95
Unit Operation Checks
9. PCB ASSY, IG CTRL-A
Outline
IG CTRL-A lights the ion-gauge measurement bulb (IG bulb) filament and generates the grid
voltages. The MAIN CTRL-A PCB offers the functions to measure the collector current to
determine the degree of vacuum. When the filament lights, thermoelectrons are
discharged toward a grid held at a potential of +150V, resulting in a current from the grid to
the filament (grid current). As the filament is lit by a power supply with a floating potential,
the grid current flows toward earth potential in the IG CTRL-A circuit. This circuit adjusts
the current flowing through the filament to maintain a 1mA grid current flow to earth potential.
The grid current flows through a Zener diode, which increases the filament potential above
earth potential by the amount (Zener voltage + voltage drop in the current detector
resistance), giving a potential of +25V at a 1mA grid current. The degree of vacuum is
determined from the amount of positive ions captured at the earth-potential collector
electrode.
Operation Check Method
If an abnormality occurs, follow the procedures below to check the operation of the IG
CTRL-A PCB to efficiently diagnose the problem.
In case of abnormal degree of vacuum measured values, see the item Checking the
ion-gauge vacuum measurement circuit for the MAIN CTRL-A PCB (page 62).
Caution
A voltage over +150VDC is supplied to the PCB. To avoid electric shocks or damage
to the assembly, touch only the points instructed.
1. Checking the installation
One of the four screws locating the IG CTRL-A PCB acts to earth the PCB to the chassis.
Current flows may be interrupted if this screw is loose. Be sure to correctly tighten these
screws.
Check there is no looseness in the connector socket contacts where the IG bulb pins are
inserted.
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