SSBTRANSCEIVER BOARD CONSTRUCTION MANUAL

MST2
SSB TRANSCEIVER BOARD
20M
CONSTRUCTION MANUAL
MST2 20M Construction Manual – Issue 1
Page 1
CONTENTS
1
Introduction .............................................................................................................................................................. 4
2
Block Diagram .......................................................................................................................................................... 5
3
Circuit Description ................................................................................................................................................. 6
3.1
Carrier Oscillator ............................................................................................................................................................... 6
3.2
SSB generator ...................................................................................................................................................................... 6
3.3
Crystal Filter ........................................................................................................................................................................ 7
3.4
Transmit mixer ................................................................................................................................................................... 7
3.5
Power amplifier .................................................................................................................................................................. 7
3.6
Power Supply and RX/TX switching ......................................................................................................................... 8
3.7
Receive mixer ...................................................................................................................................................................... 8
3.8
Product Detector ................................................................................................................................................................ 8
3.9
Audio Power Amplifier .................................................................................................................................................... 9
3.10
Audio AGC ............................................................................................................................................................................. 9
4
Kit Supplied Parts ................................................................................................................................................ 17
5
Individual Parts List............................................................................................................................................ 19
6
Off Board Parts ...................................................................................................................................................... 22
7
Construction........................................................................................................................................................... 23
8
9
7.1
General ................................................................................................................................................................................ 23
7.2
Construction Steps ......................................................................................................................................................... 24
Enclosure ................................................................................................................................................................. 33
8.1
Rear Panel .......................................................................................................................................................................... 33
8.2
Front Panel ........................................................................................................................................................................ 33
8.3
Mounting the PCB ........................................................................................................................................................... 33
8.4
Front panel label ............................................................................................................................................................. 34
8.5
Fitting the DDS VFO ....................................................................................................................................................... 36
Wiring up ................................................................................................................................................................. 37
10 Testing and alignment ....................................................................................................................................... 39
10.1
General ................................................................................................................................................................................ 39
10.2
Power on ............................................................................................................................................................................. 39
10.3
Receive ................................................................................................................................................................................ 40
10.4
Transmit ............................................................................................................................................................................. 40
10.5
Tone Generator ................................................................................................................................................................ 41
10.6
Audio AGC adjustment ................................................................................................................................................. 43
10.7
Carrier frequency adjustment ................................................................................................................................... 44
11 Operation................................................................................................................................................................. 46
MST2 20M Construction Manual – Issue 1
Page 2
List of Figures
Figure 1 MST2 Block diagram ..................................................................................................................................... 5
Figure 2 Carrier oscillator .......................................................................................................................................... 10
Figure 3 Balanced Modulator ................................................................................................................................... 11
Figure 4 Crystal Filter .................................................................................................................................................. 12
Figure 5 Transmit mixer ............................................................................................................................................. 13
Figure 6 Power amplifier and RX/TX switching ............................................................................................... 14
Figure 7 Receive mixer ................................................................................................................................................ 15
Figure 8 Receive audio ................................................................................................................................................ 16
Figure 9 Component overlay .................................................................................................................................... 32
Figure 10 Front panel drilling guide...................................................................................................................... 35
Figure 11 DDS VFO front panel mounting ........................................................................................................... 36
Figure 12 Wiring diagram .......................................................................................................................................... 38
Figure 13 Typical circuit voltages........................................................................................................................... 42
Figure 14 Carrier frequency and crystal filter relationship......................................................................... 45
MST2 20M Construction Manual – Issue 1
Page 3
1 INTRODUCTION
The MST2 (Minimalist Sideband Transceiver series 2) mono-band SSB transceiver board follows
the minimalist design approach of the original MST transceiver board, but adds a number of
new features and performance enhancements.
With the addition of a VFO, an enclosure and a hand full of parts you have a complete and
working SSB transceiver.
For best results mate the MST2 with the DDS VFO for drift free operation and the LED S meter
for accurate receive ‘S’ units and transmit power display.
Full kits of parts for the MST2 transceiver board, the DDS VFO and LED S meter are available
from www.ozQRP.com.
MST2 Features:
1. Sensitive Superhet receiver using a 5 pole 10MHz crystal filter.
2. 5W PEP minimum power output using a rugged power MOSFET output stage.
3. TDA7052A speaker amplifier incorporating an AGC circuit to even out received audio
level.
4. Onboard fuse and diode protection to guard against power supply over current and
reverse polarity.
5. Carrier oscillator can be ‘pulled’ to provide either USB or LSB operation.
6. Tone generator provides a constant audio tone to assist with alignment, checking SWR
and adjusting antenna couplers.
7. Microphone amplifier accepts standard low impedance dynamic or Electret microphone
with selectable on-board bias resistor.
8. Connectors for optional LED S meter or a simple front panel LED transmit power and
modulation indicator.
9. AF and microphone gain controls.
10. Plenty of audio output to drive a loudspeaker.
11. Unwanted sideband suppression typically 40dB.
12. All spurious transmit outputs below -50dBc.
13. Receive current drain (including DDS VFO) approximately 135mA with no signal.
14. Transmit current approximately 950mA at 5W output.
15. High quality double sided PCB with groundplane, solder mask and silk screen.
16. Simple and easy to build.
17. No complicated coil winding required. Uses inexpensive commercial coil assemblies for
tuned circuits.
18. Easy to adjust and set up with minimal tools required.
MST2 20M Construction Manual – Issue 1
Page 4
2 BLOCK DIAGRAM
Figure 1 MST2 Block diagram
MST2 20M Construction Manual – Issue 1
Page 5
3 CIRCUIT DESCRIPTION
3.1 CARRIER OSCILLATOR
Transistor Q1 is configured as a Colpitts oscillator and acts as the 10MHz carrier oscillator in
transmit and beat frequency oscillator (BFO) in receive. The frequency of crystal X1 is pulled
slightly below the lower crystal frequency response by trimmer capacitor TC1 and inductor L1.
This results in USB being generated. The power supply to the oscillator is regulated with a 9.1V
Zener diode ZD1.
The oscillator feeds buffer stage Q2 to provide a low impedance drive for the balanced
modulator. A lower level output of around 500mV pk-pk is obtained at the junction of Q1
emitter resistors to feed the receive product detector.
3.2 SSB GENERATOR
Transistor Q3 is the microphone pre-amplifier with a 20K ohm input impedance and gain of
around 10. C8 is included to prevent RF feeding into the amplifier. The output of Q3 is fed to the
Mic gain control via C13.
If an Electret microphone is used, R8 provides a DC bias current and is enabled by shorting LK1.
If a dynamic microphone is used LK1 is left open. Transistor Q4 is the second microphone
amplifier and has a gain of about 10. It is biased for higher current and has a low value collector
resistor to enable it to drive the balanced modulator. R16 and C17 form a low pass filter to keep
RF out of the amplifier.
The balanced modulator is a diode switching type and doubly balanced. When the carrier signal
is positive diodes D1 and D4 conduct and when it is negative diodes D2 and D3 conduct. The
result is that no RF is present at the output transformer T1. If an audio signal is injected into the
bridge the balance is upset and a double sideband suppressed carrier signal is produced at the
output of T1. Note that capacitor C21 holds the junction of D1 and D2 at ground for RF.
