Heathkit IO-4550 - Page 18

Page
16
:
[eg
=emarersrr’]
CIRCUIT DESCRIPTION
Refer to the Block Diagram and the Schematic Diagram (in
the “Illustration Booklet”) as you read this “Circuit
Description.”
Components are numbered in the following groups:
1-99 Parts on the chassis.
100-199 Parts on the vertical circuit board.
200-299 Parts on the horizontal circuit board.
300-399 Parts on the low voltage circuit board.
400-499 Parts on the high voltage circuit board.
VERTICAL
The vertical preamplifier consists of two identical circuits:
One for Channel Y1 and the other for Channel Y2.
Components in the Channel Y1 vertical preamplifier circuit
are designated by a —1 suffix, while those in the Channel Y2
vertical preamplifier are designated by a —2 suffix. (For
example: A Channel Y1 divider resistor is R101-1, while the
same divider resistor in Channel Y2 is R101-2). Components
without a suffix do not relate to a specific channel. Since
both channels are identical, only Channel Y1 is described in
this ‘Circuit Description.”
INPUT CIRCUIT
When
Y1
input
switch
SW1
(AC-GND-DC)
is
in
the
DC
position,
a
signal
applied
to
the
Y1
input
connector
is
coupled
to
the
input
attenuator.
When
the
Y1
input
switch
is in the AC position, the signal is coupled through capacitor
C1, which passes only AC signals. This permits an AC signal
superimposed on a DC potential to be seen without the DC
component being displayed. The GND position of this
switch disconnects the input signal and grounds the
attenuator input. This allows the trace to be adjusted to a
zero reference without disconnecting the test leads from the
circuit under test.
Because the second (Q109-1/Q110-1) and vertical deflection
amplifiers (Q111-Q114), which will be discussed later,
operate at a fixed gain, any signal applied to them must be
within a usable range (approximately 80 mV/cm).
Therefore, the primary purpose of the vertical input circuits
is to reduce or increase the input signal by a known factor to
this usable level.