RESOLVED : the problem is solved.
No sound randomly , after startup , sometimes ,
I use the original power supply
It is a known issue which NDSP, as always, denies. I have had it with multiple units, a friend has and there was a thread once on this forum which since then has been deleted.
I forwarded it to support multiple times as well, which has lead to nothing but their standard answer: āunfortunately we can not replicate this issueā.
On some forums this problem is described but there is no solution! Why? It is annoying for a professional musician!
I would assume it is a software related bug and therefor only solvable by NDSP. But as long as they arenāt working on it, no chance.
Btw, itās also annoying for amateurs 
I have this same issue with my unit.
the problem is solved. 
I found a solution that solved the problem for the moment. I put WD 40 SPECIALIST CONTACT CLEANER (in French: WD 40 NETTOYANT CONTACT ) in all the inputs and outputs and power supply: it is a ādry liquidā that removes impurities and removes oxidation from electrical or electronic equipment) and everything works normally again. In summary: oxidation? dirt?
In practice I put the product in the sockets on the rear panel several times, also inserting a jack or the power supply socket with an in/out movement.
I found a solution that solved the problem for the moment. I put WD 40 SPECIALIST CONTACT CLEANER (in French: WD 40 NETTOYANT CONTACT ) in all the inputs and outputs and power supply: it is a ādry liquidā that removes impurities and removes oxidation from electrical or electronic equipment) and everything works normally again. In summary: oxidation? dirt?
In practice I put the product in the sockets on the rear panel several times, also inserting a jack or the power supply socket with an in/out movement
Hey guys, I had this issue for several weeks now and got it fixed. It is an absolutely weired story but I want to share it with you.
Last year I played a festival with my second band. Our quad cortex had issues booting up. Just flashing footswitch leds but a dead screen. We had to borrow a quad cortex from another band which was kind enough to give it to us. I took our qc home and tried to fix it. Rebooting/restarting several times, resetting settingā¦ no success. I removed the backplate to check the memory sd card. Removed and reinserted it. No success still no reboot. I picked the quad cortex up and put it on the front back edge at a slight angle and let it drop. Just a little bit to give it a small clap. And it worked. Since then there is no booting issue. I could not believe that it worked but it didā¦
A few weeks ago from today, my own quad cortex (which is another device, not the one that had the booting issue) started to have issues with the analogue inputs 1 and 2. I tried everything mentioned above. I tried several power outlets (which was the fix for this issue a few months ago) but no success. I remembered that I fixed the booting issue by dropping it from a slight angle. I tried it with my quad cortex and guess whatā¦ It workedā¦
What I wanna say with both stories: I do not know if it really fixed the issues but it has to be a huge coincidence if both quad cortexes randomly started to work again. In my opinion some issues are hardware issues and might have to do with the electronics inside the quad cortex.
This post should not encourage you to be violent against your gearā¦ obviously. I want to share it with you so you can decide if you want to try that ābug fixā for yourselvesā¦
Greetings!
David
PS: Some other users said they had the same issues and they have send theirs quad cortexes to NDSP to get it to check. NDSP could not confirm the issue. Maybe the quad cortexes got some little claps during the transport and using some magicā¦ it works againā¦
This story does not seem to convince me that the QC is a high quality product. Hope Iām wrongā¦
Then again, I had to RMA my Fender Tone Master FR-10 because of a bad volume pot and other weirdness.
I guess every product can have itās quirks. But twice with the same customer?
Iāve been struggling with this for a while now and have finally found a solution (well a work around anyway). Maybe 50% of the time my unit would power up in this state. with the same output levels as you have. Iād then reboot the system and everything started working again. So frustrating, especially at gigs!
What I discovered is that it seems to be related to the noise gate on the input. I turned that off and now I have a 100% success rate when booting the system up.
Steve
Iāve been having this issue for a while now and itās driving me nuts.
Neural support have been very slow to respond and all they do is ask me to send a video of the problem. Not sure how that helps, as itās very easy to describe - there is often NO SOUND on start up!
Can you explain what you mean about turning off the Noise Gate on the input - do you mean you have to turn off the Noise Gate for every preset that has one on the input? Because for me thatās most presets, which is a LOT of tweaking just to fix this problem, and we really shouldnāt have to do thisā¦
Unfortunately it reverted back to old (intermittently broken) behavior - I think my work around was just luck. I finally gave in and shipped it back for a repair. It should be returning later this week.
Steve
Was it inside the āwarrantyā period?
I use inverted commas because there are some things I think should be covered within a certain period, like say footswitch coming loose or something, but a bug that is clearly affecting many users, and appears to have been introduced by a firmware update, such as this issue of having no sound, should be covered for life, or it will ruin NDSPās reputation
The repair was covered. Shipping was not, which was quite expensive.
It got to the stage though where Iād need to reboot 6 or 7 times before Iād get an output, so basically unusable in gigs. Thankfully I have amps and pedals so didnāt have to cancel anything.
Oh man - are you in the USA? And where did you have to ship to?
I donāt get how they can cover the cost of a repair but not the shipping for it to be repaired?!
I just came to say thanks. The solution of cleaning the inputs and outputs with WD40 worked like magic. Iāve already played 10 shows with 0 errors when starting the Quad Cortex. Thanks!!
Just a word of caution. WD40 was designed to loosen frozen hardware by desolving rust and other corrosion. WD40 IS NOT A CONTACT CLEANER! It may clean the contacts but, because it is somewhat corrosive, it can damage electronic components and even the plating on the contacts youāre cleaning, over time. I would recommend Deoxit or any quality contact cleaner.
Been having the same problem for a while now! its driving me nuts! Sometimes the input level graphic looks like thereās something connected but like itās stuck at a db level. But right now I get no audio out of any output but USB, which I can record onto my daw (but no hear while Im playing) I donāt think is routing problem in my daw cuz I restart the QC and sometimes I get audio out all the outputs! This is driving me nuts! I havenāt got this problem so far in a show but Im afraid it will happen any day now!
I nearly had to cancel a gig last night because mine did this YET AGAIN!
Like on your unit, I have no inputs connected yet the dB meter on the I/O page shows all the way in the reds on all the inputs.
Iām sending it back to Neural as it is still under warranty. This appears to be a big problem however, so I do take issue with their policy that the owner is responsible for the cost of shipping a unit back to be fixed under warranty. I canāt think of any other manufacturer that thinks it is ok for the person who has bought their device to pay to ship it back when it is still under warrantyā¦
My main issue now is that the useless thing wonāt backup to the useless Cortex Control app. I have a cloud backup but would prefer to have a hard copy too. That said, Iām at my wits end with this now, so Iād rather get it fixed/replaced and risk not being able to restore anything to it when it comes back, that carry on with the constant doubt over whether it will crap out on you right before a gig.
The stress of that last night was enough to make me forget to hit the record button on our mixer, as we wanted to record the set - I only realised this halfway through.
If this kind of user experience continues, Iāll be picking up a Helix Stadium later this year 
I tried below and improved. (not perfect)
1 Factory reset; no effect for this trouble.
2 Cleaned all I/O with a contact cleaner; I guess it would be a placeboā¦but who knows?
3 Roll back to the previous version of CorOS and updated to CorOS3.1.1 again; it worked for me, but it needs opening the QC, making backup the disc image of the SD (before update), then rolling back, not for everyone.
As you say, Cortex Controll should have this function.
CorOS is encrypted by the serial, but NDSP knows the encryption key.
Achieving this, they should link the user account and Serial No.
They have to attach a user registration paper like the DAW apps, so Iām not expecting this right nowā¦
