5A-5. Power Supply Troubleshooting

HYDRA

Service Manual

Refer to the Schematic Diagrams in Section 8 during the following troubleshooting instructions. Also, these diagrams are useful in troubleshooting circuits not specifically covered here.

Warning

To avoid electric shock, disconnect all channel inputs from the instrument before performing any troubleshooting operations.

5A-6. Raw DC Supply

With the instrument connected to line power (120V ac, 60 Hz) and turned ON, check for approximately 14V dc between A1TP1 (GND) and the "+" terminal of capacitor A1C7 (or the cathode of either A1CR2 or A1CR3). (This voltage is approximately 30V dc at 240V ac line.) If no voltage or a very low voltage is present, check for approximately 24V ac across the secondary of the power transformer (or approximately 50V ac at 240V ac line).

The voltage at the output of A1U19 (also A1TP7), should be about +5.3V dc. At 120V ac, 60-Hz line power input, the Hydra Databucket line current is approximately 24 mA. At 50-Hz, 120V ac line power input, there is a 5 to 10% increase in this current figure.

5A-7. Power Fail Detection

The Power Fail Detection circuit monitors the Raw Supply so that the Microprocessor can be signaled when power is failing. A reference voltage of nominally 1.3 volts dc (internal to A1U10) is compared to the voltage at A1U10-4. If A1U10-4 is less than about 1.3 volts dc, the power fail output (A1U10-5) should be low. This corresponds to a raw supply voltage of about 8 volts dc (A1C7). If the raw supply voltage is greater than 8 volts dc, the power fail output (A1U10-5) should be high. If the power fail output is near 0V dc during normal operation, the Microprocessor will sense that power is failing and will not be able to complete a scan operation.

5A-8. 5-Volt Switching Supply

Use an oscilloscope to troubleshoot the 5-volt switching supply. With the oscilloscope common connected to A1TP1, check the waveform at either A1U9, pin 4 or A1T1, pin 2 to determine the loading on the 5-volt switching supply. The output voltage of the 5-volt switching supply at A1TP2 (VCC) is normally about 5.0V dc with respect to A1TP1 (GND).

∙Normal Load:

The signal at A1U9-4 (with respect to A1TP1) is a square wave with a period of 9 μs to 11 μs and an ON (voltage is low) duty ratio of about 0.38 with the line voltage at 120V ac. The amplitude is usually about 15V p-p. The positive-going edge will be "fuzzy" as the duty ratio is varying to compensate for the ripple of the raw supply and the pulsing load of the inverter supply. See Figure 5A-2 (NORMAL LOAD).

∙Very Heavy Load or 5V Supply Shorted:

Under heavy load (example: A3 A/D Converter PCA has a short circuit) it could load down the power supply voltage such that the current limiting feature is folding the supply back. For example, if the supply is folded back due to excessive current draw, unplug the ribbon cable at A3J10 on the A/D Converter PCA. When tracking down power supply loads, use a sensitive voltmeter and look for resistive drops across filter chokes, low value decoupling resistors, and circuit traces. Also check for devices that are too warm. On the A3 A/D Converter PCA, all devices run cool except A3U5 microprocessor and A3U8 FPGA, which run warm, but not hot.

5A-8

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Fluke 2635A 5A-5. Power Supply Troubleshooting, 5A-6. Raw DC Supply, 5A-7. Power Fail Detection

Models: 2635A 2625A

HYDRA