Heathkit HW-12 - Page 57

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| GENERAL DIFFICULTIES | POSSIBLE CAUSE
Receive RF burns when removing
antenna connector,
Transceiver chassis "thot" causing
electrical shock with linear ampli-
fier connected,
Filaments stay lit when Trans-
| ceiver is turned OFF. |
Meter reads backwards.
| te
1, Ungrounded high voltage connected to EXT
Transmitter tripped on by noise when in |
VOX operation,
RELAY socket, See Installation section of |
manual,
1, Improper power supply connections,
Power Supply (HP-20) still turned on,
HP-10 Power Supply not modified.
1, Meter improperly wired,
Po —tss—sSYS CO
Meter switch improperly wired,
S METER ADJ control not set properly.
| No output from VFO, V14. 1, Wrong or faulty tube in socket V13 or V14,
This Circuit Description is a detailed explana-
tion of the Transceiver circuits that were cov-
ered only briefly in the General Circuit De-
scription on Page 5, All circuit references in
this description concern the Schematic Diagram.
SCHEMATIC DIAGRAM
The
circled
letter-number
designations
on
the
Schematic
Diagram
are
used
to
identify
re-
sistors,
capacitors,
chokes,
etc,
Each
of
these
designations
is
related
to
the
tube
stage
in
which
it
is
used,
by
the
first
number;
For
instance,
the
resistors
in
tube
stage
V1
are
designated
R10,
R11,
etc.
In
tube
stage
V12
they
are
marked
R120,
R121,
etc.
This
system
of
circuit
component
designation
is
used
through-
out
the
Schematic.
Numbers in diamonds on the Schematic refer to
the color coding of the cable assembly wires,
Numbers 1 through 9 indicate solid colors;
numbers 10 through 18 refer to wires with a
white background and a single color stripe; and
numbers 20 through 28 refer to wires with a
white background andtwo identical color stripes.
The numbers can be related to wire colors by
using the same color code as used for re-
sistors: brown = 1, red = 2, orange = 3, etc.
MICROPHONE AMPLIFIER V1A
Voice signals from the microphone are coupled
through capacitor C12 to the grid of micro-
phone amplifier tube VIA. The amplified signal
at the plate of V1A is coupled through C14 to
the Mic Gain control, and through capacitor
C102 to the VOX (voice operated transmitter)
circuit, The setting of the Mic Gain control
determines the amount of modulation, Since
V1A supplies signals for modulation and for
VOX, it operates during both receiving and
transmitting. Capacitor C10 bypasses to ground
any RF signal picked up by the microphone
push-to-talk switch lead,
AUDIO FREQUENCY CATHODE FOLLOWER
The audio signal from the Mic Gain control is
applied to AF (audio frequency) cathode follower
stage ViB. This stage matches the tube im-
pedance to that of the balanced modulator, By-
pass capacitor Ci9 keeps the modulator RF
voltages from reaching V1B. During receiving,
the relay cuts off VIB and a number of other
transmitter stages,