(I wrote this several years ago, and I no longer have my Wilderness Sierra or the manual that goes with it. You can still leave a comment below with questions about this article, but I may not have the materials at hand any more to give you a good answer.)
Step 1: Jacks, Controls, and Panels
Step 2: Voltage Regulation
Step 3: The VFO
Step 4: The Pre-mix Oscillator
Step 5: The Transmit Switch and Transmit Mixer
Step 6: The Driver and Keyline Buffer
Step 7: The RX Bandpass Filter and Mixer
Step 8: The Crystal IF Filter, ABX, and IF Amplifier
Step 9: The Product Detector, AGC, and AF Amplifier
Step 10: The Sidetone Oscillator
Step 11: The RIT
Step 12: The Transmit RF Amplifier
The recent Elmer 101 series on the Small Wonder Labs SW+40 was met with critical acclaim by the members of the Internet QRP Club. I personally learned most of what I know about ham radio transceivers from the experience of building the SW+40 while following the step-by-step instructions and analysis which was part of Elmer 101. Numerous subscribers to QRP-L are indebted to the authors of Elmer 101 for providing such an excellent learning experience.
Several years ago, I decided it was time to build myself a Wilderness Sierra transceiver. Its purpose would be to become my main rig for backpacking and other portable adventures (including Field Day, of course) and, of course, to provide me some entertainment in constructing it. After all, where’s the fun in operating a rig you didn’t build? But to make this even more fun, I thought I’d conduct my own Elmer session and see if I couldn’t teach myself a few new things while I built this rig. What follows is the result of this little expedition.
I make no claims to be a design engineer, and it’s not likely that skilled electronics designers in the audience will learn much from my experience. However, if you’re still learning, like me, then perhaps you’ll find this narrative informative and entertaining. You might note a resemblance to Elmer 101 in organization and style; be assured that this is no coincidence. After all, imitation is the most sincere form of flattery!
Please note that I’ll refer frequently to the assembly and operating manual for the Sierra, rather than repeat what’s in it. It’s not my intent at all to replace what is already a fine manual provided by Wilderness Radio. My intent is only to supplement it with a “build a little, test a little” approach for assembly. Be sure to read through the Wilderness manual prior to beginning your construction.
Step 1: Jacks, Controls, and Panels
I chose to begin building by mounting and soldering all the jacks and controls which support the front and rear panels. That way, once I installed the panels themselves they would serve as a convenient support for the PC board during installation and soldering of the parts. This is the same way that the NorCal 20 is constructed.
Start with “Jumpers” on page 13 of the manual. Install the bare wire in the “G” holes in the first step, and solder. Skip over installation of W1 and W2 and jump straight to “Controls and Connectors”. Follow the instructions there to install C54 (the main tuning capacitor). Continue through the manual to install J1, J2, J3, J4, R1, R17, R18, S1, and S2. Skip the One Last Look section for now (it’s a bit premature, eh?). Complete the parts of “Chassis Assembly” on page 14 up through the first step on page 15. When you’ve completed these steps, you should have all panel-mount jacks and controls installed, and the front and rear panels installed. The panels now hold the board conveniently for soldering.
If you’d like, you can continue through this section by installing all the knobs and the tuning dial. I favor this approach personally because it’s easier to handle the controls during testing steps if the knobs are in place. On the other hand, you can certainly defer this til later if you’d prefer. You can also install the latches and feet on the top and bottom covers at this time, but don’t assemble the covers on the radio yet. We have a bit of soldering to do first.
Step 2: Voltage Regulation
Install the following components:
These three components are responsible for supplying the rig with a regulated 8V supply. Voltage regulation is important for oscillator stability, and the 8V regulated supply feeds several circuits in the Sierra. U9 is the actual voltage regulator IC. Its job is to take an input voltage between 10.5V and 23V and output a constant, regulated 8V. D6 will only conduct current if the power supply is connected with the correct polarity. This is to protect your rig in case you hook up the battery backwards. The purpose of C44 is to provide some prefiltering of the input voltage. It supplies a path to ground for any AC component of the supply voltage. It’s worth noting that the output of the voltage regulator is also usually tied to ground with a capacitor for the same reason. In the Sierra, you’ll see many instances of this at various points in the circuit.
