Fluke Biomedical 943-35 - Manual (Page 20)

Maintenance, Calibration, and Troubleshooting
Troubleshooting
4
4-5
Preamplifier input is easily damaged. Ground pin 3
of Z1 before applying power and measuring
photomultiplier tube anode or dynode voltage.
3. Using a DVM, measure the total DC resistance of the dynode string. The value should be 9.67
megohms ± 15%. If no reading is obtained, check dynode resistor interstage connections.
4. Connect the cable to the readout device.
5. Turn on channel power and apply 900 Volts to the detector. Measure the anode resistor voltage
(R13) using an electrostatic voltmeter. The value should be 900 Volts.
6. Remove channel power.
If the "Preamplifier Check-out" is to be performed,
omit step 7.
7. Reassemble the detector according to the instructions provided in "Detector Assembly".
Preamplifier Checkout
The high voltage must be removed for this test.
If the "Dynode Test Measurement" was performed,
omit steps 1 and 2.
1. Turn off channel power at the readout device.
2. Disassemble the detector according to the instructions provided in "Detector Disassembly".
3. Remove the grounding jumper from pin 3 of Z1.
4. Connect an oscilloscope to the input circuitry of the readout (50 ohms terminated).
5. Turn on channel power.
6. Using a pulse generator, inject negative pulses of 0.50 Volts amplitude, 1 μs pulse duration, at a
frequency of 1 kHz, to the node of R13 and C6 (+) and ground (-).
7. Output pulses should be -1.6 VDC ± 20% amplitude. If pulses are not present, check Z1 and
associated circuitry.
If the preamplifier and dynode tests are positive, the detector malfunction is probably in the
photomultiplier tube. Remove all power, replace the photomultiplier tube and reassemble the detector
according to the instructions provided in "Detector Assembly".
CAUTION
NOTE
CAUTION
NOTE