The Idea & Design
The
circuit is quite simple and straight forward. I don’t’ claim that this is my
design. The basic idea was from an article by W6IBC which employed two numbers
of SA 612 and had very few coils & components. The circuit has been
modified to ensure the availability of all components from the local market, simplicity
of VFO design, deletion and addition of RF amplifiers to have better noise
level, to provide digital read out, higher audio output and to have a RF
amplifier train to deliver an out put of 1 watt from the exciter. I would like to thank OM Vaidyanathan (VU2PRV) who
kindled the interest to me in taking up this project and helping in development
of the circuit design.
The Circuit Description
The circuit
employs two numbers of NE612 / SA612 and a crystal filter between the two ICs with
an IF amplifier stage. Both the ICs have dual function in the Transmit &
Receive mode. The IC1 doubles as receive and transmit
mixer and IC2 acts as product detector and balanced modulator.
As shown in
the circuit the IC U2, a mixer operates between audio and IF as a product
detector in receive mode and as a balanced modulator in transit mode. This also
acts as a BFO/Carrier oscillator with a crystal and a trimmer to adjust the
BFO/ Carrier oscillator frequency.
The
signal received from the antenna after the band pass filter is mixed with the
VFO signal. The out put is amplified further by a single stage (2N3904) before
the filter. A Cohn type 10 MHZ filter
using 6 numbers of 10 MHZ crystals is used for good skirt selectivity. The
filter circuit component details and values are given in a separate circuit.
As
the IC U1 NE 612 requires very low VFO injection, a single stage BFW10 FET is
used in the calpits mode. The VFO output is taken through a 6.8pf capacitor. It
is preferable to have all the capacitors in the VFO section of styroflex type
to minimize drift. An additional stage with BC549 transistor is employed to facilitate
connecting to a digital read out through a 100pf capacitor from the emitter of the
transistor. This eliminates the noise generated by pic IC of the digital
read out. A separate small pcb is used for the VFO circuit so that it will be
easy for mounting near the 330pf tuning gang. A stabilised DC voltage of 5
volts is powering the VFO derived from 78L05. The VFO coil comprises of 40
turns ( 36SWG) on a small IFT former.
The
bi-directional signal flow through the crystal filter not only simplifies the
circuit but also minimizes the number of components in this stage. A 74 HC4053
IC is used for change over of the bi-directional signal flow between transmit
and receive modes. Even though the entire circuit operates on 12 volts a 7805
is used to power 5 volts to all the ICs except the audio amplifier IC LM380
which operates on 12 volts. A condenser mike capsule is used with a single
stage transistor BC549 as preamplifier and the output is fed to the U2 NE612
IC. The audio out put from pin 5 of U2 NE612 IC is further amplified by a
single stage transistor 2N3904 and fed to the LM380 audio amplifier IC. The
speaker can be 4-8 ohms impedance.
The
PTT switch energises when pressed a 12 volts relay RL1. Two sets of the RL1
relay contacts are used for change over of filter connections through 74 HC
4053 IC, and powering the audio stage and cutting the power to mike amp stage.
When the relay RL1 is in the de- energized condition, it will be receive mode
and when the relay RL1 is energized it
will change over to transmit mode. The relay RL1 in the transmit mode also
energises another relay RL2 which is used for Antenna change over. Both RL1 and
RL2 are small PCB mounting type relays which are mounted and soldered in the
exciter PCB itself.
The
RF amplifier train consists of three stages using 2N2222A, 2N2219 and BD 139
Transistors. The out put from the BD139 transistor can be connected to a single
stage or push pull amplifier as desired . The PCBs for the VFO & main
exciter are available as a set and the push pull amplifier PCB set is available
as an optional item. The power supply has to be decided depending upon the
final power amplifier stage ie a single stage or push pull amplifier. How ever the exciter
requires 12 Volts supply.
Construction
The
coil details of T1 to T4, L2,T5 and T6
are given in the circuit. T1 to T4 are wound using the small IFT formers. The
coil L1 at the output of 2N3904 is an inductor of 47micro Hendry. The 250 micro
Hendry RFCs in the RF amplifier stage can be the readily available inductors of
higher current rating or it can be prepared by winding 20 turns of 28 SWG wire
gauge on a ½ inch torrid. The T5 consists of 30 turns of primary & 4 turns of secondary
with 28 SWG enameled wire on a ½ inch dia torrid. The T6 is wound using a ½
inch TV Balun core consisting of 6 turns primary and 4 turns secondary winding
with 28 SWG enameled wire.
It
is to be noted that some of the 74HC4053 IC malfunction. Please use a IC base
for all the ICS. So after completing the entire assembly and while testing the receiver the IC 74HC4053
can be removed from the base and jumpers can be used to the respective pins on
the IC base like 4-3, 15-1 & 14-13.The specialty of this circuit is that
there are no tuning of coils involved except the band pass filters,. The exciter
with push pulls amplifier delivers easily 40 watts power.
The
photographs of the rig constructed by me are given for reference. All the type of
components used on the PCB , the front panel and push pull amplifier sections
are clearly shown for easy understanding.
It
is possible to use the same PCB for 3.5 Mhz or 14 Mhz by suitably modifying the
BPF coils and adding suitable amplifier stages.
Thyagu
(VU2PTR)
Completed the RX of the rig. Had problem in TX. Now I can fix the problem following your instruction.
ReplyDeleteTo day I and my friend VU2LMO made an experiment with the band pass filter coils of this TVCR. We replaced the pipe cores with bumb bell shaped IFT coils. The reception gain increased dramatically and near zero noise.
ReplyDeleteDear Jaydip Banerjee ,I am vu2lf , Laxman, from chennai,My cell No.9600531859 , can u whatsup me the photo of the ift which u have used ptr2k10, tks in advance,73's
ReplyDeleteDear Jaydip Banerjee ,I am vu2lf , Laxman, from chennai,My cell No.9600531859 , can u whatsup me the photo of the ift which u have used ptr2k10, tks in advance,73's
ReplyDeletePTR2K10 cabinet available ?
ReplyDeleteGreetings!
ReplyDeleteI joined HAM community recently, and my call sign is VU2CSF. I would like have some of your kits, like Digital SWR cum Power meter, LC meter, Dip meter, VHF swr meter etc. for my home brewing. Expecting a reply from you. My cell - 07205892406/07873007007, email - csp.ogimac@gmail.com
Thanks and regards, Chandra Sekhar (VU2CSF)
Hi Thyaga, do you have a PCB for this transceiver? if you have can you please let me know I need one Thanks
ReplyDelete