Since all the questions from part 6 have stopped, I guess we
for part 7. The web site has been updated with all the part 5 and part 6
stuff, along with the quiz answer ;-)
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
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
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
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?