Due to variations in component parameters the balance is not exact and so trimcap TC2 and
trimpot VR1 are adjusted to bring the modulator into balance. In practice up to 50dB of carrier
suppression can be achieved.
Diode D5 is used as an RF switch. With no DC current flowing through the diode it is high
impedance to RF. In TX mode around 6mA of DC current flows through D5 and it becomes a low
impedance path for RF. This feeds the output of the balanced modulator into the crystal filter. In
receive D8 performs a similar function.
A 555 timer (U1) is configured for astable operation with a frequency around 1KHz. A triangle
waveform is available at the junction of the timing components C12 and R15. This is filtered by
R11 and C10 to provide an approximate sine wave for transmission. This is coupled to the top of
the Mic gain control via R9 and C9. U1 is not oscillating until pin1 is taken to ground by closing
the Tone switch. The level of tone and hence the transmitter output can be varied by adjusting
the Mic gain control. The tone generator serves as a handy tool during alignment and when
checking SWR and adjusting antenna couplers.
MST2 20M Construction Manual – Issue 1
Page 6
3.3 CRYSTAL FILTER
The crystal filter is a 10MHz 5 pole ladder type using closely matched crystals on the same
frequency. Capacitors C24 to C29 are selected to provide an approximate 2.7KHz bandwidth.
Resistors R23 and R24 terminate the crystal filter in the correct resistance to give low ripple in
the pass band.
3.4 TRANSMIT MIXER
The transmit mixer is based around U6 a SA612 balanced mixer. The 10MHz LSB signal from the
crystal filter is fed single ended into pin1 while the other input on pin 2 is grounded to RF by
C59. The VFO signal of around 300mV pk-pk is fed into pin 6. Transistors Q8 and Q9 act as
emitter follower buffers providing a balanced low impedance feed for the transmit band pass
filter. The transmit band pass filter is comprised of T5, T6 and associated capacitors and tuned
to the sum of the VFO and carrier frequencies. The transformers used here are actually 10.7MHz
IF transformers with the integral 47pF capacitor removed. A smaller external capacitor is used
instead to increase the resonate frequency to match the 14MHz transmit frequency.
The output of the transmit band pass filter is link coupled to the pre-driver built around
transistor Q10 which has both series and shunt feedback. The collector load is a broadband
transformer (T7) with a 10 to 3 turn ratio. R50 determines the DC collector current, while R51
and C69 set the AC gain.
3.5 POWER AMPLIFIER
Transmit signal from the pre-driver is applied to the driver stage built around transistor Q11. A
BD139 works well here when biased with about 60mA of collector current. The design is well
proven using both shunt and series feedback to provide low input and output impedance and
good stable gain on the low HF bands.
The power amplifier (Q12) is an IRF510 MOSFET and has been used in many designs. It is a
good candidate for the HF bands and provides up to 5 Watts PEP of power from a 13.8 V drain
supply. The output from the driver is applied across resistor R58 and becomes the AC drive
component for Q12 gate. Zener diode ZD5 and trimpot VR3 provides a stable and variable DC
gate voltage to place Q12 just into conduction for linear service. There is a short ramp up of the
gate voltage when switching to TX mode as capacitor C74 charges and is included to provide a
smooth gate voltage transition.
The drain load for Q12 is a broadband bi-filar wound transformer (T9) and was found to
provide maximum output into a 50 ohm load. The waveform from Q12 can be high in harmonics
and so a 5 pole low pass filter is included to reduce the level of harmonic and other spurious
energy to an acceptable level.
As a visual indication of power output and modulation, the transmit signal is sampled by
capacitor C85 and ground referenced by R60. The signal is rectified by D11 and filtered by C86.
This drives transistor buffer Q13 to drive an external LED S meter or a front panel LED via
current limiting resistor R61.
MST2 20M Construction Manual – Issue 1
Page 7
3.6 POWER SUPPLY AND RX/TX SWITCHING
When the PTT is operated the TX/RX relay is energized and the transmit signal is passed to the
antenna. When the PTT is not operated the relay switches the antenna through to the receive
circuits. The relay also switches power to the TX and RX sections as required. The power supply
is also made available on separate connectors for the external VFO and LED S meter.
Diode D10 and a 2A fuse provide both over current and reverse polarity protection. If the
supply is connected in reverse D10 will conduct and the fuse will blow. If however, the supply is
not capable of supplying much more than 2A, the fuse may not blow, but the supply will be
limited to a safe voltage and no damage should occur.
3.7 RECEIVE MIXER
Signals from the antenna are applied to a bandpass filter formed with two transformers T2, T3
and capacitors C50, C51 and C52. The antenna is link coupled to T2 while the output is fed from
a tap on the tuned primary winding of T3 to for improved impedance matching.
The mixer U5 is another SA612. The input is protected with a pair of back to back diodes and fed
single ended into pin 1. Pin 2 is grounded to RF by C54. Zener diode ZD3 provides a stabilized
6.8 volt supply. VFO signal is injected into pin 6 at about 300mV pk-pk. The balanced output
which contains the difference signal of 10MHz is fed to broadband transformer T4. The output
of T4 is passed to the crystal filter when DC current flows through R43 and into D8.
3.8 PRODUCT DETECTOR
The 10MHz SSB intermediate frequency (IF) signal from the crystal filter is applied to the
product detector U2. The product detector is formed with another SA612 and mixes the IF
signal with the 10MHz BFO signal to produce an audio output. The BFO is adjusted slightly
below the crystal filter response so that Upper Side Band signals are detected correctly.
A balanced input audio amplifier is formed with one half of a NE5532dual low noise op-amp
(U3a). A reference supply for the non-inverting input is obtained from R26, R27 and C35. The
high frequency response of U3a is limited by C34, C38 and C40, while capacitors C36 and C37
reduce the low frequency response.
The output of U3a is fed via a 1uF coupling capacitor to the AF gain potentiometer. The audio
signal is also made available via R34 on a separate connector for a LED S meter to display
receive signal strength. Transistor Q5 is turned on in TX mode and shorts the audio signal input
to the LED S meter to ground. This stops switching transients from being displayed.
As the receiver gain is fixed between the antenna and the AF gain control, the audio level across
the AF gain control is directly proportional to the receive signal strength. The LED S meter
measures this audio level and accurately displays the receive signal strength on an LED bar
graph.
MST2 20M Construction Manual – Issue 1
Page 8
3.9 AUDIO POWER AMPLIFIER
Audio fed from the wiper of the AF gain control is amplified by the other half of the dual op-amp
(U3b) which is configured for a gain of 5. The amplified signal is then applied to the audio power
amplifier (U4) to drive a loudspeaker. This is a TDA7052A device with a Bridge-Tied Load (BTL)
output. This configuration has a number of advantages for operation at low supply voltages, and
also allows the speaker to be directly connected to the chip without the need for a large
coupling capacitor.
Both speaker wires are connected directly to the IC. Connecting a speaker wire or
external load to ground may damage the IC.
3.10 AUDIO AGC
The main reason for choosing the TDA7052A is the ability to alter the gain over a very large
range by varying the DC voltage at pin 4. If pin 4 is left floating an internal source provides
about 1.1V resulting in a maximum gain of +30dB. As pin 4 is pulled low the gain decreases, and
if pulled all the way to ground the device is effectively shut off. By varying the amount of current
pulled from pin 4 the gain can be continuously varied. This feature is used here to provide an
Automatic Gain Control (AGC) circuit to even out receive audio and limit blasts from the speaker
on very strong signals.
