Monroe Electronics Electrostatic Voltmeter - Isoprobe® - model 279L - Manual (Page 14)

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E. Probe Oscillator Adjustment:
Connect an oscilloscope probe to TP5 and adjust R53 for a 6 Vp-p sine wave.
F. Demodulator Phase Adjustment:
1. Move the oscilloscope probe to TP3 and adjust the ZERO 1 pot on the front panel for a null (straight line) on
the oscilloscope.
2. Apply +10V to the test plate and adjust R65 for the best representation of a negative going half-wave
rectified sine wave with an amplitude of about 0.2V.
3. Switch the test plate voltage to –10V and the oscilloscope should display a positive going half wave.
G. FET String Bias Adjustment:
1. Switch off the rear panel power switch.
2. Remove JP4.
3. Tie TP8 to TP6.
4. Connect a DVM acrossR309 of the 279L/71 board.
5. Connect P300. WARNING! When the power is on, high voltage will be present on the high voltage
rectifier.
6. Switch on the rear panel power switch.
7. Adjust R55 for 40mV on the DVM.
8. Move the DVM from R309 to R311.
9. Adjust R56 to match the N-FET bias reading. Switch off the rear panel power switch.
H. High Voltage Adjustment:
1. Remove the ground from TP8.
2. Remove the jumper at JP8.
3. Replace jumper JP4.
4. Connect the DVM from chassis ground to end of R303 nearest the edge of the 279L/71 board and the DVM
should read about +390V.
5. Move the DVM input lead to the end of R314 nearest the edge of the 279L/71 board and DVM should read
about -390V.
6. Remove the DVM.
I. Speed of Response:
1. Set the gain pot, front panel recessed, for about midrange.
2. Connect the oscilloscope to the rear panel output connector.
3. With the 279L in standby, STBY/OPER switch out, check the range of the ZERO 2 pot on the front panel
meter. It should swing plus and minus with a total of about 10V. Zero the front panel meter with ZERO 2.
4. Push the operate switch and then zero the front panel meter with the ZERO 1 pot on the front panel.
5. Apply a repetitive +300V pulse to the test plate.
6. Adjust the gain pot, the integrator gain pot (R54) and the oscilloscope controls until a critically damped step
response between 0% and 100% is observed. Rise and fall times from 10% to 90% should be typically
5ms.