That's awesome, thank you. Filling out the form now. :)

The parts arrived and have been fitted. They have completely fixed the problem, but have now shown that there's a second issue. First the new rubber wheels.

Removing the drive from the unit made it easier to work on. The top two rubber wheels were perished and removing them made one crack badly and the other snapped. This is after I've fitted the two new top ones, and also the new bottom one.

These are the old ones I removed. I didn't cut the broken one, it snapped when I tried to stretch it whilst removing it from the pulley.

This has totally fixed the slipping and rhythmic knocking noise. But, now that the player will play cassettes again, it has shown that there is another problem.

Again, sorry for not knowing the nomenclature, but the spring loaded sprockets that engage the spools in the tape are the new problem. They are spring loaded, but when pushed down (e.g. when inserting a cassette) they do not spring back up. This leads to poor engagement of the gears, and the tape spools jumping and making a ratchet like noise when trying to play. Careful prepositioning of the tape spools allows a tape to be engaged correctly (after several attempts) and has shown the unit to be otherwise faultless.

I have tried a bit of IPA on a cotton bud, but that didn't help. I'm reticent to try adding some lubrication due to how difficult to remove it would be if that's a bad idea.

Ultimately I'm not sure if their moment is restricted (e.g. gummed up), or if the strength of the springs has decreased. Has anyone seen a similar issue before, or is this a known problem?

The tri-wing parts are spring loaded.
They move up and down a hexagonal shaft - this shaft cracks down one side and the tri-wing binds.
The ideal solution is to replace the reel table part (with the hex shaft), but simply glueing the tri-wing to the hex-shaft in its upper position
works surprisingly well.
Careful not to glue any other parts together.

Martin

Much appreciated, thank you. It's working perfectly now. 😁

Your advice on this forum has helped me a great deal. I repaired a Beocord 2000 easily by replacing the belts, then on a Beocord 3300 (bought for HX Pro). It was trickier - all three belts were melted, there was a +4dB permanent output, fixed by replacing the 220uF caps on the Dolby board. I replaced the rubber parts on the idler wheel and the table reels and thought this would fix the rewind problem, but it was the sprocket. Easily fixed by glue !