The power supply for U4 is set to +8V by a 7808 regulator. This is done for two reasons. Firstly
the TDA7502A can become unstable at high supply voltages, but more importantly to fix the
voltage at the output pins under no signal conditions. With no signal this voltage is half the
supply voltage (+4V), but when audio is fed to the speaker the voltage at pin 5 will swing above
and below the 4V quiescent point. The base of transistor Q7 is DC connected to pin 5 by a
resistor and a trimpot. The trimpot (VR2) is adjusted so that transistor Q7 is just below
conduction when there is no audio. When a signal is received the positive audio peaks at pin 5
will start to turn on Q7 and cause some current to be pulled from pin 4 and lower the gain.
When the audio decreases, Q7 will begin to turn off which raises the voltage on pin 4, and
increases the gain. This action continually attempts to adjust the audio level and provide AGC
action. Capacitor C48 stores the charge in between positive cycles to avoid Q7 turning off during
negative peaks and causing distortion. For such a simple circuit the dynamics are very good and
make a great addition to the receiver.
In TX mode transistor Q6 is turned on hard and pulls pin 4 immediately to ground. This shuts off
U4 and prevents any spurious audio from the transmit circuits being heard in the speaker.
When returning to RX mode C48 charges slowly and provides a smooth click-less transition.
MST2 20M Construction Manual – Issue 1
Page 9
Figure 2 Carrier oscillator
MST2 20M Construction Manual – Issue 1
Page 10
Figure 3 Balanced Modulator
MST2 20M Construction Manual – Issue 1
Page 11
Figure 4 Crystal Filter
MST2 20M Construction Manual – Issue 1
Page 12
Figure 5 Transmit mixer
MST2 20M Construction Manual – Issue 1
Page 13
Figure 6 Power amplifier and RX/TX switching
MST2 20M Construction Manual – Issue 1
Page 14
Figure 7 Receive mixer
MST2 20M Construction Manual – Issue 1
Page 15
Figure 8 Receive audio
MST2 20M Construction Manual – Issue 1
Page 16
4 KIT SUPPLIED PARTS
QTY
Value
Designator
Capacitors
2
1.8pF ceramic disc NPO
C51, C65
5
22pF ceramic disc NPO
C22, C50, C52, C64, C66
1
47pF ceramic disc NPO
C6
2
82pF ceramic disc NPO
C25, C28
10
100pF ceramic disc NPO
C2, C3, C4, C24, C26, C27, C29, C55, C61, C85
3
220pF ceramic disc
C38, C40, C43
4
220pF 100V C0G ceramic MLCC
C81, C82, C83, C84
6
1nF ceramic disc
C8, C17, C31, C45, C53, C58
1
10nF ceramic disc
C21
2
47nF polyester MKT
C12, C34
6
100nF polyester MKT
C10, C11, C36, C37, C41, C44
28
100nF ceramic MLCC
C5, C7, C16, C20, C23, C32, C33, C46, C54, C56, C57,
C59, C60, C62, C63, C67, C68, C69, C70, C71, C72, C73,
C75, C76, C77, C79, C80, C86
5
1uF 50V RB electrolytic
C9, C13, C14, C19, C42
3
10uF 25V RB electrolytic
C15, C48, C49
2
22uF 25V RB electrolytic
C30, C74
5
220uF 25V RB electrolytic
C1, C18, C35, C39, C47
1
1000uF 25V RB electrolytic
C78
2
40pF trim capacitor
TC1, TC2
Resistors
1
4.7Ω 1/4W 5%
R56
1
10Ω 1/4W 5%
R54
3
22Ω 1/4W 5%
R20, R51, R55
7
100Ω 1/4W 5%
R4, R6, R16, R18, R21, R49, R58
1
150Ω 1/4W 5%
R50
4
220Ω 1/4W 5%
R1, R14, R23, R24
2
330Ω 1/4W 5%
R44, R61
6
470Ω 1/4W 5%
R7, R25, R31, R42, R52, R57
5
1K 1/4W 5%
R3, R19, R38, R45, R46
4
2.2K 1/4W 5%
R12, R22, R43, R47
2
3.3K 1/4W 5%
R13, R53
4
4.7K 1/4W 5%
7
10K 1/4W 5%
5
22K 1/4W 5%
R8, R28, R29, R60
R9, R11, R15, R26, R27, R34, R48
R33, R36, R37, R39, R40
2
56K 1/4W 5%
R5, R41
3
100K 1/4W 5%
R2, R17, R35
3
220K 1/4W 5%
R30, R32, R59
1
470K 1/4W 5%
R10
1
500Ω vertical multi-turn trimpot
2
20K horizontal trimpot
VR1
VR2, VR3
MST2 20M Construction Manual – Issue 1
Page 17
QTY
Value
Designator
Semiconductors
9
1N4148 signal diode
D1, D2, D3, D4, D5, D6, D7, D8, D11
1
1N4004 1A power diode
D9
1
1N5408 3A power diode
D10
1
9.1V 500mW Zener
ZD1
4
6.8V 500mW Zener
ZD2, ZD3, ZD4, ZD5
11
2N3904 NPN transistor
Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8, Q9, Q10, Q13
1
BD139 NPN transistor
Q11
1
IRF510 power MOSFET
Q12
1
7808 +8V 1A regulator
REG1
1
NE555 timer SMD
U1
3
SA612 RF mixer SMD
U2, U5, U6
1
NE5532 dual op-amp SMD
U3
1
TDA7052A audio power amp
SMD
U4
Coils
1
15uH RF choke
L1
4
IF transformer 42IF123
T2, T3, T5, T6
1
FT37-43 4T:3T
T1
3
FT37-43 10T:3T
T4, T7, T8
1
FT50-43 5T bifilar
T9
2
T50-6 11T
L2, L3
1
2M 0.4mm enamelled wire
Hardware
1
12V DPDT DIP relay
K1
2
M205 PCB fuse clip
F1
1
M205 2A fuse
F1
1
2 way screw terminal block
PWR
6
2 pin 2.54mm pitch header
MIC, TONE, SM IN, SPKR, VFO, PTT
4
3 pin 2.54mm pitch header
MIC GAIN, AF GAIN, VFO PWR, SM PWR
6
10MHz crystal
X1, X2, X3, X4, X5, X6
Crystals
MST2 20M Construction Manual – Issue 1
Page 18
5 INDIVIDUAL PARTS LIST
Designator
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
C25
C26
C27
C28
C29
C30
C31
C32
C33
C34
C35
C36
C37
C38
C39
C40
C41
C42
C43
Value
220uF
100pF
100pF
100pF
100nF
47pF
100nF
1nF
1uF
100nF
100nF
47nF
1uF
1uF
10uF
100nF
1nF
220uF
1uF
100nF
10nF
22pF
100nF
100pF
82pF
100pF
100pF
82pF
100pF
22uF
1nF
100nF
100nF
47nF
220uF
100nF
100nF
220pF
220uF
220pF
100nF
1uF
220pF
Type
25V RB electrolytic
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic MLCC
ceramic disc NPO