Now, making sure that the power switch (back panel) is off, connect a 13.8V supply through J3. Get out your voltmeter and connect the ground to the ground jumper installed in step 1. Turn on S2 and use your voltmeter’s positive probe to check the voltage into D6 (at the non-banded end). It should be the same as the supply voltage. Now check the voltage at the other end of D6. It will read about 0.25V below the supply voltage. Typical diodes have a voltage drop of about 0.6V, but D6 is a Schottky diode. Schottky diodes have a metal-semiconductor junction instead of the usual PN junction and have a smaller voltage drop across the junction. Why would we want the voltage drop across this diode to be as small as possible? (Hint: the supply voltage, not the regulated voltage, is used to drive the PA transistor.)
Next, measure the voltage at the output of U9 (the lead closest to the front panel). You should read approximately 8V. If not, check the input (the lead closest to the back) to see if you are getting about 13.5V. If you have troubles with this part of the circuit, the best way to trace the problem is to find the point at which you no longer have the voltage you would expect. Possible problems are incorrect installation of D6 or U9, or a bad solder joint or solder bridge.
Step 3: The VFO
The variable frequency oscillator (VFO) is the heart of the frequency selection circuit in the Sierra. Variable capacitor C54 is used to vary the capacitance of the Colpitts oscillator circuit, and with it the oscillator frequency. Later, the output of the VFO is mixed with other oscillator frequencies to produce useful frequencies for transmitting and receiving signals.
This is pretty-much a classic series-tuned Colpitts oscillator. The ARRL Handbook and W1FB’s Design Notebook both discuss this circuit, and none of that discussion is repeated here. It’s worth noting, however, that it’s fairly easy to calculate the approximate oscillator frequency of this circuit using
where f will be given in Hz if L is in henries and C is in farads. There is only one inductor in the oscillator, L7, whose value is about 19 mH. There are six capacitors. C52, C53, and C54 are in parallel and an equivalent capacitance can be found by simply adding their capacitances. For the moment, let’s assume that C52 and C54 are in their fully-meshed configurations, so that they have their highest capacitance values (24 and 40 pF, respectively). The equivalent capacitance of C52, C53, and C54 is then 244 pF. Combining this equivalent capacitance with C56, C57, and C58 in series gives the total capacitance for the oscillator of about 166 pF (remember that capacitors in series combine like resistors in parallel, and vice versa). Given that L is 19 mH and C is 166 pF, solving for f in the equation above gives 2.833305 MHz, which is right in the ballpark of what we would expect.
To construct the VFO section, install the following parts:
- L7 (follow the instructions on page 13 of the manual)
- C52 (follow the instructions for C52 on page 11 of the manual)
- C56, C57, C58 (these are the polystyrene capacitors–follow the instructions on page 9 of the manual)
- R19 (it can be installed either way)
When you’ve installed all the parts, you can use an oscilloscope to examine the VFO signal. A good test point is the hole labeled “V” between C57 and C58 on the board. For my Sierra, I saw a very nice sine wave with an amplitude of about 2.5V peak-to-peak. I also used my commercial rig to tune in the VFO signal when both C52 and C54 were fully meshed (this works best if you use a short wire as an antenna and lay it alongside the Sierra). Mine came out at 2.8825 MHz. Why is it not 2.833305 MHz, like I calculated above?
Incidentally, using a commercial rig is a very convenient way to detect the expected signals in the transmit section of your Sierra. Often you’ll need to run your makeshift antenna very near the part of the circuit under scrutiny. But DON’T connect the Sierra directly to the antenna of your commercial rig. Just put your antenna in proximity to the circuit. Simply zero-beat the signal in your commercial rig to determine the frequency.
We’re not going to bother adjusting C52 yet, because the addition of the RIT circuit will affect the output frequency of the VFO. But feel free to monkey around with C52 and note its affect on the oscillator frequency. Try to predict what will happen, for example, if you reduce the capacitance of C52.
Step 4: The Pre-Mix Oscillator
In this step we’ll assemble and test the components which make up the pre-mix oscillator (PMO). Much of this portion of the circuit resides on the Sierra’s plug-in band module for each band. We’ll begin with the band module for 40m.
Since the Sierra is a multiband rig, in order for the incoming signal (whose frequency varies by band, of course) to mix with the VFO frequency to form the IF frequency (which is always the same, regardless of band), the VFO frequency must be changed for each band. Rather than create a separate VFO for each band, we use a single VFO and mix it with the signal from a separate oscillator to create a signal of the desired frequency. This separate oscillator is the PMO.
