These are a few of my findings after spending time working on and modifying the Australian design award winning Motorola Syntrx radio. Most work to date has been done on VHF Hi band versions. Please do not ask me for a EPROM image or programme to make one. I can not help you with that.
The information given here is intended to help licenced Amateur radio operators to modify these very impressive surplus comercial radios for personal use. Ive done the best I can with my new HP scanner and the hand written notes I jotted down. DO THESE MODS AT YOUR OWN RISK .
The prom can be replaced with an EPROM. So far 2764's 27128,27256 have been tried. I strongly recommend the use of the transistor switch circuit on the OE line of all eproms. Only early versions of the 2764 seem to work without this. Following is a table of PIN to PIN connections for a 2764 to a Motorola Prom . Make up a board however you like but I use a 20 conductor ribbon cable and an 20 pin IC IDC header plug to go into the Syntrx Prom socket. I clip off the 2 spare header pins near the JU409 link end of the Prom socket and let the header hang over that end. Read the notes following the table carefully to include the transistor switch. The pinouts of a 27128,27256 are very close with only a couple of differences. Dont connect pin 28 of the Eprom through to pin 18 on the Syntrx, use a fly lead to bring +5v to pin 28 and key the Eprom using the OE (output enable) Pin. The Syntrx Prom pin 18 feeds the transistor circuit provided below to key the OE pin. Dont forget to remove JU409 from the Syntrx (link next to the old prom socket) if you want to go beyond 20 channels .
The VCO may not pull in well on the lower 2m channels. I have NOT had to fiddle with the VCO on UHF versions as yet. So this only applies to VHF versions. Just tune it in and if it wont go down low enough then ADD 5p6 across C243 for the TX and 3p9 across C254 for the RX. I usually go direct to the underside of the mainboard and remove the flat metal VCO shield. I recomend to use some sort of mechanical support on these added caps as the radio easily goes microphonic with vibration. I use a dab of Hotmelt glue betwen the main board and the caps. Dont use too much though. For info C243 is 6p8 and C254 is 4p7 .
A word on microphones. If your microphone does not have a scan switch between the power and channel change switches then you can fix that. Take the microphone to bits and have a look. There is a spot on the PCB for a switch. You need a board mounting (preferably) , SPDT centre off toggle switch. From behind the front cover of the microphone with a sharp knife cut away the plastic covering neatly where the switch will protrude. You will see what I mean. Be careful to get the switch flush with the others so it lines up with them and looks right from the front. You will have to desolder the metal bracket holding the switches onto the board and get the new one in the middle between the others. Put it back together and get the software sorted out with some scan groups and it should work. If the switch is up then it will scan (provided the microphone is in its holder clip). The centre position is normal and down is local. By local I mean it will TX on the repeater outputs when on a duplex channel and RX will not change. Thats it for scanning microphones.
Packet and accesories connections.
This is what we do here. The pinouts for P102 (accessories) connector are as follows in our setup.
Pin 1 - ground
Pin 2 - Speaker level audio out
Pin 12 - Switched 13.8 volts
Pin 13 - Currently TEST I/O ( but SAPOL re-use it for combined
PTT and mic audio OUT )
Pin 14 - Mic / PTT IN (we use this one)
You can download all the pictures as Zipped JPEGS here if you like. About a 512k file