Converting the Codan 6801 for Amateur Use

How to convert a simple, reliable commercial HF transceiver into a multi-band, Amateur Radio transceiver.

These notes are of interest to Amateurs who wish to convert their own Codan 6801 or similar transceiver. Conversion will be made much easier by reference to a copy of the Codan 6801 Service Manual.

Plenty of other transceivers are suitable for conversion, and many of the notes and suggestions here are applicable to other models. See the Recycled Radios page for ideas.

The Transceiver

Xcvr prior to conversion
The Codan 6801 comes in two models, a commercial land-based model, the 6801, and a marine model, the 6801S, which is essentially similar. There are two versions, 12V DC and 24V DC operated. In this article "Codan 6801" means any of these models. The Codan 6801 is fully solid state, and the design dates from the late 1960s. At the time, it would have been very much state-of-the-art.

The 6801 includes such nice features as thick film hybrid ICs, an excellent noise blanker, a voice operated smart squelch, double balanced mixers, real AM, plenty of optional extras, which means plenty of room inside the unit, and plenty of places to add in the extras! The Codan is very easy to work on, and the documentation is good. The unit is rugged, highly reliable and suited for long term unattended operation.

Briefly, the specifications of the transceiver are:

Frequency range: 2 to 16 MHz, transmit and receive
Split frequency operation possible
14 total frequencies maximum
10 channels provided
Modes: USB (A3H)
AM (A3J) - option (compatable double sideband with AM filter)
LSB (A3H) - option
CW (A2J) - option (tone generator used in SSB mode)
Power Output: SSB 120W PEP (24V) 100W PEP (12V)
AM 30W (24V) 25W (12V)
CW 60W (24V) 50W (12V)
Clarifier / RIT: TX, RX or both
Physical: Dimensions 400mm x 360mm x 160mm
Weight 6.8kg

Block Diagrams

Large areas of the transceiver design are used in both transmit and receive. This is done using diode switching, and significantly reduces the complexity of the unit. In the following drawings, circuit blocks that are common to the transmitter and receiver are shown in red.

Codan 6801 Receiver block diagram
The 6801 Receiver
The receive signal passes backwards through the transmitter low-pass filters, adding useful attenuation to out of band signals. The broadcast filter is a high pass 1800 kHz filter which reduces the broadcast band energy reaching the receiver and also attenuates any potential IF leakage at 1650 kHz.

The Preselector contains the AGC controlled dual gate FET RF amplifier and two channel filters, one each side of the amplifier, tuned to the working frequency. The filters are diode swiched by the channel switch. The Q of these circuits eliminates mixer image responses. The receive signal then passes to a double balanced mixer to convert it to the IF frequency, 1650 kHz. The preselector and mixer are reversed in direction and re-used on transmit.

The mixer output is buffered and fed to the noise blanker, which operates on the wideband signal before the crystal filters, and consists of a noise amplifier, detector and a balanced PIN diode gate. The signal then passes through the crystal filter, with optional filters diode switched, then through the AGC controlled IF amplifier. The filters and IF amplifier also operate in transmit, although the AGC is disabled.

A conventional diode envelope detector provides AM demodulation and AGC. The transmitter modulator serves as the product detector for SSB and CW modes. The same 1650 kHz crystal oscillator is used for carrier reinsertion on USB and LSB - the filters are changed between sidebands.

The audio signal selected from the two demodulators passes through a voice detecting mute. The principle of its operation is based on the characteristic energy density of different parts of the human voice spectrum. The output goes to a conventional audio amplifier and loudspeaker or headphones.

Codan 6801 Transmitter block diagram
The 6801 Transmitter
The microphone amplifier includes audio AGC. The output drives the balanced modulator, creating DSB with or without carrier, which is reinserted in AM mode. The signal is amplified in the IF amplifier and sent to the crystal filters to remove the unwanted sideband (SSB) or any wider than necessary products (AM).

The IF signal is changed to the channel frequency by the first mixer and then sent to the Preselector, to remove the unwanted image frequency and any channel oscillator leakage from the mixer. The signal then passes to the low power stages of the power amplifier.

The first power amplifier stage is an ALC controlled class A broad band amplifier. This is followed by the two stage push-pull class AB power amplifier. The transmitter high power signal passes through the transmitter low-pass filters, which are chosen via the channel switch to cut off before the second harmonic of the transmitter signal, and then on to the ALC detector and the antenna.