Let’s look at an example on the 40m band. Let’s say we want to tune in a signal which is at 7.040 MHz. Since the Sierra’s IF is 4.915 MHz, we need to mix the incoming signal with another having a frequency of either 2.125 MHz ( 7.040 – 4.915 MHz) or 11.955 MHz (7.040 + 4.915 MHz). In the Sierra, an 11.955-MHz signal will be supplied by the output of the PMO. In order to make the 11.955-Mhz signal, the PMO will mix the signal from the VFO (3.045 MHz) with the PMO’s own oscillator (for 40m, this is a 15-MHz signal).
Now for a signal on 20m, say 14.040 MHz, we’ll need an output of 18.955 MHz from the PMO to move our incoming signal to the IF. In this case, the PMO provides its own oscillator signal of 22 MHz, but everything else works the same as before.
You’ve probably guessed by now that the PMO’s internal oscillator frequency is determined by parts which reside on the band module. This is indeed true—-crystal X8 and variable capacitor C70 determine the internal oscillator frequency of the PMO. X8 determines the frequency, while C70 is used to fine-adjust the frequency slightly (pulling the frequency of the crystal).
The real heart of the PMO, of course, is the NE602 mixer chip. X8 and C70, along with the feedback network formed by C68 and C69, drive the frequency of the NE602’s internal oscillator. The input from the VFO is sent through C59, which filters any DC component, and then through the low-pass filter formed by R22 and C61 to reduce the harmonics from the VFO. The NE602 mixes its internal oscillator signal with that from the VFO to get an output signal (actually several output signals—sums and differences of the two inputs and their harmonics).
The output of the NE602 is taken from pin 5, fed through isolation capacitor C63, and then through two parallel tank circuits formed by C64 and L8, and C66 and L9. These two tank circuits act as a bandpass filter to remove all but the desired PMO output frequency. C64 and C66 are trimmer capacitors and will be adjusted so that the bandpass of the filter is at the desired frequency (you can easily calculate the bandpass of the filter given its capacitance and inductance using the same formula that we used in step three to calculate the VFO frequency). Note that the bandpass filter is also located on the band module.
After passing through the bandpass filter, the oscillator signal arrives at Q2, which along with R23 and R24 forms a common-drain amplifier circuit. This amplifier has a high input impedance and low output impedance and probably serves to provide the proper impedance to downstream circuits. It also helps to isolate the PMO circuit from other circuits in the Sierra.
Let’s build the circuit. First, install the following parts onto the main board of the Sierra:
- C59, C61, C62, C63, C68, C69, C72
- R9, R22, R23, R24
Now install the following parts onto the 40m band module board. Pay special attention to the Band Module Assembly instructions beginning on page 16 of the manual:
- C64, C65, C66, C70
- L8, L9
Plug the band module into the Sierra and apply power. Using a commercial receiver, look for the PMO oscillator signal around 15 MHz. Once you find it, use a tuning tool to adjust C50. Note how the oscillator frequency changes as you adjust C70. Next, tune your commercial receiver to about 12 MHz and search for the PMO output signal. Once you’ve found it, note how you can affect the amplitude of the signal by adjusting C64 and C66, the trimmer caps in the bandpass filter. Adjust these two caps as best as you can for maximum amplitude.
Finally, you should be able to verify that the PMO output frequency plus the VFO frequency is equal to the frequency of the internal PMO oscillator (15 MHz for the 40m module).
Step 5: The Transmit Switch and Transmit Mixer
We begin this step by adding the components which turn on the transmitter when the key is pressed. Q4, a 2N4126 PNP transistor, conducts when the keyline is taken to ground. When the keyline is grounded, current flows through R12C and R12D. The voltage drop across R12C makes the emitter more positive than the base, and this causes the transistor to conduct. Once the transistor is conducting, it can supply 8V at its collector to other parts of the circuit. Indeed, this line feeds U8, turning it on when when the key is pressed.
