Part 11 
by Dick Kelly, W6BKY, w6bky@aol.com

The oscillator shown in the previous article is, primarily, for checking out the 80 meter receiver module, but it can also serve other purposes. For example, you will need a signal to check out the 3.58 MHz IF amplifier circuit in the 40 meter receiver section of the transceiver. You will also need a 3.58 MHz heterodyne oscillator (sometimes called a "carrier oscillator) for the transmitter section of the 40 meter transceiver. This oscillator (with minor modifications) can do the job. Not only that, it seems a waste to build all this 80 meter circuitry and not put it "on the air". All you need is power, transmit/receive (T / R) switching, an antenna, and you’re ready for QSOs. More about that, later. For now, let’s get to the task at hand, which is checking out the 80 meter receiver, AKA product detector. 

First, be sure the 3.58 MHz crystal oscillator is working. Apply +12VDC to the oscillator circuit and measure the voltage at the collector; it should be about 12 volts. Next, check the voltage at the base of the Q1; it should be about 4.5 volts with the key OPEN, and about 4.0 volts with the key CLOSED. If these voltages are NOT correct, you have a wiring error or a bad solder joint. Find and correct any wiring errors, then …

Key the oscillator and listen for the signal on a known good receiver: Set the receiver to CW mode and tune to about 3.58 MHz. I say "about" because, even though this is a crystal oscillator, the frequency may be off a bit (mine oscillates at 3.578950 MHz, which is 595 Hz lower than the specified crystal frequency). Apply power and key the oscillator. If you don’t hear it at 3.58 MHz, tune around until you find it. If you can’t find the oscillator signal within plus or minus 2 kHz of 3.58 MHz, (guess what) you probably have a wiring error or bad solder joint. Find and correct any error(s), then proceed.

Once you know the 3.58 MHz oscillator is working, disconnect power and set it aside. 

Remember the NE602 80 meter receiver you built (how could you forget?!). Well, you are now ready to check it out!

Gather the equipment and materials listed below.

* Complete 80 Meter Receiver Module (both "front end" and "rear end")

* Power Supply

* RF Signal Generator (the oscillator you just now built, or other signal source)

* General Coverage Receiver with CW capability; preferably one with digital read out, or a well calibrated analog dial.

* 80 meter receiver schematics from previous articles.

Before you plug in the NE602, check the resistance from ground to Pin 3. You should read zero Ohm. All other pins should read open. 

Next, apply power (do NOT plug in the NE602, yet) and check voltages at the NE602 socket:

Pin 8: 6 volts

Pin 3: Zero volts 

Disconnect power, then plug in the NE602. Re-apply power and listen for the NE602 oscillator on a general coverage receiver tuned to about 3.58 MHz. NOTE: You are listening for the NE602 VFO oscillator, NOT the 3.58 MHz crystal oscillator. It is important to use a general coverage receiver because the frequency of the VFO may be off by several KHz, or even several tens of KHz (outside the Ham bands). If you are lucky (I’ll take luck over skill any time!) you will hear the VFO as you turn the NE602 VFO control from fully CW to fully CCW. If you are only skillful and do not hear the signal, set the VFO control to about mid-range and tune the general coverage receiver until you find the signal. Be bold in your search! You may find the VFO oscillating a l-o-n-g way from 3.58 as 

I did with my first try at this circuit. 

If you find the VFO signal within the range from about 3.5 to 3.7 MHz, you can probably bring it to 3.58 by "tweaking" the slug in L1. NOTE: The tuning slug is brittle and easy to break. Be gentle! Use a non-metallic tool to turn the slug. If you don’t have an alignment tool, fashion a tool from a piece of wood or plastic. A discarded toothbrush handle sharpened to a screw driver point makes a good alignment tool. 

If you find the signal below 3.5 MHz, reduce the value of C9; if the signal is above 3.7, try adding a few pF in parallel with C9. You may have to try several values in order to bring the signal to 3.58 MHz.

When you have found the VFO signal and adjusted it to about 3.58 MHz with the VFO control set to about mid-range, you are ready to listen for a signal with the NE602 80 meter receiver. 

1. Turn the general coverage receiver off; you 

2. Connect a test probe or about three feet of 

80 meter receiver.

3. Also, connect a piece of wire to the output 

NOTE: There is no physical connection between the output of the oscillator and the antenna jack on the receiver. The signal radiates from the oscillator and is picked up by the piece of wire in the receiver antenna jack.

(4) Set the volume control to about mid-range.

(4) Apply power.

(5) Key the 3.58 MHz crystal oscillator.

You may hear a tone. If you do not, hold the key down and tune the 80 meter receiver through its range. If you still hear nothing, (guess what) there is probably a wiring error and/or a bad solder joint. Check the wiring around the NE602, pins 1,2,4, and 5 (you have already verified the oscillator is working, so pins 3,6,7,and 8 are OK). Also, be sure you have good signal and ground connections between all modules. Correct any mistakes, and try again. 

I’ll assume your 80 meter receiver is working and move on to the next step, which is to put a (very) modest 80 meter QRP station on the air. If you do not want to put your 80 meter QRP station on the air, you can skip ahead and begin getting ready for the main event, which is the 40 meter transceiver. For those who want to go "on the air" with the 80 meter rig, here’s one way to do it …

There are several ways to implement the transmit/receive (T/R) switching. Since this station is only an interim goal (remember, the goal of this series of articles is a 40 meter transceiver) I tried to keep switching as simple as possible. Figure 1 shows the set-up I used. 

I have chosen a straightforward method using a double-pole, double-throw (DPDT) switch (my switch was, of course, salvaged from a TV set). Any DPDT switch will work fine. As you can see, one pole is for antenna switching and one pole is for product detector (NE602) power switching. This brute force switching technique is not very sophisticated, but it is easy to build and fail-safe. Transmitter power switching is not required because closing the key turns the circuit ON, and opening the key turns it OFF. My main power on/off switch is attached to the audio volume control. If your volume control does not have a switch attached, any single-pole, single throw (SPST) switch will do.

The "Transmitter" is, of course, the 3.58 MHz oscillator you used to check out the receiver. Wire the switches, connect a 50 Ohm 80 meter antenna and a key, apply power, and you’re ready to go on the air! Don’t expect to work any DX with this rig, but you can use it to check into your local 80 meter CW net and tell ‘em all about your new home brew rig. What? You don’t have a local 80 meter CW net?! Well, call a buddy, and start one!

IMPORTANT NOTE: 3.58 MHz is outside the Novice band; you must hold a general class license, or higher, in order to use this rig legally. That’s the law of the land, and you’ll have to deal with it. There is another law you must deal with, and this law is one that you can’ break, even if you want to. Why not? Because it is a law of physics, and, unlike political laws, it simply can not be broken. This law says that you must have an efficient 80 meter antenna in order to get good results with this little rig. Don’t have a good 80 meter antenna? That’s OK, because ANY antenna will work better than no antenna at all, and you can still have lots of fun.

Sharp eyed readers, such as yourself, may be wondering about that 56 Ohm load resistor across the output of the oscillator circuit. Well, I forgot to remove it before I did my first "on-the-air" test, and I was able to contact my buddy across town. He gave me a "weak, but readable" report. When I removed the resistor and tried again, he gave me the same report. Evidently, it makes little or no difference whether the resistor is there or not as far as local contacts are concerned. Strange, but true!

I hope you have had as much fun as I building these first few modules. It is now time for bigger and better things, and next time I will introduce you to the rest of the 40 meter transceiver.

‘Til next time, 73, Dick, W6BKY