The ALC system operates on a mixture of forward power, reflected power, and final stage voltage swing, so will control the transmitter accurately with any load SWR. The ALC also has a thermal input which reduces the transmitter power to keep the heatsink at a maximum allowable temperature in case of overload.

Codan 6801 Channel oscillator block diagram
The Channel Oscillator
The channel oscillator (diagram above) consists of a crystal oscillator with crystals selected by the channel switch, and a levelling system to keep the signal constant over a wide range of crystal frequencies. The oscillator frequency is normally 1650 kHz higher than the chosen USB or LSB (suppressed carrier) or AM (carrier) frequency. It is possible to operate with the channel oscillator frequency 1650 kHz lower than the channel frequency, and transmit or receive on the opposite sideband. This is useful since LSB filters for surplus Codan transceivers are quite rare.

Codan 6801 channel switch diagram
The Channel Switch
Several functions take place on the 10 position channel switch (see diagram above). There are three wafers used to control activities for (a) transmit only, (b) receive only, and (c) transmit and receive. These wafers supply power to the diode switches used to control the channel crystal and preselector channel coils, so it is possible to set one channel to receive and transmit on different frequencies.

The design is completely flexible, so it is also possible to arbitrarily set any crystal or preselector to any of the switch positions. There are in fact provisons on the circuit board for 14 crystals and 14 filter sets, so of the 10 channels, up to four can have independent transmit and receive frequencies, and in addition, the set can be arranged to use the same transmit or receive frequency on more than one channel. This was very common in the marine application.

Two further wafers on the channel switch directly select the transmitter low pass filters. Once again, the filter in use can be arbitrarily selected by manually wiring the filters and jumpers between switch positions, for complete flexibility.

A spare wafer is provided, which in some versions is wired to a rear panel connector, to provide an automatic antenna or antenna tuner selector.

To allow such a wide range of coverage, a very wide range VFO is required for the first mixer. For upper sideband, the VFO runs above the receive frequency, so a range of 3.45 MHz to 16.65 MHz is required. For lower sideband, the signal is inverted by running the VFO below the receive frequency, so a range of 0.15 to 13.35 MHz is necessary. Image rejection is likely to be poor on the 160m band in LSB mode, since the image is only 300 kHz away.

Conversion Notes

Checking out the concept
It was quickly realised (read the bit above about the block diagrams and the channel switch) that the transceiver could be easily made into a general coverage amateur transceiver, by using the channel switch as a 'band switch'. This would be used to set the centre frequency for the Preselector, which allows a small range about the centre frequency, and select the transmit LP filter. Experiments showed the Q of the Preselector to be about 25, so the useful bandwidth before drive dropped off badly worked out to be 50 kHz on 160m, 100 kHz on 80m, 200 kHz on 40m, and plenty on the other bands.

With some simple logic, level converters and five small relays (to control the transmit low pass filters) the bandswitch could be completely dispensed with and a fully remote controlled all band rig made. Rather than go to this effort with the 6801, look for a 7727TB, which already includes relay operated filters.

The tune-up and check-out phase
First the rig should be checked out on a dummy load on the original marine or commercial frequencies. Audio AGC can be detrimental when using data modes for keying, so C128 was removed. This effectively leaves the microphone amplifier running at full gain. With careful choice of a suitable microphone and fitting a normal plug rather than the original MIL one, plenty of audio will be available, with no hum at all. The gain can be reduced by manual DC control of the gain of the microphone amp at IC5 pin 3. The option headers include a pin (shorted to ground) which drops the gain to minimum.

The rig should then tuned up temporarily on an Amateur frequency (for example 3.695 MHz, using an ex-marine 2045 kHz crystal), and a few crystal controlled QSOs held to check that everything works. Note that LSB operation results with low side injection, so you don't need a separate filter or carrier crystal for LSB operation.

The new band plan
Allocate the bands required on the channel switch. Here is a suitable layout:

  1. 160m 1.8 - 1.85 MHz
  2. 80m 3.50 - 3.60 MHz
  3. 80m 3.60 - 3.70 MHz
  4. 80m 3.70 - 3.80 MHz
  5. 80m 3.80 - 3.90 MHz
  6. Unassigned (about 5 MHz)
  7. Unassigned (about 6 MHz)
  8. 40m 7.00 - 7.20 MHz
  9. 30m 10.0 - 10.3 MHz
  10. 20m 14.0 - 15.0 MHz
Each of the ranges should ideally be assigned to a logical order of crystals and preselector coils (in order from left to right), and the assignment suggested modified to suit your requirements (and perhaps available coils, since they are now impossible to find!). In the suggested plan, multiple 'bands' are assigned for 80m, where the coil bandwidth is insufficient to cover the whole Amateur band. The coils should be painstakingly tuned up to the centre of the required range, using a signal generator in place of the channel crystal. You can leave crystal controlled channels in place and use a VFO on other 'bands'.