ceramic MLCC
ceramic disc
50V RB electrolytic
polyester MKT
polyester MKT
polyester MKT
50V RB electrolytic
50V RB electrolytic
25V RB electrolytic
ceramic MLCC
ceramic disc
25V RB electrolytic
50V RB electrolytic
ceramic MLCC
ceramic disc
ceramic disc NPO
ceramic MLCC
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
25V RB electrolytic
ceramic disc
ceramic MLCC
ceramic MLCC
polyester MKT
25V RB electrolytic
polyester MKT
polyester MKT
ceramic disc
25V RB electrolytic
ceramic disc
polyester MKT
50V RB electrolytic
ceramic disc
MST2 20M Construction Manual – Issue 1
Designator
C44
C45
C46
C47
C48
C49
C50
C51
C52
C53
C54
C55
C56
C57
C58
C59
C60
C61
C62
C63
C64
C65
C66
C67
C68
C69
C70
C71
C72
C73
C74
C75
C76
C77
C78
C79
C80
C81
C82
C83
C84
C85
C86
Value
100nF
1nF
100nF
220uF
10uF
10uF
22pF
1.8pF
22pF
1nF
100nF
100pF
100nF
100nF
1nF
100nF
100nF
100pF
100nF
100nF
22pF
1.8pF
22pF
100nF
100nF
100nF
100nF
100nF
100nF
100nF
22uF
100nF
100nF
100nF
1000uF
100nF
100nF
220pF
220pF
220pF
220pF
100pF
100nF
Type
polyester MKT
ceramic disc
ceramic MLCC
25V RB electrolytic
25V RB electrolytic
25V RB electrolytic
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic disc
ceramic MLCC
ceramic disc NPO
ceramic MLCC
ceramic MLCC
ceramic disc
ceramic MLCC
ceramic MLCC
ceramic disc NPO
ceramic MLCC
ceramic MLCC
ceramic disc NPO
ceramic disc NPO
ceramic disc NPO
ceramic MLCC
ceramic MLCC
ceramic MLCC
ceramic MLCC
ceramic MLCC
ceramic MLCC
ceramic MLCC
25V RB electrolytic
ceramic MLCC
ceramic MLCC
ceramic MLCC
25V RB electrolytic
ceramic MLCC
ceramic MLCC
100V C0G ceramic MLCC
100V C0G ceramic MLCC
100V C0G ceramic MLCC
100V C0G ceramic MLCC
ceramic disc NPO
ceramic MLCC
Page 19
Designator
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
Value
1N4148
1N4148
1N4148
1N4148
1N4148
1N4148
1N4148
1N4148
1N4004
1N5408
1N4148
Type
Signal diode
Signal diode
Signal diode
Signal diode
Signal diode
Signal diode
Signal diode
Signal diode
1A power diode
3A power diode
Signal diode
F1
2A
M205 Fuse + PCB clips
K1
L1
L2
L3
DPDT
15uH
11T
11T
PCB mount DIP relay
RF choke
T50-6 toroid
T50-6 toroid
Q1
Q2
Q3
Q4
Q5
Q6
Q7
Q8
Q9
Q10
Q11
Q12
Q13
2N3904
2N3904
2N3904
2N3904
2N3904
2N3904
2N3904
2N3904
2N3904
2N3904
BD139
IRF510
2N3904
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
NPN transistor
Power MOSFET
NPN transistor
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
220Ω
100K
1K
100Ω
56K
100Ω
470Ω
4.7K
10K
470K
10K
2.2K
3.3K
220Ω
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
MST2 20M Construction Manual – Issue 1
Designator
R15
R16
R17
R18
R19
R20
R21
R22
R23
R24
R25
R26
R27
R28
R29
R30
R31
R32
R33
R34
R35
R36
R37
R38
R39
R40
R41
R42
R43
R44
R45
R46
R47
R48
R49
R50
R51
R52
R53
R54
R55
R56
R57
R58
R59
R60
R61
Value
10K
100Ω
100K
100Ω
1K
22 Ω
100Ω
2.2K
220Ω
220Ω
470Ω
10K
10K
4.7K
4.7K
220K
470Ω
220K
22K
10K
100K
22K
22K
1K
22K
22K
56K
470Ω
2.2K
330Ω
1K
1K
2.2K
10K
100Ω
150Ω
22Ω
470Ω
3.3K
10Ω
22Ω
4.7Ω
470Ω
100Ω
220K
4.7K
330Ω
Type
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
1/4W 5%
Page 20
Designator
REG1
Value
7808
Type
+8V regulator TO-220
SK1
SK2
SK3
SK4
SK5
SK6
SK7
SK8
SK9
SK10
MIC
MIC GAIN
TONE
SM IN
AF GAIN
SPKR
VFO
PTT
VFO PWR
SM PWR
2 pin polarised header
3 pin polarised header
2 pin polarised header
2 pin polarised header
3 pin polarised header
2 pin polarised header
2 pin polarised header
2 pin polarised header
3 pin polarised header
3 pin polarised header
T1
T2
T3
T4
T5
T6
T7
T8
T9
TB1
TC1
TC2
4T:3T
42IF123
42IF123
10T:3T
42IF123
42IF123
10T:3T
10T:3T
5T Bifilar
PWR
40pF
40pF
FT37-43 ferrite
10.7MHz IF transformer
10.7MHz IF transformer
FT37-43 ferrite
10.7MHz IF transformer
10.7MHz IF transformer
FT37-43 ferrite
FT37-43 ferrite
FT50-43 ferrite
2 way terminal block
Trimmer cap
Trimmer cap
U1
U2
U3
U4
U5
U6
NE555
SA612
NE5532
TDA7052A
SA612
SA612
Timer
RF mixer
Dual low noise op-amp
BTL power audio amp
RF mixer
RF mixer
VR1
VR2
VR3
500 Ω
20K
20K
Multi-turn trimpot
Horizontal trimpot
Horizontal trimpot
MST2 20M Construction Manual – Issue 1
Designator
X1
X2
X3
X4
X5
X6
Value
10MHz
10MHz
10MHz
10MHz
10MHz
10MHz
Type
Crystal
Crystal
Crystal
Crystal
Crystal
Crystal
ZD1
ZD2
ZD3
ZD4
ZD5
9.1V
6.8V
6.8V
6.8V
6.8V
500mW Zener diode
500mW Zener diode
500mW Zener diode
500mW Zener diode
500mW Zener diode
Page 21
6 OFF BOARD PARTS
The following is a suggested list of parts to install the MST2 transceiver board in an enclosure
along with the DDS VFO and LED S meter. These parts are not included in the kits.
The enclosure listed is the type used in the prototype, however any suitable plastic or metal
type with the approximate dimensions can be used. When choosing consider the following:




QTY
1
1
1
1
3
1
2
1
1
1
1
4
1
1
6
4
4
4
4
4
1
2
2
2
1
1
Metallic rear panel to act as a heatsink for the transmit PA.
Placement of mounting screws to match the PCB mounting holes.
Height of the enclosure to allow clearance for the DDS VFO PCB and LCD.
Sufficient space at the front of the enclosure to mount the DDS VFO and LED S meter.
Item
Basic Transceiver
Plastic instrument enclosure. 200mm x 155mm x
65mm with aluminium panels.
