Built an Arradio module over the weekend with a new friend I met trough muusikoiden.net/tori. I posted a prompt to borrow an Arradio module to study its circuitry which led to a process where we managed to source parts and pcbs to build a few units. To confirm my earlier notes, the tda7088 pin1 to 3,3v power line via 33k on a switch (currently 30k) does indeed work as a channel latch mechanism and toggling it changes the tuning behaviour. We changed the original design by adding polarity protection diodes 1N5817 (instead of the ferrites, but still including them) and I added a 1k resistor to the output (cut the trace on the pcb and soldered a 1k smd resistor as a bridge). Neither additions might not be necessary but I feel more confident exploring the circuit with these changes. I’ll proceed to test the unit with different radio sub-modules and possibly designing a new pcb layout. The current unit also has a I/O switch which I’ll need to study too… It cuts the power from the FM receiver sub-module and I’m not sure if it has (or needs) surge protection circuitry.
Updates:
Drilled a hole to the faceplate to accommodate a sub-mini switch hooked to toggle the latching mechanism. I tested if the circuit “Scan” feature could be activated with a button (3,3v to pin15) but this only produced glitches. I think the tuning potentiometer control voltage should be squeezed with a resistor for it to work (or even disengaged). The Conrad “UKW Retro Radio” schematics which the sub-module is based on show a 200k resistor between the tuning pot(s) and the RE2 trace which lead to the “Reset”/pin16 (perhaps serving as a voltage divider?). The “Reset” also works for manual tuning. The new “retro radio” board wiring examples show a 1m resistor after the tuning pot(s). I should test the minimum voltage the chip needs for manual tuning and add a voltage divider accordingly. The “Scan” is on pin15 and I think a constant voltage at at “Reset”/manual tuning prevents it from working. I’ll test having 0 voltage present at the RE2 trace will enable the “Scan”.
I don’t have any radio reception in the basement I work in, which defeats the purpose of having a radio. Spend a few days building antennas. I had most luck with a 2x 75cm dipole antenna made from insulated scrap copper but I’d have to set it up 10-15 meters out to our housing companies backyard, which might cause suspicions. Touching base with the radio-hobbyist scene.