Intermittent distortion/drop-out on 424 MkII

[kansas_parker]

I recently bought a used 424 MkII and there is an intermittent distortion and signal drop-out. I've messed around with cables, outboard gear, individual channels, etc. and it appears that the issue is in the master signal chain. As you can hear in the attached recording, it carries over onto tape, as well as through the phones and line out jacks. I opened it up and cleaned out all of the pots and sliders w/contact cleaner. I've got limited experience with electronics, but I'm not averse to DIY solutions. I just don't know where to start on this. Any advice is greatly appreciated.

https://www.mediafire.com/file/olplz8a082fie2r/424_Signal_Distortion.wav/file

 

[kansas_parker]

Also, it will play pre-recorded tapes from a previous 424 I had without any distortion. So it's got to be somewhere on the input side.