Fluke 5720A service manual

Troubleshooting 5

Component-level Troubleshooting

J201. The +PA supply (at +180V)should go into current limit. The oscilloscope should show a repeating pattern in which the +PA supply climbs to about +180V then drops to 0V. If a failure is detected, check Q203, VR246, VR213, U201C, and associated components.

10.Check the unregulated -250V Supply. Using a DMM, measure the dc voltage at TP212 and verify it is -280V ±10%. If a failure is detected, measure the voltage at the anode of VR231 and verify it is-140V ±20%. If this voltage is incorrect, check VR231, VR230, VR229, VR225, and Q209. If the voltage at the anode of VR231 is correct, then check Q215, CR234, CR235, CR239, CR243, and associated components.

11.Check the regulated -PA Supply at 0V. Using a DMM, measure the dc voltage at TP204 and verify it is less than 1.0V. If a failure is detected, check Q212.

12.Check the regulated -PA Supply at -180V. Jumper TP207 to PACOM (TP203), measure the -PA Supply at TP204 and verify it is -190V±10%. If this voltage is correct, skip to step 13, otherwise continue as follows. Measure the output of the Current Limit circuit. Connect a DMM to pin 2 of U201 and verify it is +12±4V. If this voltage is incorrect, then the -PA Supply is in current limit. Check the output for shorts and check the current-limiting circuit contained in U201D, Q211, Q214, and associated components. If the voltage at pin 2 of U201 is correct check Q210, Q216, VR232, VR238, and associated components.

13.Check the unregulated -PA Supply at -360V. Jumper TP207 and TP208 to PACOM (TP203), measure the voltage at TP212 and verify it is-480V ±10%. If a failure is detected, check Q209, VR225, VR229, VR230, CR234, CR235, CR239, CR243, and associated components. Next, check the regulated -PA Supply at TP204 and verify it is -370V±10%. If a failure is detected, check Q210, and VR226.

14.Check the Current Limiting of the -PA Supply. Jumper TP207 to PACOM (TP203)

and remove the jumper connecting TP208 to LO HI I (TP206). Connect an oscilloscope to the -PA Supply at TP204. Connect a 1 kΩ 10W resistor (such as Fluke P/N 157933) between PACOM (TP203) and the -PA supply output at Jumper J202. The -PA supply (at -180V)should go into current limit. The oscilloscope should show a repeating pattern in which the -PA supply climbs to about -180V then drops to 0V. If a failure is detected, check Q214, VR247, VR233, U201D, and associated components.

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