The rest of the circuit in this step will remind you of the PMO circuit in step 4. The transmit mixer consists of another NE602 mixer chip (U8), the components which make up the NE602 internal oscillator, another bandpass filter, and the transmit buffer transistor. Components C38, X7, RFC4, and C39 determine the frequency of the NE602 internal oscillator. C38 is used to tweak the frequency of the oscillator to fine-tune the transmit offset. The frequency of this oscillator is 4.915 MHz. Remember that, in the last step, our PMO output for the 40m band module was 11.915 when the VFO was at 3.085 MHz. In the transmit mixer we mix the PMO output and the internal oscillator output and take the difference of the two, which in this example turns out to be 7.000 MHz. This is the beginnings of our transmitted signal.
The bandpass filter at the output of U8 resides on the band module and is very much like that of the PMO. You might notice, however, that there are extra capacitors in parallel with the trimmer caps. Why? The simple answer is to increase the capacitance of the tank circuits making up the bandpass filter. Increasing the capacitance has the effect of lowering the resonant frequency of the filter. One could also have taken the approach of increasing the inductance of L3 and L4 or, I suppose, changing the trimmer caps to larger values. I suspect that the extra capacitor was cheaper and easier to find than a larger trimmer cap. If you wind L3 and L4 for yourself (36 turns on a small core) you’ll find that you’d be hard pressed to get more turns on the core to increase its inductance, and there isn’t room on the module for a larger core. So, adding the extra capacitor is the easy way to get the right bandpass.
The remaining part of the circuit consists of Q5, along with R11, R37, C82, and L11. These parts make up the buffer between the transmit mixer and the driver stage. The purpose of the buffer is to isolate the transmit mixer from the effects of successive stages. Like Q2 in the PMO, this amplifier has a high input impedance and low output impedance and probably serves to provide the proper impedance to downstream circuits.
To construct this circuit, first install the following parts on the main circuit board:
- C29, C30, C31, C38, C39, C40, C42, C50, C82
- Q4, Q5
- R11, R12, R37
Now connect a key to the Sierra and apply power. Use your DMM to measure the voltage at the collector of Q4. It should read 0V with the key up and about 8V with the key down. Now, hold the key down and try to tune in U8’s oscillator at 4.915 MHz on a commercial transceiver. Once you find it, adjust C38 and note how the frequency changes. Note too, that the signal disappears when you let the key up. So far, so good.
Now, install the following parts on the band module:
- C32, C33, C34, C35, C36
- L3, L4
Now plug in the band module and apply power again. With the key down, use your commercial transceiver again to try to find the 4.915-MHz signal. If you have trouble, try adjusting C32 and C36. Once you find it, adjust those two trimmer caps for maximum amplitude. Now see if you can find your 7-MHz signal in your commercial transceiver. This may take a bit of hunting unless you’ve calibrated your VFO and PMO already. Once you find it, adjust C32 and C36 again for maximum amplitude. If you have trouble finding the signal, try tweaking C32 and C36 a bit. I had a hard time finding the signal until a did a bit of adjusting of these trimmer caps.
Step 6: The Driver and Keyline Buffer
The driver section of the Sierra’s transmitter provides the first real gain for the transmitted signal. Q6 provides amplification of the output of the transmit mixer for the final stage, the power amplifier. The keyline buffer section, made up of Q8 and Q9, turn on the driver stage when the transmitter is keyed, and at the same time they isolate the driver section from other parts of the radio which are keyed by the keyline.
Let’s start with the keyline buffer circuit. MOSFET transistors Q8 and Q9 have the characteristic of turning on when the voltage at the gate is about 2.5 V greater than the voltage at the source. When they are on (conducting from drain to source) the resistance from drain to source is very low, on the order of a few ohms. Thus, these transistors make an easy-to-control switch, and this is exactly what they’re used for in this circuit. When the transmitter is not keyed, there is no path to ground for current which would come from the 8V supply and pass through resistor R12B. Thus, the gate voltage is 8V, and Q9 is turned on. Current flows from drain to source in Q9, and nearly all of the 8V supply is dropped through R12A. This means that the gate voltage at Q8 is essentially zero, and Q8 is turned off. This isolates the driver Q6 from DC ground and essentially keeps it turned off because the emitter voltage is essentially the same as the collector voltage (8V), and is higher than the base voltage.
What happens when we press the key? Now current can flow through R12B, and the gate voltage of Q9 goes to zero, turning off Q9. Since current no longer flows from drain to source in Q9, the voltage at the drain of Q9 is now 8V. This, of course, means that the gate voltage of Q8 is now 8V, and Q8 is turned on. Now, since current can flow from drain to source in Q8, this allows Q6 to turn on and start amplifying the signal coming from the transmit mixer.
