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CB radio oldie for 10m FM

Modification of an old AM-CB radio for 10m FM amateur radio

For the modification of such a device two things are necessary above all: First, the frequency range have to modify, then the device receiver and transmitter side will be changed from AM to FM modulation. As an example, here is the alteration of the pictured M-511 mobile device described, which was manufactured in the 1970s by DNT. In the original state, the 27 MHz transmission frequency was generated by mixing the frequency of a crystal oscillator in the 37 MHz region with that of approximately 10.6 MHz. The receiver was a dual conversion superheterodyne, wich used a varying IF of about 10.6 MHz. The first oscillator, which worked at 37.6, 37.65, 37.7 or 37.75 MHz depending on the selected channel, was the same used during transmit operation. The varying IF was brought to a final IF of 455 kHz, depending on the respective channel with the crystal frequencies 10.140, 10.160, 10.70 or 10.180 MHz. The block diagram of such a device is shown here:

To change the frequency range, one could replace the 37 MHz crystals with those with frequencies in the 39 to 40 MHz range. After changing the 27 MHz resonant circuits for 29 megahertz, the change in the frequency range would have essentially already been done. If necessary, the capacities must be exchanged to change the RF resonant circuits. For the somewhat higher frequencies of the 10m band, usually the next or after next lower E12 series value is the appropriate, e.g. 47pF or 39pF instead of 56 pF. A simple practical tip: In many devices, it is sufficient to screw out the tuning cores and break through in the middle, to tune then each of the RF circuits only with a half of the core.

The modification by changing the crystals will usually fail because those with suitable frequencies are not tangible and a custom order exceeds the cost of such a tinker project. A simple way is, to let swing the first oscillator below the input frequency. Thus, much easier to obtain 18 MHz crystals could then be used. It then reached the frequency range of 28.6 MHz, where in those days also AM operation was made. In order to use such quartz crystals, the oscillator wich worked at the 37 MHz band in its original state, had to be rebuilt as shown below:

Meanwhile, the AM operation in the 10m band has become rare anyway. Furthermore it almost now only happens now above 29 MHz. Therefore, from the outset, I had plans to rebuild the unit for the 10m FM operation. That is why I took a different approach by changing the first intermediate frequency of the device to 9.045 MHz. For converting the first to the second intermediate frequency can then use a 28.5 MHz overtone crystal, which is operated on the fundamental wave. Formerly used in cheap 10m AM hand-held radios, such a quartz crystal on the fundamental wave can usually be trimmed to exactly 9.5 MHz with a serial trimmer. The crystal with a frequency spacing of 455 kHz required for transmit operation is obtained by a similarly utilized CB transmission crystal for channel 15 (27.135 MHz / 3 = 9.045 MHz). Another crystal, which shall be trimmed for repeater operation (100 kHz shift) to a frequency of 8.945 MHz, was originally intended as a CB receiver crystal for channel 29 (27.295 Mhz - 455 kHz = 26.840 MHz). It is no problem that the harmonic frequency is about 5 kHz too high, because the crystal for FM modulation by a capacitance diode has to be pulled down with a series coil anyway. In the block diagram shown here of the device thus changed in its frequency scheme, the conversion for FM is already taken into account:

The purpose of the whole action was that now CB double superhet crystals can be used as channel crystals, as they are present here and there and occasionally still be offered. Often they are also found in old CB radios, especially the Kaiser brand. To be able to use them, the oscillator operating in the 37 MHz range has to be modified to the 20 MHz range. In the sample device only two capacitors on the conductor side of the board had to be soldered. After adjustment of the oscillator coil core for example is with such a receiver crystal for the CB channel 14, which has a frequency of 20.625 MHz, transmit and receive operation on 29.670 MHz possible! Each additional crystal allows RX and TX operation on an additional channel, repeater shift included. For 29.690 MHz, however, you need the somewhat rarer crystal for channel 15A (20.645 MHz).

The left picture shows the wiring diagram in the original device with the special 23-channel tap-changer. On the right, the wiring diagram is shown after the conversion. The 23-channel switch has been removed and replaced with a standard six-position rotary switch.

In order to make the device FM-capable on the receiver side, an FM demodulator must be connected following to the IF amplifier. The audio cable between the AM demodulator and the volume potentiometer must be disconnected from the AM demodulator and instead connected to the audio output of the FM demodulator. Receiver modification for FM is described in more detail here elsewhere.

For FM transmit operation, the modulation amplifier of the CB device can be used. Its output signal is fed via a 10kOhm resistor to a capacitance diode, the mass side is in series with the 8,945 or 9,045 MHz crystals. The positive leads of the TX driver and output stage will be separated from the modulation transformer and connected directly to plus, so that amplitude modulation is not continued at the same time. It is also possible to connect a carbon microphone directly to the FM connection via a 1 μF capacitor or, better still, a corresponding transistorized replacement capsule from older telephones. In this case, a resistor (approx. 1 kilo ohms) must be connected from plus to the connection point capsule-to-capacitor.

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