A few things to add here as I have had our Control 24 apart more times than I care to remember...
Check the frequently used buttons (Save, Undo etc...) they should feel exactly the same as the less used buttons (Function etc) if they are not giving a positive click, they may be on the way out... The broken ones can be fixed if you are out of your warranty & live on cold coffee & solder fumes, but if in doubt send it to a pro Digi repair place who can obtain & fit new buttons...
DON'T buy this unit for the Pre's, Monitoring Section or Line Mixer... This unit is an excellent control surface, but don't let it anywhere near your audio signal path (I am not talking about the new C24 here, I have not used that model & cannot comment) The Pre's are based on the frankly garbage Focusrite Platinum series, the line mixer has always been pretty terrible & the monitoring section is admittedly feature rich & the best of the Control 24's audio functions, but it ages badly and can develop some scary faults that you may not notice until it's too late... And if we are honest here it doesn't get remotely close to a Goldpoint Attenuator or Dangerous Music unit even when new...
The flickering scribble strips can be fixed permanently by hard soldering the loom to the PCB, but again you will kill your warranty & have to de solder if you wish to remove that component (not a major deal)
If you are getting an older unit and wish to use the audio functions, the electrolytic capacitors are not the best & may not of aged well, so be prepared to re-cap any in the audio path, personally I wouldn't bother, you are better putting the money to good quality dedicated Pre's or monitoring controller... Like many, I used to use the Control 24 pre's for talk back, but it's got to noisy for even that now, so they don't get used at all...
Let me state again 'This unit is an excellent control surface' I don't have a downer on it at all, but if you are looking to buy one of these, you should be going into it with your eye's open, and without any real expectations for the audio side... Then if you find a use for the audio features, it will be a pleasant bonus...
I really wish that the would make a Control 24 (or C24)as just a control surface without the other cheap & nasty audio bolt-on's and a price to reflect the components that have been removed... I am sure they wouldn't be able to sell them fast enough... The unit as it is, is at a price point that excludes home hobbyists etc yet the Pre's they load it with suit that market perfectly... It seems insane...
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