Testing & Alignment Procedure for the
PTR2K10
7 Mhz Transceiver
1.0 General :
It is assumed that the Builder is a
call sign holder having valid License and having requisite knowledge about
operation principles of SSB circuit, handling measuring equipment, prior
experience in construction / home brewing of electronic circuits and good
soldering practices.
After completing soldering of all the
components of exciter, clean the entire pcb with IPA solution so that all the
excess soldering paste and other dirt will be removed. This is very essential
as some sections may create problem due to presence of soldering paste. Also
check for dry soldering. Don’t forget to solder the cross over jumpers marked
as “J” on the pcb with hook up wire as, even one is left out it will give raise
to problem. Then check correctness of polarity of all the electrolytic
capacitors and diodes. Check the pin out for all transistors. Always use appropriate IC base for all ICs
and suitable heat sinks for the voltage regulators and transistors. While
winding the band pass coils check and
ensure that the starting point of the windings of T-1,T-2,T-3 &T-4 are
soldered correctly to the pin of lowest
voltage (cold) point. The T-3 & T-4 need not be soldered in the beginning
which can done later after testing the receiver section. The reason is
explained in the later part.
2.0
Testing of Audio stage :
The easiest stage to test is the Audio
section. Solder wires for the positive, negative of the 12 volts power supply
to the main exciter pcb and wires for the condenser mike & loudspeaker. It
is to be noted that the LM380 Audio amplifier and the mike circuit operates on
12 volts. The LM380 will get 12 volts only in the receive mode and the mike
circuit will get power in the transmit mode. Hence for testing both together,
12 volts must be connected to the positive side of both circuits by a jumper. Now solder the ground and center point of
the AF gain control as per the circuit. The –ve of 1 mfd capacitor (C35) from
collector of BC549 (Q 4) of the mike circuit should be connected with a
shielded cable to the free end of AF gain control instead of –ve of C42.
Now power 12 volts. Check presence of
12 volts and 5 volts at the I/P & O/P of 7805 (U4) regulator using a
multimeter. If the center point of the AF gain control is touched by a finger
hum should heard from the speaker. Now say hello in to the condenser mike. You
should get the amplified audio. Increase the AF gain control and the audio level
also should get increased provided the control connections are correct. Other
wise if you increase the gain control the audio level will get reduced. In that
case interchange the end connections on the AF control. Due to feed back loud howling noise will be
heard at the maximum AF gain position. Mic gain preset (R18) can also be
adjusted for good audio. This completes the audio side checking and all
connections should be restored to normal as per circuit. Remove the 12 volts
jumper with out fail.
3.0
Testing the VFO Circuit :
The VFO circuit employs a single BFW10
(Q 101), as the VFO injection required for NE612 / NE602 is very low. After
soldering all the components, the VFO pcb it should be cleaned with IPA. Solder
the tuning gang condenser and power supply connections. Connect a counter at
the VFO O/P point. Now power 12 volts and measure the voltage at the O/P of 78L05
(U100) with a multimeter which should be 5 volts. If every thing is fine the
VFO should oscillate. If you find no frequency read by the counter check and
correct the defect. Check the frequency variation by moving the gang condenser shaft
from one end to the other side. Adjust the slug of the L-100 coil wound on the
IFT former till 2.9 to 3 Mhz frequency variation is achieved while moving the
gang from one end to the other extreme. Also there may be a need to vary the
100pf (C102) capacitor value connected in series with the gang, to achieve the
required frequency cover. With Filter
frequency as 10 Mhz this will give 7.0 to 7.1 Mhz for the 40 mtr band operation. The
capacitors in the VFO section has to be styroflex type to avoid drift problems.
The O/P from BC549 (Q100) emitter can be connected to a homebrewed frequency
counter to have digital display of the frequency selected.
4.0
The cohn Filter :
The cohn filter comprise of 6 nos of
10 Mhz crystals and 100pf disc capacitors as show in the circuit. It is very
important to select the crystals for the Filter and LSB operation. The
following procedure can be adopted for crystal selection. Construct a single
stage crystal oscillator circuit using BF194 or 2N2222A. About 30 nos of 10 Mhz
crystals are required to select the best 6 nos for the cohn filter. Mark all
crystals from 1 to 30 with marker pen. Connect a frequency counter to the O/P
of the crystal oscillator. Power the oscillator circuit and connect the
crystals one by one and note the frequency of each crystal. Out of the 30
crystals select the best 6 nos with in 100 Hz variation to get the best skirt
selectivity and good performance. Ideally if all crystals are of the same
frequency then that will be the best which may be very rare to get. Now for the
LSB operation select a crystal having 1.5 Khz more than the 10 Mhz cohn filter
crystals. For example if the selected cohn filter crystals average frequency is
10000 Khz then the carrier oscillator crystal (Y7) connected to pin no 6 of
NE612 / NE602 (U2) should be 10001.5 Khz.
5.0
Alignment & Adjustments for the Receiver section :
Before attempting to test the receiver
section remove the 74HC4053 IC (U3) from its base. Use short wire jumpers to
connect pins 4-3,1-15,and 13-14 on the IC base . This is to rule out any
problem associated with the IC, as some makes in my experience are giving poor
contacts. After alignment and adjustment the jumpers are to be removed and the
IC can be fixed back on the base. The IC 74HC4053 can change over contacts by
simply connecting the pins 9 &10 to ground by the relay contact. If it is
not happening the pin no 16 of the IC is held at 10 volts by a zener diode (D4).