I think that the driver circuit is basically a common emitter amplifier. I’m not going to take a shot at detailed analysis of this amplifier, but a few things are easy to say. First, the operation of the trimpot drive adjustment is simple: if you increase the resistance which it presents to the circuit, more of the voltage drop will appear across it and less will appear in R13, R18, and T2, causing the output voltage through T2 to change. T2 itself is responsible for the power transfer from the driver section to the PA.
Time to build. Install the following parts on the main board:
- Q6 (don’t forget the ferrite bead—see the manual for instructions), Q8, Q9
- C51, C55, C71
- R13, R14, R15, R18
Plug in your band module and apply power. If you have an oscilloscope or an RF probe, connect it at the location on the main board of the base of Q7 (which hasn’t been installed yet) and check for a 7-MHz signal on key down. The Sierra manual says to expect about 680 mV RMS at this location, but that will vary with the drive control setting, supply voltage, etc.
Step 7: The RX Bandpass Filter and Mixer
We’re going to switch over now and work on the receiver for a while, starting with the point at which the signal enters the receiver from the antenna. It first encounters a bandpass filter made up of C1 and L1, located on the band module. Past that point is the RF gain control, simply a potentiometer which can be used to limit the amount of received signal which gets passed on to the receive mixer. Next is transformer T1, which transforms the impedance to whatever is being expected by the receive mixer. Trimmer cap C2 works with the secondary of T1 to form yet another band pass filter (note that T1 and C2 are both on the band module).
Finally, we arrive at the receive mixer U2. It’s going to mix the received signal with the output from the PMO to generate a signal at the IF. For 40m, if we want to tune a 7.040 MHz signal, our VFO will need to be tuned so that the PMO output is (7.040 + 4.915 MHz) or 11.955 MHz.
One other item worth noting is the function of Q1. Q1 gets turned on (conducts) when the transmitter is keyed, because 8V is then applied at its base. This disrupts the Q of the tuned circuit made up of C1 and L1and provides a path to ground for the lion’s share of the transmitted signal which would otherwise enter (and possibly fry) the receiver.
On to construction. Install the following parts on the band module:
- C1, C2
Now install the following parts on the main board:
- R2, R25
- C3, C4, C5, C24, C46
Next, prepare and install jumpers W1 and W2 per the instructions on page 13 of the manual. NOTE: INSTALLATION OF W1 AND W2 WILL MAKE INSTALLATION OF OTHER PARTS IN THE SAME VICINITY MORE CHALLENGING. As an alternative, you might try installing a temporary jumper between the W1 and W2 holes which are very near the band module. Or, you might use simple jumper wires temporarily tack-soldered instead of the RG-174 coax. (Okay, I admit it, I installed W1 and W2 without realizing what I was getting myself into.)
Finally, install a temporary jumper between the base and collector holes of Q7 (which we haven’t installed yet). This allows us to steal some transmitted signal as a source for the receiver during testing.
Plug in the band module, apply power, and attach a scope or RF probe at the point on the board labeled “RX”. Next, key the transmitter and begin adjusting C1 and C2 for a maximum amplitude. This results in the two bandpass filters being tuned for resonance in the 40m band.
Step 8: The Crystal IF Filter, ABX, and IF Amplifier
In this step we install and test the variable-bandwidth crystal IF filter and the IF amplifier. The IF filter is made up of X1 through X4, D13 through D15, R19B through R19D, and R31. D13 through D15 are varactor diodes, which act as variable capacitors. R31 is used to vary the voltage on these varactor diodes, which are reversed biased so as not to conduct current. As the voltage across them is varied, their capacitance varies as well. You might be accustomed to seeing crystal ladder filters like this in other radios. The capacitance of the capacitors between the crystal elements has an effect on the bandwidth of the filter. Using varactor diodes in place of capacitors allows us to vary the bandwidth of the filter simply by changing the bias voltage on the diodes.
C73 and RFC1, and C74 and RFC2, are L-networks used to match the impedance of the output of U2 with the crystal ladder filter, and then to match the crystal ladder filter impedance with the input to U5. C6 and C10 are probably there to modify the series-resonant frequency of X1 and X4 to fine-tune the filter performance.
