Since all the questions from part 6 have stopped, I guess we
are ready
for part 7. The web site has been updated with all the part 5 and
part 6
stuff, along with the quiz answer ;-)
http://www.qsl.net/kf4trd
Here 'ya go.
In the transmit chain we left off without instaling the final
amplifier
transistor. This was done on purpose. With the final out we don't
have
to worry about frying the final because we forgot to connect the
dummy
load. We will install it last before the final calibration.
The reason for building the transmit circuit first was to provide
us
with a signal source for testing the different sections of the
receiver.
This should make troubleshooting the receiver easier for those
who don't
have an RF signal generator.
So, gather the following parts:
T1 IF can (the last one in the kit)
C1 47 pF
C40 47 pF
C101 .1 uF
D7 1n4148 diode
D8 1n4148
D9 1n4148
D10 1n4148
RFC3 10uH inductor (brn blk blk) in the "misc" bag of
parts
8 pin socket for U1
U1 NE612 mixer IC
Install the above components using the same caution as in the
previous
lessons. CHECK YOUR WORK!
Now we are going to cheat a little. We are going to
"borrow" some signal
from the output of the transmit mixer so that we can get a nice
strong
signal for the receive sections.
Two TEMPORARY jumpers are needed for this. I used two of the
trimmed
leads from one of the capacitors that I just installed. Solder
them
lightly, as they will need to be removed later. I soldered mine
on the
top of the board. All references are looking at the front of the
board,
with T1 in the lower left hand corner. Connect the first jumper
from J1
between pins 2 and 3 (Those are the top two). The second jumper
is from
the base hole for Q6 (left most hole) and and the top hole for
C36.
Remember that all references to "top" and
"left" are with you looking at
the top of the board with T1 at the lower left hand corner. (I
wish I
had a digital camera ;-)
What these jumpers do is bypass Q6 (which is not installed yet)
and
bypass the RF Gain potentiometer. This feeds the transmit signal
from
the base of Q6 into the input of T1.
Set T1 to mid range. This will be tweeked later.
Turn on the rig and check for smoke ;-) None? Good.
Ok, put your RF volt meter or o-scope on pin 5 of U1. An easy
place to
measure this is at the top pad of C11. The o-scope users will see
a
little (millivolt) signal leaking through from the VFO. Key up
the rig.
You should see RF at pin 5. O-scope users should see a complex
waveform
at pin 5. Mine was several volts peak to peak. This waveform
should look
similar to the one we saw at the output of U5, the transmit
mixer,
before the bandpass filter.
Here is a little theory.
>From the antenna the receive signal first passes through the
first
bandpass filter consisting of L3, L4, C37-39 (not installed).
This
cleans up the transmitted signal during transmitting and
preselects the
7Mhz band during receive. The signal is coupled through C40 to
the four
diodes whos function is to limit the signal reaching the receiver
by
clamping it to ground if it is over 1.4 volts. The only time that
it
will reach that level should be during transmit. The signal from
the
transmit that reaches the receiver provides sidetone during key
down.
The RF gain pot (not installed) is used to reduce the RF from the
antenna while receiving a strong signal. The connection of this
part is
a little unusual and it took me a while to figure it out. In
other
instances where control of a signal is required, you see the
signal
injected into the "top" of the pot, the bottom of the
pot grounded, and
the output signal taken from the wiper. The problem with this
setup is
that as the wiper postion is changed the output impedance is
changed
also. In Daves configuration the impedance is relativly steady
across
the wiper postions. This is important as changes in this
impedance will
effect the resonant circuit of T1.
T1, which is set to resonance at the receive frequency, couples
the RF
into U1. Here it is mixed with the VFO signal and is available at
pin 5.
This signal will contain two major components and several minor
components. The major components are RF+VFO and RF-VFO. At 7 Mhz
with a
3 Mhz VFO, there will be 10 Mhz and 4 Mhz. The 4 Mhz is the one
that we
are interested in as it is our IF frequency. All these unwanted
"nasties" will be removed by the next section (part 8)
the crystal
filter. More detailed info on the mixer can be found in part 5,
the
transmit mixer.
Remember that our transmit mixer oscillator (Y5) was
"pulled" lower in
frequency by RFC2 and C29. This gives us the proper offset for
receiving
on the proper side of the signal. At a receive frequency of 7.040
Mhz
the VFO will be at 3.040 Mhz and the transmit frequency should be
about
800Hz lower, or 7.0392
Ok, have at it. Please post any questions to the list so we can
discuss
them.
I have 2 questions right off the bat.
1) What is the purpose of RFC3?
2) Why is pin 2 of U1 fed from the center tap of T1 instead of
from the
top of T1?