I was thinking about using a second PD-405 and using the digital side to contact to the node. So need to find the various points of interest on the circuit board. RX/TX audio, PTT and COS.
Guess that nobody has done this before. I have downloaded the Service manual and currently working through the circuit diagrams. There are a few TP’s that seem to offer a way forward. Will keep you posted.
The PD405 has been on the bench this afternoon. It looks like a hedgehog at the moment. Lots of wires coming off the various test points. I have the incoming audio sorted(ish) and the 405 will transmit content received from AllStar to the second PD405. Will work on the TX side over the next few days. Pity we can upload images here so I could share the fun.
I had another play this afternoon and have most of the functionality working. The basic wiring is as follows. I am using the conventional cable colours used when modifying a CM108. The Black Wire (TX/Audio Out. e.g. The Sound coming from Internet/AllStar) from the CM108 connects to TP406 or TP407 on the PD405. TP406/407 are both Mic-in so we pass the audio received from the Internet/AllStar to the Mic-in connection on the PD405 which will be our node radio. Yellow Wire (RX/Audio in. e.g. The sound that we want to send back to the Internet/AllStar) connects to TP402. TP402 is the speaker out on the PD405 that will be used as the node radio. So when we transmit to the node radio from another handheld, the speaker out of the node radio is sent to the Mic-in on the CM108. White Wire (COS) connects to TP102. Finally the Red Wire from the CM108 (PTT) connects to TP103. Grounding TP103 causes the node to go into TX mode. These Test Points (TP) are all clearly shown on the PD405 Service Manula which is readily available to download from the internet. I have a little bit of work to do with biasing to get the audio levels just right. There is also a diode needed on the COS line but I will work on the final values of all components over the next few days. The TP pads are small, approx 1-1.2mm but easy to get to. Just use a flux pen to prime and then a very fine tipped soldering iron to connect the wires.
And don’t forget the all important earth wire. The 5th wire needed with the CM108 set-up. There is a good earth pad on the PD405 in the centre of the board about a quarter way down from the top. This sits to the right of the TP401/402 pads. I normally use pin 39 on the RPI 4 which is earth.There are a couple of additional grounds available on the CM108 at the very end of the board at the opposite end to the USB connection.
This is the last update. I made an error when I said COS (White wire from CM108) connects to TP102. It connects to TP404. Everything is now working now as intended. So the complete wiring and biasing is as follows. Black wire from CM108 (Audio out. Sound coming from the Internet/AllStar) connects to TP407 via a 4K7 resistor. White wire from CM108 (COS) connects to TP404 via a Zener Diode (BAT43) with cathode pointing to the CM108. Red Wire from the CM108 (PTT) connects via a 10K resistor to TP103. Finally Yellow wire from the CM108 (Audio in. The audio we want to send to the Internet/AllStar) connects to TP402 via a 10K resistor. This set-up in interesting because when I use another Hytera PD-405 to talk to the node, I can switch both the node and the second radio to analogue or digital. The circuit above doesn’t distinguish between the mode that the radio is in. However, you can still hear the digital artefact when using digital. Later today, I will upload some pictures to my QRZ page showing the circuit and the node on the bench.