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Radio Recycling
By Nicolas - M1HOG
www.M1HOG.com
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.
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?
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 CB’s are cheap!
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.
Colpitts Oscillator
• A basic oscillator design
• Built from a kit, then
reproduced using perf
board.
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.
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.
Phase Locked Loop
Basic types of PLL found in CB’s
• 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..
Introducing the Jaws MK2
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 80’s.
Inside the Jaws MK2
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.
Offset Frequency
Original Frequency
New Frequency
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
Installing the new offset frequency
Offset Frequency
Original Frequency
New Frequency
The VCO coil
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.
Tuning up the Jaws MK2
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.
Links
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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
Thank you !
This presentation is
available at
www.SADARS.org
Nicolas
www.M1HOG.com