Repare the audio output of the Lunii - My Story Factory v1

はじめに

This tutorial allows you to diagnose and prepare certain problems with the output audio of the Story Factory v1.

STOP! Immediately drop that kitchen knife! Even if you've already opened tons of things with it, it's still not a screwdriver!
As a super handyman parent, you do have a real screwdriver hidden at the bottom of a drawer, but until you find it, I'll show you how it goes.
For this, I'm going to use a size 2 Phillips (or Phillips PH2) screwdriver. A 3.5 dish will do just fine, all this and being able to reach the screw without damaging the hull of our Lunii.

Let's go ! We firmly hold our Story Factory in one hand and with the other we unscrew the 4 screws that come our way.
Make sure that the 4 screws are fully unscrewed by tilting the Lunii, they will fall out by themselves.

Then straighten the Lunii and hold the two top edges each with a thumb.
By gently spreading your fingers you should be able to open it without any resistance.
Hooray we made it! And a small interesting detail, the adhesive under the Lunii acts as a hinge.

If the speaker is not working, try connecting headphones to the audio jack.
Any set of headphones will do, not just the official Lunii headphones.
If no sound still comes out, the problem is upstream of the amplification part, and this tutorial will likely be useless to you.
If sound comes out of the headphones, continue the investigation.

Disconnect the battery.
Measure the resistance between the speaker terminals; it should be around 3 ohms.
If the resistance is too large, the speaker is probably damaged.

Turn the potentiometer to the maximum volume.
With a multimeter, measure the resistance between the two pins (audio in and audio out) at the bottom, as indicated in the photo.
If the resistance is high (in the order of a megaohm) or infinite, the potentiometer and audio jack daughter board are the problem.
If possible, replace the daughter board.
The problem may be from a blocked contact inside the audio jack, which can be unblocked with isopropyl alcohol (see this forum for more information).
If the resistance is fine, the next step describes a hack to work around the problem.

Insert a through-hole resistor of 1 kOhm or similar into the last two pins of the connector, as shown in the picture.
Reconnect the battery and test the sound and volume control.
If this fixes the audio issue, solder the resistor in place.
In my case, I used a 1 kOhm 0402 resistor recovered from a broken circuit board. The ideal resistor would be in the 0805 format.
Solder the resistor between the two areas highlighted in pink in the picture.
With this method, connecting headphones will no longer automatically turn off the main audio output. To fix this, you must disconnect the speaker cable.

Desolder the amplifier, which is highlighted in orange in the picture.
To desolder this component with only one soldering iron, bridge all of the pins on one side with solder and heat it with the iron. Once the solder is melted, lift that side of the component up. Repeat this technique on the other side of the part to remove it completely.
Reconnect the battery, turn the potentiometer to on, and test the voltage between GND and VBAT. This should be the same value as the battery voltage, between 3.7 and 4.2 V.
If possible, play an audio file and use an oscilloscope or a multimeter to verify that a signal is present on the audio input pin, as indicated by a blue arrow in the picture.