Casio Casiotone MT-70 vintage keyboard [Read Description. Video Included]

Description

Cleaned up, exterior in really good shape, buttons and functions are working, things not tested: The MS port and the AC/DC power port.
The bad: there may be a loose cap/transistor/spark plug/thingamajig that is causing some static/crackling/fuzzy audio issues with the piano, appearing when adjusting volume and lasts a few seconds afterward, and sometimes the issue occurs while playing. The accompanying volume knob tends to exhibit this issue worse, only really starting to work after turning it up past the 50% point.
i'm sure someone more skilled than me can diagnose and fix it. I have uploaded a video to youtube that shows the piano is working perfectly otherwise, with the sound issues included:
https://youtu.be/qxHcWGfdvFI
Will ship USPS without the batteries for weight reasons. Includes original manual.
Happy bidding!