Red binding post
Black binding post
SO239 panel mount socket
Knobs
SPST toggle switch
10K log pot 16mm
5mm amber LED and bezel
Microphone socket
Front panel label
Loudspeaker 8 or 16 ohm 67mm square or
equivalent
4g x 6mm self-tapping screws
3mm x 10mm screw and nut
TO-220 insulating washer and bush
2 pin 2.54mm pitch header plugs + pins
3 pin 2.54mm pitch header plug + pins
DDS VFO
10mm long x 3mm threaded spacer
12mm long x 3mm threaded spacer
6mm long x 3mm round head screw
16mm long x 3mm countersunk head screw
LED S meter
Aluminium bracket
10mm long x 3mm countersunk screws + nuts
10mm long x 3mm round screws + nuts
3mm or 4mm long unthreaded nylon spacer
2 pin 2.54mm pitch header plugs + pins
3 pin 2.54mm pitch header plug + pins
Misc
hook-up wire, shielded audio cable
MST2 20M Construction Manual – Issue 1
Comment
www.altronics.com.au H0480F or
equivalent.
POS power supply.
NEG Power supply.
Antenna socket.
Nominally 20mm diameter.
Tone control switch.
AF gain and Mic gain.
If the LED S meter is not installed.
To suit microphone. Must have a
separate PTT line.
If required.
Minimum recommended size.
PCB mounting into plastic posts.
To mount TX PA.
To mount TX PA.
Optional.
Optional.
4 single 22mm spacers as alternative
PCB to spacer
Front panel to spacer
See LED S meter construction manual
Bracket to front panel
Bracket to PCB
PCB mount
Optional.
Optional.
Page 22
7 CONSTRUCTION
7.1 GENERAL
The MST2 is built on a high quality fiberglass PCB. The PCB is doubled sided with the majority of
the tracks on the bottom side with the top side forming a ground plane.
To assist construction the component overlay is screen printed on the top side and a solder
mask is included to help guard against solder bridges.
The ground plane is substantial and can sink quite a bit of heat from low wattage soldering irons
so ensure you use a good quality iron that can sustain the power required. You may find that
sometimes solder doesn’t appear to flow through to the top side. This is not necessarily a
problem because the plated through holes make a connection to the top side automatically.
Another point to consider is that plated through holes consume more solder than non-plated
holes and makes it more difficult to remove components. So check the value and orientation of
components before soldering!
There isn’t a ‘best’ scheme for loading the components. If desired you can build sections at a
time and test them out, but it is not really necessary and in any case some sections rely on
others before they will operate. The suggested procedure is to load the smaller components and
those closest to the PCB first and then work upwards.
MST2 20M Construction Manual – Issue 1
Page 23
7.2 CONSTRUCTION STEPS
It’s advisable to print out the parts list and tick off the components as they are installed. The
PCB has a silkscreen component overlay with components designators, but you might like to
print Figure 9 as an additional reference when installing the components.
Step 1: PCB
Remove the PCB from its bag.
The PCB comes shipped in a static shielded bag to protect against static
damage. While the ICs are not particularly sensitive to static damage it is still good
practice to take the usual precautions against static discharge during construction.
It is normal practice to install the ICs last, but it is almost impossible to solder them with the
larger and nearby components in place and so they are installed first.
All the ICs are 8 pin SOIC types with 1.27mm pitch pins and are quite a bit smaller than
conventional DIP through-hole ICs they are by no means difficult to work with.
The diagram below shows how to identify the SMD IC pins.
The ICs will have one or
more of the following
identifiers to locate pin 1:



A dot above pin 1
A beveled edge down
one side starting at pin 1
and ending at pin 4.
A line across the top
from pin 1 to pin 8.
MST2 20M Construction Manual – Issue 1
Page 24
Step 2: Resistors
The resistors are all 5% ¼ watt types with easy to read colour bands.
If in any doubt about reading resistor values measure them with a multimeter first.
Pass the pigtails through from the top and bend out slightly underneath to hold them in place.
Turn the PCB over and press down slightly to press them against the surface and solder. Cut off
the excess pigtail with side cutters.
It is easier and less confusing to install a group with the same value rather than to cover a
section of the PCB with mixed values. You will also find it more convenient to install 5 or 6
resistors at a time rather than inserting them all before soldering as the pigtails will more than
likely get in the way.
Step 3: Diodes
Note the positive or Cathode end before installation. The small Zener diodes look like signal
diodes so make sure you don’t get them mixed up. You may need a magnifier to identify them
correctly. Form the leads before inserting to reduce stress on the body when pulling through the
PCB.
Zener
1N4148
The diodes are identified
with a band printed on the
case at the Cathode (+) end.
Cathode (+) end
1N4004
1N5408
MST2 20M Construction Manual – Issue 1
Page 25
Step 4: Non-polarized capacitors
Note the various types. Ceramic disc NPO, standard ceramic disc, polyester MKT and ceramic
multi-layer chip capacitors (MLCC). These are all non-polarized and can go in either way.
There are 28 x 100nf (0.1uF) MLCC and as they are quite small install these first. Follow this
with the low pass filter capacitors, then the disc ceramics and finally the MKT.
Ceramic NPO
Ceramic
Polyester MKT Ceramic MLCC
Step 5: Trimmer capacitors
The type of trimmer capacitor supplied is quite small and has one lead electrically connected to
the screwdriver adjustment slot. Use a multimeter to determine this pin and solder to the hole
in the PCB connected to the ground plane. If you find the leads too wide for the PCB holes,
simply trim them down with sidecutters before inserting.
The earth pin of the trimmer
capacitor is highlighted in
the picture opposite.
Earth pin
MST2 20M Construction Manual – Issue 1
Page 26
Step 6: Trimpots
Note that the carrier balance trimpot is a multi-turn vertical mount while the bias and AGC
trimpots are horizontal mount types.
Step 7: Transistors
The 2N3904 transistors are orientated to match the screen silk component overlay. The BD139
pins are shown below:
Printing on front.
E
B
C
Leave the IRF510 power MOSFET and the 7808 voltage regulator installation till later.
Step 8: Electrolytic capacitors
These are polarized and it is very important that they go in the correct way. Electrolytic
capacitors have a line down the side of the case indicating the negative lead and the positive
lead is generally the longer lead. The PCB component overlay has a ‘+’ mark to indicate the hole
for the positive lead.
Step 9: Connectors
The MST2 board utilizes polarized pin headers for some external connections. These are
equivalent to the MOLEX ‘KK’ range. If preferred the wires may be soldered directly to the PCB,
but the connectors make for a professional looking build, plus allow easy disconnection and
testing if required. The connectors have a vertical polarizing piece and the connectors are
installed with this piece towards the centre of the PCB.
The power connector is a 2 way screw terminal block and the terminal openings face towards
the edge of the PCB.
The fuse holder clips have a small tab located on side of the upper parts contacts that are
intended to locate the fuse within the holder. Ensure the clips are installed so these tabs are on
the outer ends of the holder.
MST2 20M Construction Manual – Issue 1
Page 27
Step 10: IF transformers
The four IF transformers can only go in one way and are simply fitted into the holes in the PCB
and soldered. They do however require the internal capacitor to be removed so that the primary
winding can be resonated on 20M with an external capacitor.
This picture shows the IF transformer
from underneath. Notice the tubular
resonating capacitor located in the
recess at the bottom of the former.
Using a small tool such as a jeweler’s
screwdriver or scriber lever up one
end of the capacitor. Grab the capacitor
with long nose pliers and pull out
completely. The connecting wires are
very fine and will break off easily.
