Heathkit HW 5400 Manual 2

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HEATHKITE
MANUAL

HEATH COMPANY - BENTON HARBOR, MICHIGAN

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Heathkita

FILTER CIRCUIT BOARD

The filter circuit board contains six low-pass filters
which roll off the high frequencies during both
transmit and receive modes. Each filter comprises
two pi-sections.

Relay K551 opens during the transmit mode to pre-
vent the RF from the power amplifier from entering
the receiver front end [on the RF circuit board).
Relay K552 is connected across the lstolt supply
and is used to operate the relay in an external linear
amplifier.

Transistors Q551 and Q552 form an overvoltage pro-
tection circuit for the Transceiver. If the input volt-
age should happen to exceed 16 volts DC, diode

D552 conducts and turns on transistor Q552. This
turns off transistor Q551 which shuts down the
Transceiver by de»energizing the master on-off relay
(K1) located on the rear of the chassis.

A series-regulating circuit is formed by transistors
Q555, Q556 and diode D555. Transistor Q556 senses
the voltage at the emitter of transistor Q555 and ad-
justs the voltage drive level at the base of transistor
Q555 as the load varies. Diode D555 forms the refer-
ence voltage for transistor Q556. Transistor Q553
supplies the drive for transistor Q554, which
supplies the bias voltage to the power amplifier in
the transmit mode.

BFO CIRCUIT BOARD

The BFO circuit board generates the LSB. USB. and
CW signals for the product detector and the balanced
modulator (located on the audio circuit board). The
BFO circuit board also interfaces with the controller
to generate several DC logic voltages that operate
many stages throughout the Transceiver.

In the LSB mode, crystal Y801 is selected by diode
D808 and connected to the series-tuned circuit
formed by capacitor C819, coil L804, and diode D809.
Transistor Q814 forms the BFO oscillator which has
the required feedback provided by capacitors C811
and C812. Buffer transistor Q818 drives the low-pass
filter that is formed by capacitors C805. C806, 0807,
and coil L801. This filter reduces any harmonic con-
tent in the signal produced by the BFO. Crystal Y802
is used during USB operation and operates in the
same manner as the LSB circuit.

Varicap diode D809 varies the frequency of the RFC
oscillator for proper [F shift and CW offset. Diode
D812 allows the IF shift voltage to pass during the re
ceive mode so diode can operate properly. Diode
0813 and resistor R834 cause the SSB oscillator to re-
turn to center during the transmit mode. Diode D814

and resistor R837 produce the desired 700 Hz offset
when the Transceiver is transmitting CW.

In the USB mode, the controller circuit turns on trans-
istor Q801. Thus turns on transistor Q802, which pro-
duces an USB voltage that turns on crystal Y802. This
also turns on diode D802 which turns on transistor
Q807. Transistors Q803 and Q804 control the LSB
voltage and operate in the same manner as transistors
QBOI and QBOZ.

During the narrow CW (transmit) mode, the control-
ler circuit turns on transistor Q811 via diode D807. In
the wide CW (transmit) mode, the controller turns on
transistor Q1311 through diode D806. Transistor Q811
turns on transistor Q812 during the CW [transmit]
mode, which feeds a DC voltage to CW offset control
R837. Transistor Q811 also turns on transistor Q809,
which provides a 9-volt source for other circuits in
the Transceiver. Also during CW operation, transistor
Q809 turns on transistor Q806 which keeps transistor
Q807 off. Transistor Q807 is only turned on in the
USB and LSB modes and turns on transistors Q805
and Q8138. This generates a DC voltage in the transmit
mode to operate the SSB centering and other stages.

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CONTROLLER CIRCUIT BOARD

The controller circuit board performs the following
functions:

1, Refreshes the display.

2. Programs a synthesizer integrated circuit
in each of the three phase-locked loops.

3. Receives inputs from the shaft encoder
circuit board and keypad (if this acces-
sory is installed) to alter the frequency.

4. Polls the front panel switches for the de-
sired band, mode, etc.

5. Monitors the phase-locked loops for an
unlocked condition.

6. Allows the Transceiver to go into the
transmit mode when you request it to do
so, but first ensures that the phase-
locked loops are locked and the fre-
quency you have selected is within cer-
tainlimits.

7. Stores display and memory frequencies
for each hand even when the Transceiver
is turned off [if the memory keep-alive
voltage is present at the rear panel Ac-
cessory Socket].

8. Mutes the receiver when required.
9. Performs diagnostics on itself when the
Transceiver is turned on [if backup

power was lost).

The various circuits on the controller circuit board
will now be described in detail.

