Error Analysis: WM-F5 loud hissing/sizzling after few seconds of tape playback

Hello everyone,

I was lucky to get a hold of a few WM-F5s (three in total). One of them was an absolute donor unit - broken hinges, leaked batteries all over the board (and beyond), a real donor unit.

The second unit was luckier and had even functioning radio capabilities. Unfortunately, upon inspection of the unit, someone did not grasp the concept of how to disassemble the Walkman and actually broke the upper board piece which was held by the volume screw apart.

That unit was then sent to an expert who managed to restore the connections between both pieces as well as he could. Fees aside, which I was fully accepting to pay because I just wanted a working F5, I then noticed that the Walkman has the odd habit of starting to hiss very loudly during tape playback. Though it is not quite the stereotypical type of hissing, but more like a sizzling/static sound which cannot be ignored after a few minutes of playback. The pal repaired the center gear, too.

Oddly enough, this is only happening when playing tapes - tried that with Type IV XRs and a Type 1 HF but the results remain the same.

I attached two .ogg file links of that sound - one during music playback (playing ALEX & Tokyo Rose's Akuma, can only recommend this Synthwave Opus) and one during fast-forwarding the tape.

https://jumpshare.com/s/cFFIwijVEx3nGORxb3Wo - During Rewind/FF
https://jumpshare.com/s/159wJgDAA3OxzJXx09RQ - During Music Playback


During music playback it is quite audible during the ~20 second mark, while it remains fully audible during the fast-forwarding.

Any ideas what else might be at fault here? The pal who repaired it has no idea and neither have I. I suspected the headphone jacks first, but both have the same issue.


tl;dr: I got a WM-F5 repaired which had a broken MB piece right below the volume control and now it's playing tapes with loud hissing/sizzling background noise after said piece was re-attached by a pro. What are the possible issues here?