A Rig for the MAS Contest by KD1JV

Actual size is 3.5" wide and 2.5" deep.

The M.A.S., (Minimal Art Session) is a contest started by Dr. Harmut "Hal" Weber, DJ7ST. (Now a SK)The idea behind this contest is to encourage Hams to build and operate a rig using a minimal number of parts. To be eligible, a transmitter must use no more than 50 parts (pretty easy to do) and a transceiver must use no more than 100 parts (a bit harder to do). To encourage the use of a very small number of parts, bonus points are awarded by the percentage of the actual number of parts used less than the maximum allowed number. Thankfully, things like hardware, knobs, connectors, headphones, key and the like are not counted as parts. Also, the transmitter low pass filter is considered to use 3 parts, even if in actual fact it uses more parts to ensure spectral purity of the transmitted signal.

However, if you use any IC's, the number of parts integrated into the IC must be counted. It is impossible to determine how many resistors, transistors and diodes (capacitors are usually not integrated onto a chip, but sometimes there can be) are inside a chip since the manufacture usually does not bother to tell you and if there is a simplified diagram of the insides, it does not show all the parts. And even if this number can be determined, even a simple IC can have dozens of parts integrated. So, this effectively eliminates the use of any modern IC's in a rig designed for the MAS contest. Not being able to design with op-amps, audio amps like the LM386, mixers like the SA612 or CMOS logic gates, is a definite handicap to designing a simple, but effective rig which is actually capable of making contacts.

Despite the limitations of not using IC's as noted above, I decided to give it a try. Some very simple and very low parts count rigs have been devised over the years, one of the most well known is the "Pixie". The problem with these overly simplistic designs is they simply do not work very well. The chances of actually making a contact with one of these rigs is slim to none. In my opinion, they are a waste of time and of natural resources. What good is a extreamly low parts count rig if you can't make contacts with it? If I am to spend time and effort to design and build a rig, I want something which at least has a good chance of actually talking to someone! If your gonna work a contest, you got to be able to hear people coming back to you!

The rig I came up with uses 51 parts for a complete transceiver, giving a 50% bonus. The transmitter puts out about 2 watts, the only supr is -50dBc at the second harmonic, and uses just 16 parts. The receiver is a regenerative type detector, with a RF pre-amp and two stage, high gain audio amp, for a total of 35 parts in its minimual configuration. MDS is about 0.5 uV if you have good hearing. Since the MAS contest is an 80 meter event, this rig is designed for 80.

Optional Parts: Parts marked with a "*" on the schematic below are optional parts. These are D3, a reverse polarity protection diode and a fine tuning control. Since the rig does not produce a side tone on its own, an optional side tone generator is also described. These parts are included in the printed circuit board layout to make the rig more useable in general use.

The Schematic:

The transmitter:

The transmitter is a simple crystal oscillator using a 3.579 MHz color burst crystal. A 2N7000 MOSFET is directly coupled to the output of the oscillator for the PA. Q3 is used to key the oscillator and PA on and off. Q4 is used as an inverter so that normal, active low keying can be used. Rise and fall time wave shaping is not included to reduce parts count, so this circuit will produce key clicks. C7 provides feedback so the circuit will oscillate. Normally, a second capacitor would be used from the emitter to ground, but the 2N7000 has enough gate capacitance to eliminate the need for that additional cap. Instead of the normal sine waves one would expect from a crystal oscillator, this oscillator was made to produce fairly narrow pulses. This improves the efficiency of the PA so that even though the 2N7000 is in a plastic TO-92 package, it does does not get alarmingly hot producing 2 watts of output. It is advisable the antenna load be preset to a low SWR before transmitting, as the 2N7000 has a 60V break down voltage and a high SWR can easily exceed this, causing the part to fail. The output low pass filter provides some impedance matching between the output of the PA and the antenna load. C1 in combination with L2 forms a trap at the second harmonic, other wise an additional filter stage would be required to meet FCC spectral purity regulations. Instead of buying a single 1500 pfd cap for C5 in the LPF, two 680 pfd caps could be used instead.

T1 is a bifilar wound transformer, which means two wires are wound around the core at the same time. (5 turns). Use an ohm meter to determine the ends of the wires A-B and C-D, then connect the ends B and D together to form the center tap, as shown in the diagram in the schematic.

The Receiver:

The receiver is a regenerative type and is a slightly modified version of the QRPKITS "Scout" regen designed by Charles Kitchen. See http://www.qrpkits.com

Q5 is the QSK switching transistor. During transmit, this transistor is turned off to isolate the receiver input from the low pass filter. Q8 is a common base RF pre-amp to keep the oscillations from the regenerative detector from being transmitted and reduce pulling effects from the antenna. The resonant circuit made up of the secondary of T2 and C16 determines the operating frequency of the receiver. Ideally, the tuning cap C17 should be an air variable with vernier drive. If you don't mind adding a few additional parts, a pot tuned varactor diode can be added for fine tuning and a small value trimmer cap (C27) used to help set the tuning range. Making C16 150 pfd and using a 50 pfd tuning cap (jumper out C28) allows for pretty much full coverage of the 80 meter band, so a vernier dial is needed or the tuning is very touchy. The schematic is drawn showing the use of a polyvariable capacitor with the varactor fine tuning. Polyvariable caps are also available from qrpkits.com.

Stability of the receiver is directly related to the stability of the input tuned circuit. NPO or C0G type caps should be used and an air variable for tuning. Using a powdered iron core for the inductor is a liability, but an air core coil would be much larger and more difficult to manage physically.

