Diagnostic Testing and Troubleshooting (2635A) 5A

Analog Troubleshooting

Table 5A-3. Power Supply Troubleshooting Guide (2635A) (cont)

Symptom

A1U18 hot.

A1U18 oscillates.

A1U19 oscillates.

A1U19 very hot.

A1U19 hot.

Fault

Shorted A1C32

Open A1C32.

Open A1C34.

-Shorted A1U22 (VCC to VSS).

-Shorted A1U23 (VCC to VSS).

Shorted A1C34.

Check the inguard supply voltages on the A/D Converter PCA with respect to A3TP9. The following table lists the components nearest the power supply test points.

Power Supply

Nearest Component

Acceptable Range

VDD

A3C8

5.00 to 5.70V dc

VSS

A3C9

-5.10 to -5.75V dc

VDDR

A3C19

5.30 to 5.95V dc

+VAC

A3CR1

4.7 to 5.7V dc

-VAC

A3C26

-4.8 to -5.7V dc

Check that the inguard Microcontroller A3U9 RESET* line is de-asserted. Check VDD at A3TP1, referenced to A3TP9.

Check that the microcontroller crystal oscillator is running. When measured with a high input impedance oscilloscope or timer/counter, the oscillator output at A3TP10 should be a 3.6864-MHz sine wave (271.3 ns period), and the divided-down E clock output at A3U9 pin 68 should be a 921.6 kHz-square wave (1.085 μs period).

Check outguard to inguard communication. Setup an input channel and enable monitor measurements on that channel, causing the outguard to transmit to the inguard approximately every 10 seconds.

On the Main PCA, look for outguard-to-inguard communication (5.0V (VCC) to near 0V pulses) at A1TP15, referenced to A1TP1. On the A/D Converter PCA, check for 5.35V (VDD) to near 0V pulses at A3TP8, referenced to A3TP9.

At the start of outguard-to-inguard communication, the A/D Microcontroller (A3U9) should be RESET. Check for this reset pulse (5.35V (VDD) to near 0V, lasting approximately 1 millisecond) on A3TP1 with respect to A3TP9.

Check for the following inguard-to-outguard communication activity:

PCA

Test Point

To

Pulses

A/D Converter

A3TP7

A3TP9

5.55V (VDDR) to 0.7V

Main

A1TP8

A1TP1

5.0V dc (VCC) to 0.0V

5A-13

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Fluke 2625A, 2635A service manual Power Supply Nearest Component Acceptable Range, Test Point Pulses
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2635A, 2625A specifications

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