Yaesu FT-1907 & FT-1900 mmdvm DMR mod

Hi all, in this article i will be explainning how to connect your Yaesu FT-1907(uhf) or Yaesu FT-1900(vhf) to an mmdvm board. Actually this very transceiver was used as an echolink node for approx 2 years 24/7 with no issues at all, even though the transceiver was located in a hot and humid room during the summer that the air temp was reaching 37C! Yes i know!

Yaesu FT-1907 main board

When i decided to stop the echolink node due to low traffic in my area this trasceiver was no more in use. I never though back then that in the near future this will come handy for DMR! After i finished the upgrade of the DSTAR FOX controller to an all mode digi controller (mmdvm) i though why not to make it a repeater. I was already using the mmdvm for a wide area public hotspot with a Yaesu FT-7800 which by the way even some negative thoughts that it does not work well with DMR ,was performing excellent.

DSTAR fox multimode controller

So i had my TX radio and i needed a RX radio. The spare Yaesu FT-1907 UHF transceiver will be used again, after many years of inactivity! The issue...just one. This radio is not packet ready, and does not support 9600 connectivity out of the box. It needs to be modified.

After some reasearch i found that the mod is really easy. Steps as shown bellow:

1) Locate discriminator audio for TX and RX

2) Ground

3) PTT

4) RSSI (for future use)

Lets start by locating the clean raw audio from the discriminator for RX first. We are looking for an IF demodulator IC and this is a 16 pin chip solder on top the pcb(not through). In the main pcb it is located at the lower right corner and it is the Q1028(NJM2591V) chip. 

location of the Q1028 chip on the main board (blue circle)

NJM2591V

We will use the following "legs" of the chip. Numbers 9, 12 and 15.These are AF OUT for receiving the digital audio signal from our handheld to the transceiver sending it to be decoded to the mmdvm, then we can also connect the RSSI pin to the mmdvm board to have RSSI data displayed to our mmdvm screen and finally the Ground pin. For ground we can use whichever DC grounding point on the pcb, except the RF ground. 

Be carefull to use proper cable to do the connections on the main board and also you 'll have to twist the cables with the ground cable. This is to avoid any RFI with the audio signals. Finally avoid soldering directly on the chip, prefer to solder at the next component were you are going to have more space and you won't damage the chip by mistake.

AF OUT and GND connected and twisted. No RSSI used above.

...to be continued with the TX and PTT connection part!

73s

Proceed at your own risk!!!