Motorola GM600 Flat Audio

A while ago DH6BB sent me a Motorola GM600 radio to have it programmed for the usage with DAPNET. DAPNET is a network for pager transmissions and implements the POCSAG protocol. So you need a radio which allows for flat audio input. Starting the RSS discovered that there is a “data channel” option while configuring the only possible conventional channel. However this option was greyed out.

After fiddling around for a while I found out that this item gets enabled if you load (different) network info into the codeplug/radio. After doing this I expected to be able to feed the POCSAG signals into pin 5 of the ACC port.

Flat Audio TX Input on GM600
Flat Audio TX Input on GM600

But the radio would still only use the audio on pin 2 which is MIC input and involves pre- and deemphasis of the audio signal which renders POCSAG transmissions undecodable.

So I searched the service manual for a good point where to feed the flat audio signal into the transmitter after the preemphasis. The optimal point seems to be where the signal leaves the ASIC chip. I used the non-ground facing pin of C0242. The position can be found in the picture above.

POCSAG transmissions basically work now. But parts of the messages cannot be decoded correctly by my pagers. This needs more investigation if the feed of flat audio can be optimized.

If anyone knows how to make the radio actually use the flat audio input pin oin the ACC connector please let me (and others) know.