Here is part 6 elmerlings....
We will build this up in two stages. First the RF buffer, then the
driver. A lot of the theory for these sections is over my head so don't
be shy. Ask questions!
For the first section gather and install the following components.
R24 500 ohm trim pot
C34 .01 microfarad ceramic (103)
Follow all the standard installation steps and check your work before
Turn on the rig and connect your temporary key. You should see a nice RF
sine wave at the base of Q5 (not installed yet). Remember that adjusting
R24 will change the voltage from 0 volts to maximum. Make sure it is
adjusted up. If you measured the output of the bandpass filter this
reading may be a little lower. That's ok.
Just a side note. Any time you make a measurement of an electronic
circuit you will effect it in some way, guaranteed. We try to minimize
the effect by using the correct measuring device. Case and point, there
were a few folks who had trouble adjusting T2 and T3 for a peak in the
rf output. The reason for this was that the 0-scope probe was changing
the characteristics of the circuit and changing resonance due to added
capacitance and resistance. So, as we move forward keep in mind that the
bandpass circuit is easily effected by circuit changes.
So we have this nice pretty 7MHz sine wave at the output of T3. The
problem is that it is sensitive to circuit changes. So we must isolate
it from the following sections. This is the job of Q4. It is configured
as a common collector amplifier with voltage divider bias. The bias on
the base of Q4 is set in the linear region of the transistor by R22 and
R23. With a 12 volt supply this makes the DC bias on the base of Q4 3.75
volts. Vr2=(12 * r2)/(r1 + r2)
For information and an excellent explanation of bipolar transistor
biasing please refer to chapter 8 page 20 of the 1998 ARRL handbook. It
is a must read section!
In brief, the common collector amplifier (a.k.a. emitter follower) has
some interesting characteristics. It has a gain of less than one
(Hmmmm), a very high input impedance and a very low output impedance. So
it does not provide any voltage gain, but it does isolate the bandpass
filter and the RF driver section (next section).
If you tweeked T2 and T3 in the last section, put your probe on the hole
for the base of Q5 (not installed yet). Now key up and see if you can
tweek up the voltage with T2 and T3. I went through it again and was
quite a bit off. This is because my scope probe changed the resonance of
T3 and bumped the whole circuit a bit. My scope probe is rated at 10 meg
ohms and 11.8 picofarads of capacitance.
Now while trying to peak T2 and T3 again I accidently bumped the ground
shield on my scope probe to the top part of R20 (the side not connected
to V+). Can anyone guess which component I fried when I keyed up, and
why? (This is homework ;-)
Ok, now on to part 2. First wind the primary of T4. Please follow the
directions on page 13 of the manual. My experience was that 3" of wire
was not enough for my coil. I would use 3.5" (I had to rewind mine,
yuck). Install it and the secondary as per the instructions.
Gather the following components and install them:
R29 51 (the books parts layout drawing has this incorrectly labled as
C114 .1 (104)
C35 .01 (103)
Check your work before continuing, paying special attention to T4.
Put your scope probe on the base pin of Q6 (not installed yet). Key up
and you should see your 7Mhz wave. Mine was distorted severly until I
turned R24 down a little (I had it cranked all the way up). If you have
an O-scope you will notice that the bottom half of the wave is
distorted. D6 clamps the negative half of the waveform at -0.7 volts.
This part of the circuit has several things I don't understand but I'll
try to explain what is obvious. The purpose of Q5 is to drive the final
amplifier Q6 (not installed yet).
The final amplifier q6 is a class C amplifier, which means the active
device (the transistor) conducts for less than 180 degrees of the
signal. The signal is kept going by the resonant circuit in the
collector. Q6 just gives it a "kick" every cycle to keep it going. More
on this when we put in the final amp.
R25 and R26 set the DC voltage on the base of Q5 at about 2.1 volts.
This sets the DC emitter voltage at about 1.4 volts (2.1 - 0.7 base
emitter diode drop). This sets the DC emitter current at 23 milliamps
(1.4 / (51+10)). C114 bypasses R28 at RF frequencies so the effective
emitter resistance at AC is 10 ohms which changes the AC operating
characteristics (and increases AC gain??). Again refer to the ARRL
handbook for details.
The load for this amplifier is the primary of T4 which is in the
collector circuit. The manual describes it as an 8:1 transformer. I'm
guessing that this ratio provides more current drive to the base of the
final Q6. One of the gurus is going to have to take us through this part
Ok folks, I know this was slow in coming and I apologize. Life has been
a little on the crazy side on this end. Please fire up the questions and
lets really work this section out. There is lot's of bipolar transistor
stuff we can learn here.
Next we start the receiver section. Can't wait to build the "ears" ;-)