SHAFT ENCODER

The shaft encoder consists of two slotted infrared
switches (mounted on a separate circuit board) and
a wheel that has alternating clear and opaque areas.
This arrangement produces two pulse trains as the
wheel rotates that are in quadrature (they are ap-
proximately 90 degrees out of phase) to produce four
unique states. One of the shaft encoder outputs is
applied to the external interrupt input of the micro-
processor. This allows the microprocessor to deter-
mine the direction of rotation. When this input goes
high, the microprocessor determines whether it
should increase or decrease the frequency. It also
determines whether or not you are using the fast-
tune metallic finger insert in the Main Tuning knob.
Schmitt trigger transistors Q703 though Q706 pre-
vent false triggering due to vibration.

LOOP SELECT LOGIC

Three phase-locked loops are used in this Trans-
ceiver to ultimately produce a single injection fre-
quency. Each of these loops uses a frequency synthe-
sizer integrated circuit, which contains programma-
ble divide-by-N and reference dividers and a digital
phase detector with a lock-detection circuit. To pro-
gram one of these IC's, each of the internal dividers
is programmed by applying a 3-bit binary address
code (to select one of the seven digit latches) along
with a 4-bit binary digit code, and then the strobe
input of the particular IC to be programmed is
strobed. This process repeats for each of the seven
latches within the IC.

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Port 4 of the microprocessor supplies the required
address/data for each latch and the strobe output of
the microprocessor supplies the required strobe
pulse. The address and data inputs for each of the
three synthesizer ICs are connected in parallel across
this port. Due to the loop select logic, only one of
these latches receives the strobe pulse at a time.

Since only seven of the eight lines from port 4 are re-
quired, three for address and four for data, bit 7 is
available to help select between the three 103. One bit
can only select between two ICs, however, so a trick
must be used. Because there are only seven digit
latches in each IC, the latches ignore a binary code of
111 (decimal 7). Integrated circuit U708A decodes
this binary code and uses it to set or clear flip-flop
U701B. depending upon the level of bit 7 at port 4.
This provides four possible states when the latch
address/data is strobed out on port 4. If integrated Cir-
cuit U701B is in the cleared state, and bit 7 is low, in-
tegrated circuit U711A passes the stobe pulse to inte-
grated circuit U703.

If integrated circuit U701B is in the cleared state and
bit 7 is high, integrated circuit U711B passes the
strobe pulse to integrated circuit U712. If integrated
circuit U701B is in the set state. however, integrated
circuit U71 1C passes the stobe pulse to integrated cir-
cuit U705 (regardless of the level on bit 7). Since in-
terated circuits U703 and U712 must be reprogram-
med each time you make a frequency change, inte-
grated circuit U7OIB is normally kept in the cleared
state. NOTE: Integrated circuit U705 is reprogram-
med oniy when you make a band change (except for
the 10-meter band, which is divided into four seg-
ments).

UNLOCK DETECTION

Lock detection logic within each synthesizer integrat-
ed circuit produces very narrow negative-going
pulses on its output [pin 13) whenever it detects a
phase-lock condition. If a large phase error occurs be-
tween the two inputs to the internal phase detector,
the pulses increase in width. An external pulse width
detection circuit senses this increase. NOTE: Since
each lock detection circuit is identical, only the cir-
cuit associated with integrated circuit U703 is de-
scribed in the next paragraph.

When the loop is locked, transistor Q711 is held on,
which prevents diode D703 from conducting. If the
loop unlocks, the wider pulses at the base of Q711
allow it to turn off long enough for capacitor C731 to
charge through resistor R731 and diode D703. When
the charge on this capacitor reaches 0.7 volt, transis-
tor Q709 turns on. This applies a low to an input of
integrated circuit U708C and takes its output high,
which is inverted by integrated circuit U7OBB. If
alignment jumper] is installed, the low on the output
of U7OBB is applied to an input of microprocessor
U710 and causes it to display a PLL message, force
the Transceiver into the receive mode, and mute the
receiver. Transistor Q709 lights LED D703 to indicate
that this loop is unlocked.

CRYSTAL OSCILLATORS

Three crystal oscillators provide the frequency accu-
racy and stability of this Transceiver. Two of these are
parallel-mode oscillators and are located on the con-
troller circuit board. One of these oscillators operates
at 8.04 MHz and is made up of transistor Q701, crystal
Y701, and the associated components. The output of
transistor Q701 drives transistor Q702 to 'I'IL logic
levels, which integrated circuit U701A divides by Z.
This 4.02 MHz signal is used as a clock for micropro-
cessor U710 and feeds the reference divider chain in
integrated circuit U703. Integrated circuit U703 buf-
fers this signal and also applies it to the reference di»
vider chain in integrated circuit U712 (pin 16). Verac-
tor diode D701 in this oscillator allows you to pull the
oscillator frequency when you use the RI'I (receiver
incremental tuning) function.

The other oscillator on this circuit board, formed by
transistor Q718, crystal Y702, and the associated
components, operates at 10 MHz and operates very
similarly to the 8.04 MHz oscillator. The output of
buffer transistor Q719 drives the reference divider
chain in integrated circuit U705.

LOOP FILTERS

Since the active loop filters for each of the phase-
locked loops are similar, only the filter for loop 1
(PLL-1) is described below.