The electrical connections between the power supply and the CRT
are shown in the figure below.
The leads from the electron-gun section of the tube are color
coded for identification. The wires from the deflection
tion plates are like TV antenna wire (flat, brown, twin leads). The
distinction between horizontal and vertical plates is lost since the
tube is easily turned about its long axis. However, one set of
plates is much closer to the screen than the other. The current to
the heater (source of electrons) comes from the
6.3 V AC supply at
the terminals H-H of the power supply. The voltages to the
accelerating and
focusing electrodes are provided by the 'B' and 'C'
terminals of the power supply. (The letters come from the old days
of battery-operated, vacuum-tube radios which required
three batteries labelled
A, B, and C.) The connection between 
and 
is already
made inside the power supply and so you don't need to include
that wire. The 'B' side of the supply is the most powerful, and is
able to deliver 100 mA at 400 V DC.
NOTE: The voltages and currents delivered by the
supply can give a nasty shock. Please make sure
that the voltage controls are turned down before
touching any un-insulated parts of the circuit!
The 'C' side (negative supply) will deliver only 1 mA at
V DC.
Any attempt to draw current from it will cause the voltage to drop
and might damage that side of the supply. The currents drawn from
the supply by the electrodes are almost zero. Even though the 'C'
side is a negative voltage supply, it is connected so that the total
accelerating voltage 
is the sum of the absolute values of 
and 
:
is greater than either 
or 
.
The control grid (
) voltage is held slightly negative by
means of a biasing battery. You are given a 1.5 V battery for this
purpose. Be sure to hook it up with the polarity as shown. This
voltage controls the beam current and hence the brightness of the
spot on the screen. You may want to try different voltages to
observe the effect.
The connections to the deflection plates are made on a separate
connector board. One of each of the plates must be grounded to the
terminal
(black wire). The ground connection permits any accumulated
charge on the plates to drain off. If they are ungrounded,
the charge buildup will cause the plates to rise to a very high
voltage and a large beam deflection will result. That is, the beam
will disappear. This does not harm the tube, nor is it dangerous,
it's just a little disconcerting not to have a beam spot.
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Voltage is applied to the plates using the voltage supply shown
in the figure at right.
The circuit gives voltages in the range from 
through zero volts to 
. The voltage is varied
by turning the
potentiometer shaft, with zero volts given when it is about at the
midpoint. The terminals of the supply are the center tap (middle
connection) of the potentiometer and the common connection between
the two batteries. A voltmeter, 
, between these terminals
indicates
the deflection voltage. The supply can be connected to the
plates with either of its terminals at ground potential. However,
it is common practice to say that positive deflection voltages give
either an upward or rightward deflections. The voltmeter, 
,
must be reversed in the circuit to measure negative voltages.
To start, ground all four deflection plates (1 to 2 and 3 to 4)
and turn on the power supply to the STANDBY position. This sends
current to the heaters, but not to the B and C supplies. Notice the
red glow from the heater behind the cathode. After a minute turn the
supply switch to the ON position. Turn the B supply voltage up to
about 350 V and then raise the C supply voltage until the spot is
visible and adjust it to give the best focus. Vary both supply
voltages, B and C, to see that the spot can be focused for different
combinations of 
and 
. Each combination of
and 
gives a different total accelerating voltage.
Due to the magnetic field of the earth, the electron beam may be deflected enough to cause it to hit one of the deflection plates, thus preventing it from reaching the screen. If this happens, use the weak bar magnet supplied and place it on the table or on the wooden support or somewhere and in some position so that the beam is reasonably well centered on the screen. This may take some fiddling to find a satisfactory position for the magnet. When taking data later on, be sure that the bar magnet doesn't move during the measurments.
On the power supply, the left-hand meter indicates the output voltage of either the B or the C supply depending on the position of the meter switch. However, you must read the meter using different scales for the two supplies. When the meter switch is pushed to the left to measure the C output voltage, read the meter using the lower 0-150 scale: when the switch is set to the right to measure the B supply voltage, use the upper 0-400 scale. The right-hand meter indicates only the current being drawn from the B supply; there is no meter to measure the C supply current since it should always be near zero. Note that the meter switch does not affect the output voltage of either supply, it only determines which one is connected to the meter.