0015V. A failure indicates the Switch Matrix (A8) assembly is probably at fault.

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