For alignment, if you have a signal
generator the job will be simple. Connect the VFO to the exciter board with all
power supply, tuning gang connections, VFO O/P to the injection point etc. Then
feed 7050 Khz AF modulated signal to the antenna point after powering the
exciter board. Now low audio tone should be heard in the speaker at one point
while VFO gang is tuned. Now the band pass coils T-1 and T-2 slugs are to be
adjusted for peak reception. After this, connect the actual antenna cable to
ant. point removing the signal generator. Power the exciter pcb and VFO pcb.
Keep the 10 K (R1) control of VFO injection point in the middle position and tune for some strong
station. It is always preferable to carry out this testing in the morning / evening
time when the band is active with many stations. Adjust 22 pf Trimmer (C50) for
best carrier placement. Fine adjustment can be made with T-1 &T-2 for best
audio and reception. The above procedure can be repeated for a weak station as well.
That’s all.
The beauty of the circuit is, not many
coils are there involving adjustments. Now the receiver section should work
with good audio level. At this stage the jumpers on the 74HC4053 IC base can be
removed and the IC can be inserted in the base after switching the power off. Now
when you switch on power, the Audio level should be same as before, other wise
there is a problem with the IC which may warrant a replacement.
6.0
Modification of VFO circuit
:
This circuit was launched during 2010
and at that time the 40 Mtr band operations was restricted to 7 to 7.1 Mhz. Subsequently
the 40 mtr band allocation has been enhanced to 7 to 7.2 Mhz which is double
the previous band width. It is to be noted that the VFO and Band pass coils at
receiving end are to be modified for operation from 7 to 7.2 Mhz frequency.
Retaining the same gang of 330 pf the the
following changes are to be made in the VFO circuit to get 7.0 to 7.2 Mhz
frequency cover.
1.
The
number of turns of L-100 Coil wound on IFT former should be 28 using the
same
Gauge of wire.
2.
The capacitor 100 pf (C102) in series to the gang should be changed to
68 pf.
3. The two nos 1000 pf capacitors (C104 &105)
should be changed to a single 270 pf
capacitor.
The rest of the VFO circuit will
remain same. As described earlier the capacitor (C102)
in series to the gang may also need
slight variation to get the coverage from 2.8 to 3
Mhz variation in the VFO.
To increase the band width of the
front end Band pass filter T-1 & T-2, connect 27-47 K ¼ watt resistors in
parallel to the tuned circuit coil side ( 35 turns). Due to high signal level,
the transmit side band pass coils T-3 &T-4 may not require any alterations
in the number of turns except for slight adjustment of slug.
If modified VFO circuit is used please
remember that 7100 khz AF modulated signal should be fed to the antenna point
while alignment & adjustments are made for the Receiver section.
7.0
Alignment & Adjustments for the Transmitter section :
It is to be noted that the T-1,T-3 and
T-2, T-4 are similar in size and the number of turns are same. So before
proceeding with transmitter section testing, the band pass coils T-1 & T-2
can be de-soldered and soldered in the T-3 &T-4 positions with out any
adjustment of the slug cores. This will simplify the tuning procedure. Connect
a small 12 volts bulb at the T-6 O/P point.
Now power up after connecting PTT
points and the mic. Try transmitting by pressing PTT and whistling in to the
mike. The bulb should light up with good brilliance. Minor adjustments can be
made with the T-3 &T-4 slug core. Don’t adjust the slugs too much. If the
exciter delivers about 1 watt O/P the bulb will glow brightly. If the builder
is conversant with using a RF probe it can be used instead of bulb test. My
experience is bulb test is quite good as the out put can be seen by the
brightness of the bulb with out worrying about the analog value of the
multimeter reading with RF probe.
Now the exciter is ready for
connecting to the single ended or a push-pull amplifier for getting 30 to 50
watts power out put. Now solder the other two band pass coils in the T-1 &
T-2 positions and adjust them as explained in the receiver section alignment
& adjustment procedure using the signal generator.
In case the exciter out put is less
than 1 watt, due to variations in the component values, variations in winding
by individuals, core material and other factors beyond our control, some more
modifications can be done as explained below. Try one by one at a time or else
the RF amplifier may go to self oscillations. Also try as per the order given
and don’t make all changes which will lead to self oscillations. To prevent
self oscillations and spurious harmonics certain designed value of resistors
are provided in the RF amplifier chain. To get more power there is a
possibility to vary these values with in certain limits. If self oscillations
& excessive heating are noticed due to a change, then revert back to
original value before trying the next point.
1. First connect a 5 ohms
resistor to the 2N2219 (Q 6) emitter instead of 10 ohms (R32) . This should increase
the out put level.
2.
Reduce
the value of 220 ohms (R27) resistor value in the 2N2222A
(Q 5) transistor
emitter to 100 ohms.
3.
The
10 ohm (R34) resistor connected across T-5 secondary can be removed.
4.
The
value of 150 ohms (R36) resistor connected across primary of T-6 can be
increased to 220ohms or 330ohms.
5. The
220 ohms (R33) resistor connected across T-5 primary can be increased to
470ohms.
6. Reduce the value
of 1ohm (R37) resistor in the emitter of BD139 to 0.5 ohm
½ watt.
Check for the excessive heat. If the transistor is getting over heated
revert back to original condition. This
completes the entire procedure.