The results from the prototype can be seen in the following chart. Valid Amateur band frequencies are shown in red, and the transmitter bandwidth (for 25W carrier in 'TUNE') is shown as thick lines. Frequencies (MHz) are shown below the graph.

TX bandwidth graph
It is easy to see that four "channels" nicely covers 80m. 40m and above are easily covered by one channel each, although obviously the 40m tuning was off somewhat when the measurements were made. The 5MHz and 6MHz bands are of course not Amateur bands (yet!) but are used to provide good receive only coverage of short wave broadcast bands and WWV at 5 MHz. 160m is somewhat of a compromise.

The graph shows the useful receiver range as thin red lines. In all cases the receive bandwidth is considerably more than the transmitter bandwidth, since some loss in gain is easily tolerated. In effect, with the above band plan, the receiver is general coverage from 1.5 to 2.3 MHz, and from 3 MHz to about 17 MHz, although the image rejection above 15 MHz is poor without retuning the Preselector. For the best performance a dual gang variable capacitor could be used in the Preselector, to give true general TX/RX coverage from 2 to 18 MHz.

The channel oscillator can be driven easily by about 50mV p-p from a generator, by feeding the oscillator collector end of the channel crystal via a series resistor and capacitor. Remove the crystal, solder a 100 Ohm resistor and 10nF ceramic capacitor in series, solder one end into the switching diode end (rear) crystal hole, and connect the generator to the other end. The purpose of the resistor is to act as part of a controlled attenuator with oscillator transistor V15 acting as the other half.

A Signal Strength Meter
A very simple arrangement seems to work very well, and works with most models (see on the right):

+10V during receive is fed to a divider which generates the same voltage as the AGC when no signal is received. As the signal strength increases, the AGC voltage (nominally 5V at full gain) is reduced toward zero. The meter conducts through the diode (a 1N4148 or similar). The correct meter zero will vary slightly between transceivers. Adjust the 10k resistor value, or if necessary connect the meter above the resistor. You should not need to use a pot.

The resistors have a high value in order to present a Thevenin equivalent resistance of about 30k Ohm in series with the meter, giving the meter a sensitivity of about 2.8V FSD. The diode reduces the offset slightly, but its main purpose is to prevent the meter reading backwards during transmit.

The meter works well, although is compressed a bit at the bottom end (like most S meters!). The reason for this is the AGC performance of the dual gate FETs, and has nothing to do with any non-linearity in the diode. Here's a graph of meter reading vs signal strength:

Simple S meter

S Meter performance

Because of the wide range of VFO frequencies required, a DDS VFO is best. A conventional PLL synthesizer would require at least 10 different VCOs and down mixer oscillators, and a multi-band VFO is just not good enough for modern operating requirements, especially for digital mode operation. It is best to use a kit (check the internet), since the DDS chips are very difficult to handle. There are several kits using the AD9835 device, which operates to at least 15MHz. The Silicon Labs Si570 device also features in several kits, and is again quite suitable. Watch the power supply requirements - some of the kits may draw more current than the rest of the receiver, and thus not so useful for portable operation. Most of the kits use a rotary encoder for frequency change, and include an LCD display.

Ideally you want the VFO to look at the bandswitch and to be set to preprogrammed frequency on each band, or in the case of 80m, four frequencies. The last used frequency on each band would be appropriate. Some kits offer this type of feature. Without it, you will need to manually wind the VFO from one band to another.

These VFO kits typically have selectable-offset or split-frequency operation, and you can wire this facility to provide split frequency operation or USB/LSB operation, by moving the VFO either side of the IF passband. The purpose of the Sideband Switch is to tell the DDS controller whether to output a signal 1650 kHz above (USB) or 1650 kHz below (LSB) the nominal transceiver frequency.

The reference oscillators typically used in DDS or Si570 VFOs are not especially stable, but are quite suitable for SSB and digital modes. If you are considering adding a more stable external reference, remember that the 1650kHz BFO will also need stabilization.

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