Radio Recycling

1
Radio Recycling

• By Nicolas - M1HOG
• www.M1HOG.com

2
Introduction

• Disclaimer - Nothing in the talk is warranted as
  accurate, optimal or indeed sensible.
• Objective Encourage people to recycle a CB as a
  way of learning about radio systems and producing
  a useful and unusual radio.
• Practical - High level talk through of the
  process and issues.

3
Why Recycle

• Ready made framework
• Accessible - discreet components, no surface
  mount, freely available circuit diagrams
• Brings back an old radio to use (on legal bands)
• Learning experience, Low cost
• Low monetary cost - but you will need some
  time!Create a unique radio?

4
What to recycle?

• CB Radios - 11m (27Mhz) was once an amateur
  frequency now between our 10 and 12 bands!
• Potential for easy conversion to 10M
• Inspirations - T4TT series of articles in PW by
  G4CFY (Spectrum communications)
• Older AM CBs are cheap!

5
AM / FM

• AM varies the strength of the transmitted signal
  in relation to the sound be sent.
• FM varies the frequency of the transmitted signal
  in relation to the sound sent.

6
Colpitts Oscillator

• A basic oscillator design
• Built from a kit, then reproduced using perf
  board.

7
Voltage Controlled Oscillator

• A voltage-controlled oscillator (VCO) is an
  oscillator designed to be controlled in frequency
  by a voltage input. The frequency of oscillation
  is varied by the applied DC voltage.
• Used in conjuction with the phase locked loop.
• This example uses varicap a diode whose
  capacitance is controlled by a voltage from the
  PLL.

8
Phase Locked Loop

• Enables a frequency which can be changed in
  steps, to be locked against a fixed crystal
  reference frequency. Provides a variable freqency
  output with the stablity of the fixed crystal
  reference frequency.
• Using CBSIM to help understand the PLL.

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Phase Locked Loop
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Basic types of PLL found in CBs

• Discreet Only the comparator is on the chip /
  everything else (divide by inputs etc) can be
  changed.
• External Loop The control loop can be easily
  accessed / altered.
• Monolithic All functions are locked within the
  chip, no easy frequency change is possible.
• Due to the PLL chip used, it turns out the very
  first radio I used, back in 1982 can be put on
  the air again..

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Introducing the Jaws MK2
12
Jaws MK2 - Overview

• A PLL controlled 27Mhz AM dual conversion
  10.240Mhz / 455Khz Superhet transceiver, similar
  principles to the BITX 20 discussed in Martins
  talks.
• PLL - The control loop can be easily accessed /
  altered.
• Discreet components, published circuit diagrams.
  
• Jaw Mk2 CB There was no MK 1!
• Neat mobile rig with classic red LED display.
• Popular AM CB of the 80s.

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Inside the Jaws MK2
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Conversion Overview

• Move the frequency to 10M - 29.00 to 29.100 with
  original 10Khz channel spacing.
• AM operation only.
• Provide a new offset frequency for the VCO.Down
  mix into the PLL remains the same.
• Ensure the PLL locks across the new range.
• Tune up the RX and TX.

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Offset Frequency
Original Frequency
New Frequency
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New Offset Frequency

• Provide a higher Offset Frequency for the VCO
  17.055mhz replacing 15Mhz.
• Crystals - Quartz Lab
• Colpitts Crystal Oscillator Kit and reproduction
  using perf board

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Installing the new offset frequency
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Offset Frequency
Original Frequency
New Frequency
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The VCO coil
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Increasing the VCO frequency

• The VCO now has to operate about 2Mhz higher than
  before.
• Either - Reduce the capacitance or the
  inductance.
• Inductance - Only 5 turns! So one turn would
  probably be too much.
• Capacitance Removed the tiny capacitor in the
  base of the coil. Added a small 1-28pf trimmer
  across the coil pins, set it to a few pf and
  centered the coil slug. Measured the output of
  the mixer while clicking through all the channels
  - perfect lock on RX and TX across the whole
  range and a cleaner signal.

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Tuning up the Jaws MK2
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Learning Points

• Understand, test and measure the donor rig
  thoroughly before taking it apart.
• Rig up a PLL lock LED to indicate if the PLL has
  "lost grip".
• Try and avoid getting bogged down in why one
  detail does not work. It will still be there
  later.
• Even a 10x probe can squash things.
• 4 watts of RF will get everywhere.
• Highlighted the need for a proper power meter and
  signal generator, although you can successfully
  improvise.

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Links

• Jaws 2 Information
• My Jaws 2 conversion Blog M1HOG
• Scratchy Volume Controls - Greg OH2FFY
• Explanation of Jaws PLL operation - Greg OH2FFY
• Data Sheets
• LC7120 - Jaws 2 PLL.
• TA 7310P - Mixer for PLL.
• TA 7205AP - Audio Power Amp.
• LC7131 - Jaws 2A PLL Cannot be modified.
• General Information
• CB2HAM Yahoo Group - Lots of helpful advice,
  schematics for the MK 2 / MK 2a and a host of
  other conversion information. Thanks Greg.
• Double Balanced Mixer
• List of PLL modifications
• Roger Lapthorn's 10m page
• Spectrum Communications
• Explanation of PLL pin functions
• RF power meter
• Modulation meter
• Tutorials and articles from NA5N - includes a
  great introduction to scopes

24
Thank you !

• This presentation is available at
• www.SADARS.org
• Nicolas
• www.M1HOG.com