Essential Modifications Errata & Notes

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This is a collection of suggested field changes to the Minima circuit as currently published at:-

http://www.phonestack.com/farhan/minimackt.png

As discovered and reported to the Minima mailing list by builders during construction of the Minima.

Important Changes

  1. Q23 in the transmit/receive switching circuit. The 4.7K base resistor should be 47K. Fixes intermittent TX/RX switching.
  2. Q23 in the Low Pass Filter section (there are two Q23's in the circuit see Notes below). The 4.7K base resistor should be 47K. Fixes intermittent switching.
  3. Q10 in the BFO circuit. The 100 ohm emitter resistor should be 1,000 ohms (1K ohms). Q10 will run excessively hot without this change.
  4. Q20 in the Morse Code Tone Audio Oscillator circuit. There should be a 0.1uF capacitor between the emitter of this transistor and the two 1K resistors. this is to provide DC blocking of 0.2V that would otherwise reach the mixer diodes in the BFO. Fixes distortion when sending CW.
  5. LCD Pin 15 is the +5V supply for the LED backlight inside the LCD while pin 16 of the same module is the GND for the LED backlight. The current Minima circuit has pin 15 directly connected to +5V. Some LCD displays have a current limiting resistor built inside them. Many however do not!! If unsure it is strongly recommended that a current limiting resistor of 220 ohms be installed between +5V and pin 15 as a starting point. If the display is not bright enough you can take some measurements and work down in resistance value to find an acceptable level of brightness. Failure to provide any current limiting at all, on those modules without inbuilt limiting, will result in the death of the backlight. More general information is available here: http://www.pongrance.com/backlights.html
  6. In the transmit/receive switching circuit (controlled by Q23/Q24) the +12V line is not shown. +12V should be applied on the cathode of D3.
  7. On the Atmega 328P. Pin 19 is incorrectly labeled PB6, it is PB5. Pin 18 is incorrectly labeled PB4, it is PB5. To be clear, the connections from lcd pins to atmega328p pins are correct, just the name of the pin inside the chip block is wrong.

Suggested/Recommended Changes

  1. If using the standard serial port to program the Arduino in the Minima. Then place a 0.1uF capacitor in series between the Arduino ATMEGA328P micro-controller Reset pin (IC pin #1) and your serial port DTR (Data Terminal Ready) pin. Which is typically pin 4 on a DB9 PC serial port connector. This will enable the Arduino IDE to pulse a reset to the micro-controller just before it starts a new sketch upload. This makes programming new firmware very easy, requiring no manual pressing of the reset switch.

Notes

A. The Minima RF output from the LPF's is approximately 0.025mW or -16dBm. The 1 watt RF output mentioned on the Minima introduction page is the output level when using the Minima JBOT (Just a Bunch Of Transistors) RF Amplifier. This amplifier was similar to a 5 watt JBOT RF amp originally designed for use with the BitX transceivers. It was modified to include an additional stage of amplification required for the low RF drive level from the Minima but still only produces 1 watt in combination. This amp can be seen at:-

http://www.hfsignals.org/index.php/File:Qrp-linear.png

This amp has now been superseded by the Minima RF386 Amp which drives the Minima LPF output to nominally 5 watts. This amp can be seen at:-

http://www.hfsignals.org/index.php/RF386.

B. The audio output of the Minima is only suitable for driving headphones. Should you wish to listen to a speaker then you will need some amplified computer speakers. Or you will need to add-to/modify the audio amplifier stages in the Minima.

C. The current Minima circuit diagram shows two Q23 transistors. One in the LPF circuit and the other in the TX/RX switching circuit. There is no Q12 in the circuit diagram.

D. Bias adjustment of the J-FET KISS mixer. Changes in the unwanted mixer product levels are only significant at the centre tap of the T1 mixer transformer secondary.

E. An alternate version of the USB/LSB sideband switching circuit that uses diode switching rather than relay control is available here:-

http://www.hfsignals.org/index.php/Sideband_Switching_of_BFO