Fona mini short?

[zazoox]

So I made a box with a raspberry pi, a clock, a relay, and a FONA mini (with mini-jack audio in & out).

Everything was working great until I plugged the FONA audio OUT into the audio IN of the monitor. (iiyama touchscreen 27")
It seems to have killed the connection between the FONA and the RPI. The FONA has its orange LED on and that's it (before it used to be the blue one) and
I can't do anything with it (as in the RPI is not seeing it anymore). Also the outputted audio by the screen is incredibly weak now, like its amp got kinda fried.

So I thought I made a mistake somewhere in the circuit, I triple checked everything. Even asked someone else to do it. No issues were found with the circuit (as in tension issues, or weak soldering)

So I quickly ordered (yeah I'm on a deadline here) a new FONA mini and wouldn't you know, it worked perfectly !
For a grand total of 20 seconds just enough time to place a call, hear a beep and then back to the orange LED and the pi not detecting anything.

I included a photo and here is the legend from the documentation I made :
1 : Raspberry Pi 3 Model B)
2 : Carte SD
3 : FONA Modem
4 : Carte SIM
5 : RTC PCF8523
6 : Relay
7 : Fona Battery
8 : Output
9 : On/Off switch


So my questions are, and keep in mind that I have exhausted all of my good ideas, I am at a total loss so those might sound weird :

Can a FONA be "fried" but still show the orange LED?
How can a mini-jack fry anything?
Is it possible there was a loop between the audio IN and the touchscreen HDMI?
Is the FONA actually dead? Has this happened to someone before?

Any ideas ?

Cheers,

[adafruit_support_mike]

The most likely reason for that kind of failure would be some kind of ground loop.. a set of connections between two different power supplies whose GND pins are at different voltages, or a connection that creates an alternate low-resistance path for large amounts of current like the FONA's 2A burst current when the radio talks to a cell tower.

Does either FONA work if you disconnect them from the rest of the circuit and make simple connections to a microcontroller?

[zazoox]

Yeah the loop makes sense. I didn't know about the FONA'S 2A.

And none of the fonas work even outside this circuit. I've got the same result on an independent board. i.e orange led and that's it.

So my solution was to order a new fona and instead of plugging its audio out straight into the monitor I'll use speakers. I know it's the connection between the audio out of the fona and the monitor's audio in. But any ideas as to how I could test this hypothesis?

Anyway, thank you for your reply

[adafruit_support_mike]

You should be able to protect the FONA's output by putting a pair of large (100uF or so) capacitors in series between the FONA's output pins and the speaker jacks to the other device. Those will let the audio frequencies pass through, but will block any DC current.

[zazoox]

Ok so I modified the circuit like you suggested by adding a capacitor in series.

However, before plugin my third FONA into my circuit I decided to test it on a breadboard.
So without plugin any output or input, I just plugin the FONA to the Pi (3.3v, TXD to RXD, RXD to TXD, KEY to GND).
The modem works (blue LED) but doesn't see any of the 3 SIM cards I tried to insert in it (red LED blinking).

Is this a known hardware issue?
Any ideas?

[adafruit_support_mike]

Do you also have a LiPo connected to the FONA? It doesn’t work without that power source.