POCSAG transmitter with a Motorola GM1200

Previously the PE2KMV DAPNET transmitter was equiped with a Friendcom FC301 datatransceiver. Because this tiny transmitter only provides 5W power, which is not that much in the hills of southern PA, the signal was boosted by an amplifier. For two reasons this setup was far from ideal. First of all the Friendcom transceiver is a very handy part for lots of experiments. On the other hand the HF amplifier had quited service soon for some unknown reasons.

The setup as it was: at the right side the amplifier, to the left of it the datatransceiver

Therefore an old Motorola GM1200 was purchased via¬† Funk24.net in Aachen (DL). The GM1200 is a transceiver for analog trunking networks, but it can handle a couple of conventional frequencies. With the remark the radio handles a 60 seconds time out timer which can’t be disabled via the software. So mostly other firmware is quickly zapped into the GM1200, which makes a MCS2000 of it. For POCSAG transmitting this time out timer isn’t an issue, as the transmissions won’t exceed 60 seconds in any way. But by flashing the firmware another nice feater is lost: as a GM1200 the unit has an input to the modulator for data traffic. By setting a check in the RSS the microphone input will be disabled and the radio will be listening on the data input on pin 24 of the accessory connector at the bottom. The picture below shows the correct settings: the QRG, output to ‘High’, ‘Data Channel Enabled’ marked and everything else unchecked.

Furthermore you need to enable the option to switch to conventional mode. The actual switching will be done by pressing the ‘P’ key on the front panel until the radio switches to ‘Conventional’. But before this feature needs to be enabled via the RSS. Via the ‘Edit’ menu you select ‘Common’, then go to ‘Personality Independent Parameters’. It opens a new window where you check ‘Talk Around’. All other options remain unchecked.

To save power I’ve also switched off the display backlight permanently: the radio is built into a closed enclosure so it’s useless to leave the lights on. To do so go to the ‘Edit’ menu and select ‘Personality’, then go to ‘Common Radio Parameters’. At the bottom of the window the display backlight can be disabled.

Then there was some mechanical work to do. The amp and the Friendcom were removed. For the Motorola I had an original mounting bracket. But this could also be replaced by some aluminium strip bent to the correct size. Because of the weight of the transceiver and the fact it’s mounted a bit wobbly in its bracket I’ve mounted the bracket with four short M6 bolts and lockrings. A short cable with a Motorola power connector could be mounted easily to both power rails.

I also had to create a cable between the C9000 interface and the Motorola. At the transceiver side I’ve used an original 25 pin accessory connector:

  • Pin 4 = ground
  • Pin 21 = PTT
  • Pin 24 = signal

At the side of the C9000 board I’ve decided to use Dupont connectors. But here you can also use brackets with screws or just solder the wires.

The Dupont conntectors at the C9000 board

Because the original bolts to mount the transceiver to its bracket a quite bulky I’ve decided to use regular hex bolts with an extra nut to firmly lock the transceiver in place.

The final result

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