Sooooo, I finally had the time and the guts to open my boombox.
I made a few experiences, which I'm happy to share with you.
The screws on the back (for opening the front panel) are really difficult to screw back in.
As a reference, I played a cassette recorded on a different tape deck and tried to sync it with the same song played on YouTube at the same time.
The song was "No tengo dinero" by Righeira (extended version). I heard it for the next few hours and I thing it's fair to say that I heard it enough for the next couple of years.
I noticed that moving the screw in the motor block even the tiniest bit substancially altered the speed.
I only managed it to get the two songs in synch for about max. 3 minutes. After that, it developed into a hall effect and eventually the two versions drifted apart.
The previous owner told me that he had the belt renewed and that it was completely cleaned and serviced presumably up to the lube level.
I finally quit the operation and closed the box.
So the problem still exists. I lost half a day and "No tengo dinero" got wiped off of my list of favorite songs.
Any suggestions what the problem might be (apart from new belts). New motor????
Here are a few images:
First step: Opening of the front cover
I left a cassette in it, in order to test speed with my computer.
Interesting new look at it:
A closer look at the cassette mechanism:
Removal of the switch circuit board:
The key step: