Heathkit HD-10, Manual

Page: 2 / 8

CIRCUIT DESCRIPTION
SCHEMATIC DIAGRAM
The letter-number designations on the Sche-
matic Diagram are used to identify resistors,
capacitors and diodes. Each designation is re-
lated by the first number to the transistor stage
in which it is used. For example, the resistors in
transistor stage Q1 are designated R10, R11,
etc. In transistor stage Q7 they are designated
R70, R71, etc.
Letter-number designations using numbers 1
through 9 are not directly related to any tran-
sistor stage. This system of circuit component
designations is used throughout the Schematic.
Circled letter designations on the Schematic Di-
agram indicate wire connecting locations on the
circuit board.
POWER SUPPLY
With slide switch S3 in the Operate or Hold po-
sition, the AC line voltage is applied across the
primary winding of power transformer T1. Re-
sistor R8 drops the line voltage that is applied
to the neon pilot lamp, which indicates when
the keyer is on. The fuse provides protection in
case of a short circuit.
The output of power transformer T1 is rectified
by silicon diode D2 and filtered by capacitor C2
to produce a positive (+) 19 volts DC. Silicon
diode D3 and filter network consisting of ca-
pacitors C3 and C4 and resistor R9 provides a
negative (–) 16 volts DC. These voltages are
the operating and bias voltages for the transis-
tor stages.
KEY SWITCHING
The key lever is moved in one direction or the
other to actuate switch S1 or S2. Switch S1
turns on the dot generator multivibrator circuit
to produce dots. Switch S2 turns on the flip-
flop and dot generator multivibrators to pro-
duce dashes. The operation of these circuits
will be explained later.
BLOCK DIAGRAM
DOTS
Dot Clamp Circuit
When the key lever is in the center (neutral)
position, a positive voltage is present at the
base of dot clamp transistor Q3. This voltage
which is applied from the power supply through
resistors R1, R5 and R6, biases Q3 in an on
condition (conducting). When Q3 is on, a posi-
tive voltage is applied to the base of Q2, one
of the dot generator multivibrator transistors,
keeping it turned off (not conducting).
When the key lever is moved to the dot posi-
tion, switch S1 closes. This removes the posi-
tive supply voltage present at the base of Q3
by directing it to ground through resistor R6. A
negative supply voltage from the voltage divid-
er consisting of resistor R1, R2 and R5, is ap-