Each of the mixer's 26 channels are equipped with a mute button. This lets you kill the output from that channel instantly, rather than having to turn down the "Gain" dial. If the "Mute" button is damaged, for example due to being pushed in too hard, the mute function may be permanently engaged. This effectively renders the channel unusable.
Each channel has an array of balanced 1/4-inch jacks and XLR jacks. If the interior connection becomes damaged, the signal will not pass from the tip of the connected jack plug to the mixer circuit. This can make an otherwise functioning mixer fail to receive an input signal. The most likely cause of this problem is separated contact points. This is usually the result of a user carelessly inserting a 1/4-inch jack cable. You can test for separated contact points using a dummy insert bypass jack.
The FX16 has an internal digital effects module. By connecting the auxiliary outputs to the channel inputs, you route the digital effect from the module into the audio channel. The effects module is a large computer-chip mounted on the circuit board. If any point between the chip and input jack on the audio channel is damaged or loose, this can cause the effects to fail. You can partially troubleshoot this by routing the effects to all channels to determine how close to the effect module the problem is.
If the FX16 fails to power up, there may be a shorted fuse in the power section. Power surges arising from incorrectly connecting the device to an input device can cause fuse outages. If you're confident about dismantling the FX16, inspect the circuit board and look for fuses with brown discoloration. This is a sign that the fuse has blown.
The Soundcraft Spirit FX16 is phantom-powered, meaning it delivers power to a microphone device via the XLR cables. XLR cables have five pins. If one of the pin sockets on the FX16 XLR jack is damaged, the device can't send power to the microphone and therefore the microphone won't work .