- Recycled Radios -

using
Recycled Crystals

Putting your Codan on 80m at zero cost

Finding Crystals

The Codan crystal controlled SSB transceivers (and many similar commercial radios) are single conversion types and use an IF of 1650kHz. In normal operation the HF oscillator injects 1650kHz above the working frequency. If you work out the maths, you will realize that the crystal filter in the IF is therefore an LSB filter!

In order to operate LSB on the lower Amateur bands, we don't need to buy a new filter (and Codan filters for LSB operation are very rare), we simply use an HF oscillator below the operating frequency! For example, a 2.000MHz crystal will soon have the radio operating on 3650kHz LSB.

Now crystals for 2.000MHz are easy to find - they were widely used in micro controller circuits, although the quality may not be very good, but what about other frequencies?

If you recall back 20 years or so, marine radio communications were frequently around 2MHz, using AM transceivers that have long since been consigned to the junk pile. They're not much use for anything any more. However, they contain CRYSTALS. Look for the following frequencies (which were the transmit crystals):

2045kHz
2068kHz
2128kHz
2162kHz
2182kHz

and of course any others you can find between 1850 and 2250kHz. The receive crystals are too high, and don't be tempted to use old FT-247 pressure-mounted types: stay with the HC-6U style crystals. To use these recycled crystals, you will probably need to ream out the holes in the board slightly, since the Codan crystals have slimmer leads, and you may have to go to some trouble to get the crystals trimmed to frequency (and never mind the temperature compensation!)

With the crystals suggested, you can operate on:

3650kHz LSB (2MHz micro xtal)  Popular operating frequency
3695kHz LSB (2045 xtal)          ZL Codan calling frequency
3718kHz LSB (2068 xtal)          ZL Codan secondary channel
3778kHz LSB (2128 xtal)
3812kHz LSB (2162 xtal)
3832kHz LSB (2182 xtal)

It is important to adjust the crystals so that the radio is exactly on frequency - within 50Hz, especially on the calling channels and 3650kHz. The simplest way is to operate the radio on AM or TUNE into a dummy load, and measure the carrier frequency with a CALIBRATED frequency counter. Make sure all the guys in your club know your frequencies - get them to recycle a radio as well - maybe make it a club project.

Tuning Up

Follow the instructions in the Codan manual. Most manuals are readily available on the internet. If the manual for your model can't be found, use one of the others, as the circuitry is similar (e.g. 6924 Mk 2 and 8121 have the same IF/RF board, and are both similar to the 6801). The biggest problem will probably be finding enough coils for the RF stage. These are almost impossible to find, but can be wound by hand, by carefully matching the inductance and number of turns.

A word of warning - you MUST use polystyrene capacitors to tune the coils to frequency. Any other types, especially ceramic, do not have sufficient Q, and the performance of the radio will suffer. For example, it will be impossible to achieve enough transmitter drive for full power. The capacitor value for 80m for most radios will be 820pF, and you will need two capacitors per channel. The coils and capacitors in the 7727 are different, and the coils equally impossible to find.

Take great care of your coils, and adjust the slugs carefully with a plastic or fibre parallel-sided trimming tool. You could make one from a knitting needle, strip of plastic or similar. The slugs are irreplaceable, are easily split, and are then no use. Carefully identify each coil to be adjusted, and don't adjust any others!

The proper procedure is to disable the microphone compressor circuit, and tune up on transmit with a two-tone generator, watching the RF output from the exciter section with an oscilloscope. That's a bit fiddly to do, and unless your generator is transformer coupled, you can be sure of frying the microphone amplifier hybrid, which is also irreplaceable! Tuneup on receive works OK. Just use a signal generator to make sure that your are not tuning up on the image. That's unlikely if you use the correct coils and capacitors. Keep the generator level as low as possible, and adjust for maximum signal, dropping the generator level as you progress. Finally check on transmit (use AM mode)that you have sufficient drive, and if necessary tweak the coils.


Copyright Murray Greenman 1997-2005. All rights reserved. Contact the author before using any of this material.