You may need to cut off any excess
wire and ensure there are no fragments
remaining in the recess.
This photo shows the capacitor
completely removed.
MST2 20M Construction Manual – Issue 1
Page 28
Step 11: Relay
The relay can only go in one way, so simply insert into the board and solder.
Step 12: Coils
Choke
Install the RF choke in the same manner as the resistors.
Transformer T1
Take two 80mm lengths of 0.4mm enamelled
copper wire.
Wind 4 turns for the primary (bal mod) and 3
turns for the secondary (xtal filter) on a FT3743 ferrite toroid.
Scrape the enamel off the ends of the wires
and tin with solder before installing in the
PCB.
The winding direction is not important.
Transformers T4, T7 and T8
Take an 80mm length of 0.4mm enamelled
copper wire and wind the 3 turn secondary on
a FT37-43 ferrite toroid.
Take a 150mm length of 0.4mm enamelled
copper wire and wind the 10 turn primary.
Scrape the enamel off the ends of the wires
and tin with solder before installing in the
PCB.
The winding direction is not important.
MST2 20M Construction Manual – Issue 1
Page 29
Bifilar transformer T9
Take a 500mm length of 0.4mm enameled
wire and fold in half. The wires need to be
twisted together. Following is a suggested
method:
Clamp one end of the doubled wires in a
vice. Make a loop at the other end and place
on a hook shaped bit in a battery drill.
While keeping the wires taut, run the drill
on slow speed until there are about 3 twists
per cm.
Carefully wind 5 turns on a FT50-43 ferrite
toroid trying to avoid scraping the enamel
on the sharp edges of the toroid.
Scrape some enamel off the ends and use a
multimeter to find the start (S) and end (E)
of each winding.
Also check that there are no shorts between
the two windings.
S1
E2
S2
E1
Take the end of the first winding and the
start of the second winding and twist
together to form the centre tap.
Trim the leads with side-cutters and tin
with solder before installing. Push the wires
through the holes in the PCB and sit the
toroid so that it rests against the surface of
the board.
S1
Ensure the two-wire centre tap goes to the
middle hole of the T9 component overlay
closest to Q12.
MST2 20M Construction Manual – Issue 1
E2
S2
E1
Page 30
Low Pass Filter Coils L2 and L3
Take a 250mm length of 0.4mm enamelled
copper wire and wind on 11 turns on a T50-6
toroid. Spread the turns to cover about 80% of
the toroid circumference.
Note the direction of winding as this makes for
a neater alignment on the PCB.
Scrape the enamel off the ends of the wires
and tin with solder before installing in the
PCB.
Step 13: Crystals
The crystals have been closely matched in frequency, so they can go in any position. The PCB
has a solder mask which should insulate the crystal metal cases from the PCB tracks, but to be
sure mount them about 0.5mm off the PCB. Once the crystals for the filter are installed solder a
wire across the top of the metal cases and down to a spare pad in the PCB groundplane set aside
for this purpose. The carrier oscillator crystal also has a PCB ground connection hole adjacent to
the crystal.
Step 14: Antenna wires
The antenna wires are difficult to solder to the pads once the PCB is installed in the case so
solder short lengths to the ANT and GND pads now.
Step 15: Power MOSFET and regulator
The reason these are left to last is simply because they sit high and near the edges and may get
damaged as the PCB is turned over and back as other components are being installed.
The 7808 voltage regulator is installed with the metal tab facing capacitor C47.
Mount the IRF510 MOSFET with the metal tab facing towards the edge of the PCB and with
about 5mm of lead length between the PCB and body.
MST2 20M Construction Manual – Issue 1
Page 31
Figure 9 Component overlay
MST2 20M Construction Manual – Issue 1
Page 32
8 ENCLOSURE
Now that your MST2 board is completed you’ll want to put it in an enclosure and try it out. The
PCB was designed to fit inside a plastic instrument enclosure 200mm wide by 155mm deep by
65mm high. A suitable type is catalogue number H0480F available from www.altronics.com.au.
It can be purchased with aluminium panels or you can easily make them yourself from 1.5 or
2mm thick aluminium sheet. The PCB mounting holes match plastic pillars on the bottom of the
enclosure and it will be necessary to remove unwanted pillars using either a large pair of
sidecutters or carefully with a large drill.
Of course any other enclosure that accommodates the PCB will be suitable, although if using the
companion DDS VFO and LED S meter it will need to have similar minimum dimensions.
Before installing the board in the enclosure, carefully look for errors, such as components in the
wrong way and solder bridges between tracks. The risk of solder bridges is greatly reduced due
to the solder mask, but check anyway. A few moments spent here is cheap insurance against big
problems later on.
One of the more common problems is poor solder joints with enamel covered wire. Some types
when soldered will melt the enamel but most will not, so it is important to scrape the enamel off
the ends of the wires with sandpaper or a sharp knife before soldering.
8.1 REAR PANEL
The rear panel only contains the SO239 antenna socket and two binding post to connect the
power supply. As the rear panel layout is straightforward no layout drawing is supplied,
however remember to locate the antenna socket near the PCB ANT and GND pins so the wires
are kept short.
8.2 FRONT PANEL
The front panel is designed to accommodate the DDS VFO and LED S meter. It requires a
number of holes and cut-outs to be made as shown in Figure 10. The microphone hole is
dimensioned to fit a standard 4 pin connector however this may need changing to match your
own.
If the LED S meter is not installed you will need to drill a clearance hole for the TX
PWR LED bezel instead of the cutout for the LED bar display.
There are two options for the DDS VFO mounting holes. Make them countersink to have a flush
surface and no holes in the label, or if you are happy to have screw heads showing on the front
leave them straight drilled.
8.3 MOUNTING THE PCB
Temporarily put the rear panel in place and sit the PCB on the mounting pillars. If needed bend
the leads of Q12 slightly so that the metal tab is resting parallel against the rear panel. Use a
sharp pencil or scriber and run around the inside of the hole in the tab to mark the position for
the mounting hole on the rear panel. Remove the rear panel and drill a 3mm hole ensuring that
you remove all burrs.
MST2 20M Construction Manual – Issue 1
Page 33
Screw the PCB to the mounting pillars using small self-tapping screws. Slide an insulating
washer between the rear panel and Q12 tab. Insert a plastic bush into Q12 tab then pass a 3mm
screw from the rear through the bush and screw on a nut and tighten.
Check with a multimeter that there is no electrical connection between the rear
panel and Q12 tab.
8.4 FRONT PANEL LABEL
A front panel label will add a professional look to the completed transceiver. You can design one
yourself using a graphic design software package or you can simply download a completed
sample from www.ozQRP.com. If you plan to design your own then use the dimensions shown
in Figure 10 as a guide. Once you have either your own file or the downloaded file follow the
procedure below to produce the label:
1. Print the file using a colour inkjet printer onto an A4 sheet of high quality photo paper.
2. Using a sharp hobby knife and steel rule as a guide cut out the inside area of the DDS
VFO LCD window and the LED S meter window.
3. Laminate the whole sheet using an A4 laminator. If you don’t have your own then this
can be done quite cheaply at large stationary stores. The exciting thing about this is that
the cutouts convert into transparent windows.
