Tektronix TYPE CA Instruction Manual page 12

by the MODE switch. With the MODE switch
in the A ONLY or B ONLY
multivibrator is held in one of its two possible
states by returning one grid to a positive
voltage and the other grid to a negative voltage.
For example, in the A ONLY position the grid
of V3375A is held positive and this half of the
multivibrator conducts while the grid ofV3375B
is held negative and this half is cut off. When
V3375A is conducting the cathode is above
ground which causes V3384B to conduct and it
in turn pulls the grid of V3393B toward ground
lowering the plate voltage of V4334 and V4354.
This reduced plate voltage cuts off the following
stage (V4364 and V4374) and the B-channel
amplifier is held in a non-conducting state.
The converse is true of the A-channel amplifier.
The grid of V3384A is near ground potential
with reduced plate current, therefore, the plate
of V3384A and consequently the grid of V3393A
are permitted to become more positive thus
providing plate voltage for V3334 and V3354.
The A-channel
ALTERNATE
Turning the MODE switch to the ALTERNATE
position returns both grids of the multivibrator
to a negative potential. It is then a bistable
multivibrator. At the end of each sweep cycle
a negative-going trigger is generated and is
coupled to the multivibrator through the Trigger
Coupling Diode V3382. Each trigger causes
the multivibrator to " flip" from one stable
state to the other. This alternately switches
the amplifiers on and off. The switching rate
is now determined by the repetition rate of
the sweep.
CHOPPED
Turning the MODE switch to the CHOPPED
3-2
position the
then conducts.
Circuit Description - Type CA
position returns both grids of the multivibrator
to a positive voltage and the multivibrator
free runs at a rate determined by the time
constant of the grid circuits. The two amplifiers
are alternately cut off and allowed to conduct
at the free-running rate of the multivibrator.
ADDED ALGEBRAICALLY
Turning the MODE switch to the ADDED
ALGEBRAICALLY position returns both grids
of the multivibrator to a. negative voltage. Both
sides of the multivibrator (V3375) are held
sufficiently negative so that incoming triggers
have no effect on the multivibrator grids.
The cathodes of both halves of the multivibrator
follow the grids down, driving V3384A and
V3384B to cut off. With V3384A and V3384B
cut off the plate voltage rises, carrying the
grids of the following stages, V3393 A and V3393B
with it. The cathodes of V3393A and V3393B
follow the grids up. When the cathodes are
up, both amplifier channels conduct equally
in the absence of any signal.
Note
Plate voltage for the input ampli ­
fier stages of both channels is
supplied by the cathodes of either
V3393A or V3393B.
Under the conditions described above signals
applied to both inputs will be amplified equally
by either channel. Algebraic addition of the
signal occurs at the grids of the output stage,
V4383. In phase input signals add, out of
phase input signals subtract, at the grid of
each tube if the polarity switches are at the
same setting.
AA