The MC1350 (U5) provides amplification of the IF signal prior to injection into the product detector. X5, C12, C14, and D4 provide a bandpass filter at the output of U5. According to the manual, this is to clean up wide-band noise at the output of U5, which I speculate is the product of the amplifier itself.
This time all the parts are installed on the main board:
- C6, C7, C8, C10, C11, C12, C13, C14, C73, C74, C76
- D4, D13, D14, D15
- RFC1, RFC2
- R10, R26, R31, R32
- X1, X2, X3, X4, X5
Plug in the band module and apply power. For this step, you’ll need an RF probe or an oscilloscope. With the key down, repeak C1 and C2 for maximum amplitude at pin 5 of U2. Next, adjust the ABX trimmer pot for maximum amplitude at pin 4 of U5 to verify that you’re getting a signal through the ladder filter. Finally, examine the signal at the location of pin 1 of U4. The signal should be considerably stronger here than at pin 4 of U5 (due to U5’s amplification).
If you don’t have an RF probe (cheap) or an oscilloscope (not cheap), you can still verify that things are working more-or-less correctly by checking DC voltage measurements against the table on page 30 of the manual.
Step 9: The Product Detector, AGC, and AF Amplifier
In this step we finish up the Sierra’s receiver. We start with the product detector, which is U4, the NE602 mixer again. It mixes the 4.915-MHz IF signal with its own oscillator signal which is different from the IF frequency by an amount equal to about 600 Hz (adjustable using trimmer cap C16). X6, along with C16, C17, and RFC5, make up the components which determine the frequency of U4’s internal oscillator. C9 and C83 serve to filter the 8V supply for U4 and U1.
Q10 and Q11 make up a mute circuit, cutting off the audio out when the transmitter is keyed. Q10 and Q11 conduct under when the transmitter is not keyed because their gates are at the same or higher potential than their sources. This is because the keyline is held at 8V when receiving and is grounded during key down. When the key is pressed, there is a large negative voltage difference from gate to source, and the JFETs are cut off. This prevents signal from reaching the AF amp from the output of the product detector.
U3 is an LM386 audio amplifier chip and provides the final amplification for the receiver signal. The output is fed through R8, the AF gain pot, to the headphones. According to the manual, C37 and R4 help to give the amplifier a peak response around 600 Hz.
Part of the output is fed to the AGC circuit made up of half of U1, R3, R5, R20, R21, C22, C26, C76, and D1. U1A is an op-amp configured as a unity-gain voltage follower (or buffer) to isolate the AF amp circuit from the IF amplifier which it feeds. R3 sets the DC no-signal voltage level. Loud audio signals from the AF amplifier increase this voltage, ultimately causing more current to flow from the output of U1A to the AGC input of the IF amp. This causes the gain of the IF amp to be reduced to keep the overall audio output pleasant in volume. R5 and C26 affect the recovery time of the AGC circuit during loud transients.
Let’s construct the rest of the receiver. First, install L5 and L6 on the band module, along with C47, C48, and C49. These parts make up the low-pass filter at the output of the transmitter, and received signals also pass through the filter from the antenna to the receiver.
Next, install the following parts on the main board:
- R3, R4, R5, R10, R20, R21, R27, R28, R30
- C9, C15, C16, C17, C18, C19, C20, C21, C22, C23, C25, C26, C28, C37, C83
- D1, D2
- U1, U3, U4
- Q10, Q11
Whew! That’s a lot of parts. Now, plug in the band module, attach a good external antenna, plug in your headphones, and apply power. You should hear some band noise in your phones. Re-peak C1 and C2 on the band module for maximum audio in your headphones. Set the AGC trimmer pot to about midway, and set the ABX pot full counterclockwise (maximum bandwidth). Now plug in a key and key down the transmitter. Does the receiver mute? It should, as long as the key is held down. Tune around the band for some signals. Here something? Congratulations! You have a working receiver! Play around with the AGC and ABX pots if you’d like. We’ll save final adjustments until we get the entire rig built.
Step 10: The Sidetone Oscillator
We’re nearing the end. In this step we construct the sidetone oscillator. Not much to say about this circuit. The other half of U1 is configured as an audio oscillator with a pitch of about 600 Hz. Trimmer pot R29 sets the volume of the sidetone. The sidetone signal gets injected into the input of the audio amp U3 during transmit. The sidetone oscillator is turned on when the keyline is grounded.