4. Using the hobby knife cutout the other holes in the label.
5. Cut around the border of the label so that it is the same size as the front panel.
6. Place the label on the front panel and install the pots, switch, microphone socket and
LED bezel if required. You can use some spray adhesive to secure the label to the front
panel if it tends to lift off, although if using the recommended case it will stay in place
once the enclosure is assembled.
MST2 20M Construction Manual – Issue 1
Page 34
Figure 10 Front panel drilling guide
MST2 20M Construction Manual – Issue 1
Page 35
8.5 FITTING THE DDS VFO
The DDS VFO is simply screwed to the inside of the front panel. The DDS VFO PCB is set back
from the panel using threaded spacers to allow room for the LCD. Using the specified LCD the
distance will be 22mm and is made up of a 10mm spacer and 12mm spacer. Alternatively a long
spacer cut down to 22mm can used.
A countersink screw is used on the front panel to provide a flush surface for the front panel
label to sit. If preferred round head screws can be used which means that holes need to be made
in the label and also the heads will be visible from the front.
If an alternate LCD is used that does not incorporate a backlight it will not be as high and so the
length of the screws and spacers need to be reduced accordingly.
The general mounting arrangement is shown in Figure 11.
10mm x 3mm tapped
nylon spacer
12mm x 3mm tapped
nylon spacer
3mm x 16mm C/S screw
3mm x 6mm screw
Front panel
PCB
LCD
Figure 11 DDS VFO front panel mounting
MST2 20M Construction Manual – Issue 1
Page 36
9 WIRING UP
The wiring diagram is shown in Figure 12. Connections using with light duty hookup wire
employ different colours for easy identification and parallel wires are twisted together. The
connections to the AF gain control, MIC gain control and the LED S meter input should be in light
duty shielded audio cable. The VFO cable can also be made from shielded audio cable as the
length is quite short.
Most PCB connections utilize 2.54mm (0.1”) pitch polarized headers and mating plugs. These
are inexpensive, give a professional looking appearance and allow easy disconnection and
removal of the PCB if required. The plugs are comprised of a plastic housing and crimp contacts
which slide into the housing and click in place. If you don’t have a suitable crimper it is best to
squeeze the contact wings around the wire with long nose pliers to hold in place and then
solder. Be careful not to use too much heat and solder as it will make it difficult to insert the
contact into the housing.
The power supply connection uses a screw terminal block because of the higher currents
involved.
The antenna connection uses short wires soldered between the antenna socket and PCB pins to
give a low resistance connection. You will also need to install a solder tag for the earth wire
under the closest antenna socket screw.
The loudspeaker is mounted on the underside of the enclosure lid and held in place with 3mm
countersunk screws and nuts. A series of holes needs to be drilled in the lid to act as a speaker
grill.
MST2 20M Construction Manual – Issue 1
Page 37
Figure 12 Wiring diagram
MST2 20M Construction Manual – Issue 1
Page 38
10 TESTING AND ALIGNMENT
10.1 GENERAL
A suggested list of basic tools and accessories to undertake testing is shown below.








Digital multimeter.
50 ohm dummy load capable of dissipating at least 5 Watts.
QRP wattmeter or oscilloscope.
Power supply capable of 13.8V DC regulated at more than 1 Amp.
Small adjustment screwdriver.
An audio signal generator.
An RF signal generator.
A frequency counter.
A VFO is required for testing and operation and it is assumed during the rest of this
section that the MST2 board is being used with the DDS VFO which is already
configured and fully operational.
Figure 13 lists typical voltages at various points around the board and can be used
to verify operation.
10.2 POWER ON








Temporarily remove the power connector for the VFO so that the MST2 board current
can be measured alone.
Using a small screwdriver:
o Rotate the bias trimpot VR3 fully counter-clockwise so that the gate bias voltage
is zero volts.
o Set carrier balance trimpot VR1 to halfway by counting turns. You should hear a
slight click as you reach either end.
o Rotate the AGC trimpot VR2 fully counter-clockwise.
Connect a 50 Ohm dummy load to the antenna socket.
Connect the power supply to the board terminals.
If your power supply does not include a current meter, connect a multimeter in series
with the power supply positive lead and set to measure current.
Turn on the power supply and note the supply current. It should be around 80mA. If it’s
far from this turn off immediately and look for problems.
The relay should be de-energized and the board in RX mode. To verify that there are no
obvious problems do a quick probe around the board with a multimeter and check the
DC receive voltages as shown in Figure 13.
Plug in the VFO power connector. Check the power supply current. If using the DDS VFO
it should rise to around 135mA. Again, anything far from this needs investigating.
MST2 20M Construction Manual – Issue 1
Page 39
10.3 RECEIVE





Set the VFO to a frequency in the middle of the band you will be using.
Turn the AF gain control to halfway. You should hear some low level hiss come from the
speaker indicating that the audio stages are working.
Connect a frequency counter at the emitter of transistor Q2. Adjust trimmer capacitor
TC1 until the frequency is 9.996MHz. This is the carrier (BFO) frequency and is placed
on the lower skirt of the crystal filter for Upper Sideband operation.
Remove the dummy load and apply a moderate RF signal to the antenna connector and
tune the VFO until a clear tone is heard in the speaker. Carefully adjust the slugs in coils
T2 and T3 for maximum volume. As you approach maximum you will need to turn the
AF gain down and probably the RF signal level as well to avoid overload. You can
experiment with the position of the two slugs to obtain a specific bandwidth, but in
general, peaking at the middle of the band will be satisfactory.
If fitted, set up the LED S meter display as per the instructions in the LED S meter
construction manual.
10.4 TRANSMIT




Remove the RF signal source and reconnect the dummy load. It will be an advantage to
have either a QRP power meter or oscilloscope to obtain an indication of power output.
Turn the Mic gain control fully counter-clockwise.
Briefly switch to TX mode by shorting the PTT contacts. Check the power supply current.
This is the base transmit current and should be about 220mA. Anything far from this
indicates a problem and should be investigated.
Operate the PTT and using a screwdriver slowly rotate the bias trimpot clockwise. The
power supply current should rise gradually and smoothly. Keep increasing until the
power supply current is about 400mA. The increase over the base current is mainly due
to the output MOSFET starting to conduct and move into linear operation, but some will
be because we have not balanced the carrier yet and there is some power output.
The final bias current is set in a later step. Ensure now that the current is not too
high or the output MOSFET will get very hot.




Adjust carrier balance controls TC2 and VR1 for minimum power output. The null is
quite sharp, and there is some interaction between the controls, so you will need to go
back and forwards to obtain maximum balance.
With the carrier balanced rotate the bias trimpot fully counter-clockwise to turn off the
MOSFET. Note the quiescent transmit current again.
Slowly rotate the bias trimpot clockwise until the power supply current is 150mA higher
than the base transmit current. The output stage bias current is now set.
If you still notice some power output readjust the carrier balance controls for minimum.
MST2 20M Construction Manual – Issue 1
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



Apply an audio signal generator to the Mic socket set to 1KHz at around 50mV. Operate
the PTT and slowly increase the Mic gain while monitoring the power output. When
some power (about 1 Watt) is indicated carefully adjust the slugs in coils T5 and T6 for
maximum output. You may need to readjust the Mic gain to get a suitable power output
to monitor the peak. Once the coils are peaked operate the Mic gain up and down while
monitoring the power output. The power output should change smoothly up and down
and you should be able to easily achieve 5 Watts output.