In order to receive CW or SSB signals, the regenerative detector must oscillate. A feedback winding on T2 turns the circuit into an oscillator. V1 in combination with C20 is used to control the amount of feedback. Polarity of the feedback winding is important. If you can not get the detector to oscillate, reverse the feedback winding connections. When winding T2, wind the 43 turn primary first and leave as much of a gap as possible between the start and finish of the winding to have a place for the two 6 turn winding to fit onto. Then wind the two 6 turn windings next, continuing in the same direction as the primary turns. Now, pick on end of the 43 turn primary winding as the "hot end" connected to the tuning caps. The start of the 6 turn winding next to the end of the 43 turn winding should go to the j-fet and the other end to the 5.1V supply. The polarity of the winding going to the RF pre-amp does not matter, so you can pick either end for those connections.

Ideally, the regen control is set so that the detector just starts to break into oscillation. This gives the best selectivity and sensitivity. However, this point will change when returning the frequency for receiving, so in practice, set the control so oscillation is sustained over the tuning range.

R9 and C15 form a low pass filter to eliminate high frequency audio and any RF which is present on R10. Note that the drain and source terminals of a J-FET are symmetrical, so they can be interchanged. That is why the schematic looks different from the way it might normally be drawn. Q7 and Q9 form a high gain darlington amplifier. Q6 further amplifies the audio and has the headphones connected in series with the collector, so it is acting as a Class A amp. Doing it this way eliminates the need to make an amplifier which can drive a low impedance load and saves a significant number of parts. NOTE: The mounting sleeve of the headphone jack is connected to the power supply, so must be insulated from a metal front panel!

Keying the transmitter without any kind of audio muting circuit resulted in very loud clicks in the headphones. This was clearly not acceptable, so a mute circuit had to be devised. This resulted in adding R11, C19, C14, Q11 and D2. When the transmitter is keyed, Q4 is turned off allowing R3 to pull the gate of Q3 high, enabling the transmitter. D2 allows the gate of Q11 to also be pulled high, turning Q11 on and connecting C14 to ground, which by-passes the base of Q6 to ground. When the transmitter is un-keyed, the RC time constant of R11 and C19 delays the turn off time of Q11 to allow any voltage transits to dampen out and eliminates serious clicks from being heard. Some minor clicking is still audible, but it is of reasonable level and not at all annoying.

A 5.1V zener, D1 stabilizes the voltage to the RF pre-amp and regen detector. If no reverse polarity diode is used to save a part, one must be careful to observe correct polarity when connecting up power. Powering the rig with a regulated 13.8V supply is recommended, although a 12V gell-cell can also be used, though that might result in chirp, as the supply to the oscillator is not regulated. Minimum operating voltage is about 10 volts, with the power output dropping off to about 500 mw.

Note that a regenerative receiver is effectively a direct conversion receiver, so signals on both sidebands will be detected.

Since one will normally be using the receiver for CW or SSB it will be in the oscillating detector mode. There is enough RF signal present on R10 to add a sensitive frequency counter for a digital readout. This fact could also be handy in initially getting the receiver to tune in the desired frequency range. A frequency counter or a general coverage receiver can be used to help set the tuning range. I tried to make the crystal oscillator act as a "Spot" so you could tune to the transmitter frequency. However, the signal is too strong and blocks the receiver. A separate, outboard oscillator maybe made to provide a spot frequency.

DSB transceiver possibility! It could very well be possible to buffer the carrier signal produced by the regen detector and use it to drive a balanced modulator to produce DSB (Double Side Band). The output of the modulator would then be amplified by some linear amps for transmit. This could result in a very low parts count DSB phone rig!

Side tone generator:

The above "Twin T" 600 Hz side tone oscillator is included on the circuit board. If you have a keyer which can generate it's own side tone, these parts can be eliminated. The "side tone out" connection goes to the point labeled "ST IN" on the main schematic. C24 and C25 can be combined as one 0.047 ufd cap. Film type caps should be used for C21 to C25.

 

Pictures of the dead bug constructed prototype:(but is missing the audio mute parts, as these were added after the photos were taken) These photos prove it doesn't have to look pretty to work! A small finned heat sink was used on the PA in this version so that prolonged keydown periods could be done while testing.

PCB Construction and parts list:

The printed circuit layout can be down loaded by clicking HERE. It is a .pdf file and will print to scale. The view is through board, so it can be printed directly onto toner transfer film. The image is reversed when you iron the pattern onto the board.

Parts subsitution:

I used 2N4401 NPN transistors, as I have a lot of them. Other NPN's such as 2N3904 or PN2222A should work as well. The J-FET I used is a 2N3819, but other N channel J-FETs such as the J-310 can also be used. Note however, the position of the gate lead is different in most other J-FETs, as shown on the schematic. The diode used for varactor fine tuning is shown as a 1N4001 on the schematic. I used a 1N4753B, a 47V zener diode. The 1N4001 may result in a smaller tuning range and C29 may have to be made larger in value to compensate. A red LED also makes a good varactor diode. The Main tuning range is about 200 kHz, from 3.5 MHz to 3.7 MHz. If you want increased range, make C27 larger in value and compensate with the trimmer cap to put the tuning in the band. Values used in the side tone oscillator should not be changed, as this will change the oscillator frequency. Also, the values used in the transmitter low pass filter need to be as shown. You do have some leway in the other resistor values, going +/- one 5% step should not be a problem, though common values are used.

Most of the part numbers shown next to the part values are for Mouser. J2 and V1 are sourced from Digi-Key, but be advised they have a $5.00 surcharge for orders of less than $25.00. Toriods are available from www.kitsandparts.com