Install the following parts on the main board:
- R6, R7
- C27, C41, C78
That’s it. Plug in the band module and key, set R29 all the way clockwise, set the AF gain about halfway, apply power, and key the rig. Here the sidetone?
Step 11: RIT
Again, not too much work is needed to add the RIT. The key component in the RIT circuit is D8, another varactor diode. Its purpose is to vary the capacitance of the oscillator slightly to change its frequency a bit for the RIT action. R33 and R34 form a voltage divider along with either R16 or R17, depending upon whether the RIT is on or off.
U6 is an LM393 dual voltage comparator. It’s used to switch between R16 and R17, depending upon the position of the RIT switch. A voltage comparator works by comparing the voltage at the + input to the voltage at the – (or reference) input. If the + voltage exceeds the reference voltage, the output of the comparator will be high (about equal to its supply voltage). Otherwise, its output voltage will be low (ground).
Let’s look at how the RIT behaves when switched off. In this case, 8V is presented to the + input of the comparator driving R17, and 3V is present as the reference voltage. This causes the comparator output to be high, at the same voltage as the other side of R17, and no current flows through R17. For the other comparator, the inputs are reversed, and the output is low. This allows current to flow through R16 and it becomes the contributing resistor to the voltage divider controlling the voltage across D8.
What about when the RIT is on? First, note that with the RIT on and the key down, the comparators behave just like the case when the RIT is off. This is good, because we only want the receive frequency to change. On the other hand, when the key is up, a zero voltage is present at the + input of the comparator for R17, and its output goes low so R17 conducts. R16 is now taken out of the circuit because its comparator’s output is high. Now we can control the frequency of the receiver using R17.
By the way, the reason we waited so long to install the RIT was because it steals a 3V level from the IF amp, U5, which we didn’t install until after we built the VFO and most of the transmitter.
Finally C77 is present to “swamp the output capacitance of U6,” according to the manual. Without it, varying R17 would vary the frequency slightly even with the RIT off.
To construct the RIT circuit, install the following components on the main board:
- R16, R17, R33, R34
Install a band module, attach a good antenna, apply power, and find a signal. Switch your RIT in and vary R17. Do you hear the signal changing pitch?
Step 12: The Transmit Power Amplifier
This is the final construction step. All that’s left is to install the parts which make up the final amplifier for the transmitter. The power amplifier is Q7, a 2N3553 transistor. D7 is a zener diode which is in place to protect Q7 in case of less-than-optimal SWR.
L10 is often the subject of modification for Sierra owners in an attempt to eke more power out of the transmitter. L10 is often replaced with a transformer consisting of the same core with 12 bifilar turns. Once the turns are wound, the beginning of one winding is connected to the end of the other to form a center tap. This center tap is then attached to the collector of Q7, while the other ends are attached to 12V (like L10 is) and C46 (the lead of C46 which would otherwise connect to the collector of Q7 needs to be lifted and disconnected from the collector, and then connected directly to the newly-made transformer). The result of this is a better impedance match to the output low-pass filter.
Some builders also replaced the 2N3553 with other devices, such as the MRF237, MRF477, RCA SK9618, NTE342, or 2SC799. I personally did both the L10 mod and replaced the final with a 2SC799 and this resulted in 4W output on 40m using a 12.75V supply. I expect that this will increase with a higher supply voltage. In fact, the manual specifically mentions that the easiest way to get more power is to increase the supply voltage (up to 16V).
There are other mods which have been published or suggested. One is to replace the caps in the band modules with silver mica caps, but N6KR contends that there is little value in this. Others have modified the low-pass filter by taking a turn off one of the inductors. This does result in increased power as measured by the wattmeter, but no one to my knowledge has verified with a spectrum analyzer that the increased power is at the primary frequency and not an unwanted harmonic.
Now to build. Install the following parts on the main board:
That’s it. At this point, you’ll want to go through the complete alignment procedure beginning on page 19 of the manual. I used both an RF probe and a commercial rig to do these steps. The RF probe is very useful for peaking the filters, while the commercial rig is good for making certain your oscillators on the right frequencies.
If you find that things aren’t working the way you think they should, make sure you check out the troubleshooting section of the manual (page 29). The section on signal tracing can be very useful in localizing problems. Also, the table on page 30 shows the DC voltages at every pin of every active device in the radio. It’s another good source of troubleshooting information.