HYDRA

Service Manual

Table 5-3. Power Supply Troubleshooting Guide (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 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 (5.1V (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-ms) 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

0V dc to 5.1V (VCC)

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 5-6.

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.

5-14

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Fluke 2635A, 2625A service manual Test Point Pulses
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2635A, 2625A specifications

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