Repairing Yamaha MT120 4-track tape deck

Afternoon, techies! I'm at a bit of an impasse in regards to what part of the circuit I should be looking at in order to repair my Yamaha MT120. The transport functions work great, and it's in cosmetically excellent condition, but input and output audio are not working correctly. Flipping between stereo and 4track metering results in tiny spikes on the meter, but otherwise no input audio gets through/out. Playing from tape also results in either no audio, or reversed audio from tracks 3 + 4 when monitoring levels are turned up.

All the components inside APPEAR to be fine (no scorched components/holes/blown caps) but i know that's probably not the case.

Before I continue prodding (I only have a multimeter but am thinking maybe an ESR will help with this), I wanted to ask where more experienced folks would look first? My first guess was the playback amp circuit or somewhere before that, but it's hard to diagnose confidently.

Gonna post up a small vid and pictures in a few.

EDIT***:

Video demonstration:

https://mega.nz/file/Ga5jSIKK#1QYiZqLuFyntlhfgRhpOpLuu3iH7hXJwsSMYshXjGCE

Circuit board photos:

1)https://mega.nz/file/DPg3iQxK#8alUnvOjDF5yHvKedqBkAB6WpJPZSjjbK2Mcg1S15Ig

2)https://mega.nz/file/PH5HWJSJ#DfmN3SIBURLeFVQL-_Iipbyt2s-7xny-uDQvqp4csHg

Block diagram:

https://mega.nz/file/LL40RZYJ#enBXumCc6gW2ahR34yUNP-yqydeCQvui7MlEzBDTMCk