Refer to the LED S meter Construction manual and verify operation. If you are not using
the LED S meter, check that the RF power LED illuminates and changes intensity with
varying power output.
If you intend to use an Electret microphone place a short across LK1. This can be a
soldered link or you can install a 2 pin header and a removable shunt. Leave LK1 open
for dynamic microphones.
Plug in a microphone and check there is RF output when you speak and the relay
operates when the PTT is pressed. You will now be able to monitor yourself with a
receiver placed nearby and determine the correct Mic gain setting.
10.5 TONE GENERATOR



Rotate the Mic gain control fully counter-clockwise. Close the Tone generator switch.
Operate the PTT and check there is no RF output.
Slowly rotate the Mic gain control clockwise and note the power output increase. You
should be able to go between 0 and about 5W output.
MST2 20M Construction Manual – Issue 1
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Location
V DC Receive
0V
0V
6.8V Note 1
10.8V
5.25V
5.25V
8V
3.9V
1.1V Note 7
0V
6.8V Note 1
8.9V
3.9V
0V
0V
0V
0V
0V
0V
0V
0V
0V
0V
V DC Transmit
13V
12.8V Note 4
0V
0V
0V
0V
8V
3.9V
0V
6.8V Note 1
0V
8.9V
3.7V
5.3V Note 3
5.4V
0.35V
5.2V
0.17V
4.9V
1.5V
0.9V
4V Note 5
7.5V Note 6
U1 pin 8
U1 pin 1
U2 pin 8
U3 pin 8
U3 pin 1
U3 pin 7
U4 pin 1
U4 pin 5
U4 pin 4
U6 pin 8
U5 pin 8
Q1 collector Note 1
Q1 emitter Note 2
Q2 emitter
Q3 collector
Q3 emitter
Q4 collector
Q4 emitter
Q8 emitter
Q10 emitter
Q11 emitter
Q12 gate
Q13 emitter
Notes:
1. Zener tolerance +- 5%.
2. DC measurement affected by RF. Approx. 5V without crystal.
3. DC measurement affected by RF. Approx. 7V without crystal.
4. 0V when Tone switch closed.
5. No RF drive.
6. 5W output
7. No receive signal
Readings taken with a digital multimeter.
Power supply voltage set at 13.8V DC.
Figure 13 Typical circuit voltages.
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10.6 AUDIO AGC ADJUSTMENT
The audio AGC is easy to set up and the procedure outlined below has been refined after many
hours of on-air listening.




Set the AF gain control to about half way
Inject an S9 RF signal into the antenna connector.
Slowly rotate the AGC trimpot (VR2) clockwise until you can just notice a lowering of the
audio level. This sets the point where AGC starts to operate.
Increase the RF signal level and check that the rate of increase of the audio level is much
less than the RF input level change.
The AF gain control is located before the audio AGC circuit and so adjusting the AF
control for signals above S9 may not have the expected outcome. This is because the
AGC circuit is attempting to adjust the audio and keep it constant. For signals below
S9 the AF gain control works in the expected manner.
If the audio AGC is not required simply rotate VR2 fully counter-clockwise.
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10.7 CARRIER FREQUENCY ADJUSTMENT
For proper SSB operation it is important to align the carrier frequency with the frequency
response of the crystal filter.
Shown in Figure 14 are three possible conditions that can be found in practice. The DSB signal
produced by the balanced modulator contains both LSB and USB sidebands. These extend
equally outward from the carrier. The carrier oscillator is shown as a dotted line but it is
suppressed in the modulator. Also shown is the crystal filter frequency response.
While the examples shown are for transmission, the same concept applies to
reception.
The suggested carrier frequency for USB operation is 9.996MHz which places it on the lower
skirt of the crystal filter frequency response. This is shown in the top diagram in Figure 14.
Due to small variations in individual crystal filters the carrier frequency may need changing
slightly to match the filter. The middle and bottom diagrams in Figure 14 show the two
scenarios where the carrier frequency and the crystal filter response are misaligned.
There are a number of ways to correct this but without instruments the easiest is to listen to a
good strength station of known quality while gently adjusting trimmer capacitor TC1. Adjust
until the received audio sounds natural and the balance of high and low audio frequencies
appears correct.
Note that you will need to adjust the VFO frequency to retune the received signal
while making adjustments because the received signal will have moved off
frequency by an amount equal to the change in the carrier frequency.
Alternatively you can adjust the carrier frequency while transmitting into a dummy load and
monitoring yourself on a nearby receiver. While speaking adjust TC1 until you sound ‘normal’.
You can also switch between sidebands on the receiver to hear the reduction in the opposite
sideband.
If you move the carrier frequency and using the companion DDS VFO you will need
to measure the carrier frequency and re-program the IF frequency configuration
item to ensure the display reads accurately. See the DDS VFO construction manual
for details.
MST2 20M Construction Manual – Issue 1
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This diagram shows the carrier
frequency correctly aligned with
the crystal filter passband. The
whole of the USB is passed but the
LSB is filtered out.
In this diagram the carrier
frequency is shifted low in
comparison to the crystal filter.
This results in the low frequency
components of the USB being
filtered out. This would make the
signal sound thin and lack body.
In this diagram the carrier
frequency is shifted high in
comparison to the crystal filter.
This results in the high frequency
components of the USB being
filtered out. This would make the
signal sound muffled and have too
much bass.
Note also that some of the LSB
signal is passed by the filter.
Figure 14 Carrier frequency and crystal filter relationship
MST2 20M Construction Manual – Issue 1
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11 OPERATION
Operating the MST2 is very easy as there are no complicated controls and software menus to
navigate. It’s simply a matter of connecting a power supply, microphone and antenna and
talking. Operation of the DDS VFO is covered in the DDS VFO construction manual, and the LED
S meter in the LED S meter construction manual.
In RX mode apart from the VFO there is only the AF gain control to adjust. The MST2 does
incorporate a form of automatic gain control (AGC) where the audio level to the speaker is
limited for input signals above about S9 level. This is performed in the audio power amplifier
which is after the AF gain control. Advancing the AF gain control will not seem to act in the
normal way for large signals, as the AGC circuit will attempt to keep the speaker level constant.
For signals less than S9 the AF gain control will act in the normal way as the AGC circuit has not
been activated.
To transmit press the PTT button on your microphone and talk. The LEDs on the LED S meter
will illuminate giving an indication of the power output.
If the LED S meter is not installed the front panel RF LED indicator will illuminate on voice
peaks. It can also be used as a rough guide to power output as the current through the LED, and
therefore its brightness, is dependent on the peak RF output. If the LED is illuminated at a
constant level when talking, it indicates that you are driving the transmitter too hard and
causing clipping of the RF signal.
Overdriving will create distortion and excessive harmonic generation and must be
avoided.
To check your signal either connect the MST2 to a dummy load and monitor yourself with
headphones on a nearby receiver, or have a friend that lives close by listen to your signal. The
idea is to increase the Mic gain progressively while the receiver sweeps across your
transmission looking for distortion and unwanted spurious byproducts. Set the Mic gain control
just below the point where these are noticeable.
Congratulations your new SSB QRP transceiver is ready to put on the air.
Have fun!
MST2 20M Construction Manual – Issue 1
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