Pioneer HPM 60 Hochtöner kratzt

Do you mean the silver foil speaker (Partno: HP015A)?

If it is the rather flat model without a transformer at the back then see below:

It may be corroded inside. It is possible to dis-assemble the speaker my unscrewing the 2 screws at the speaker-terminals and very carefully pry the speaker open with a small screwdriver around the edge of the plastic front.
The impedance of the speaker is extremely low (don't know HOW low but probably a fraction of an ohm) and it carries high currents through the silver foil membrane. Any corrosion will raise the impedance and the speaker will sound wrong, eventually be completely silent.
You need a 'glassfazerpinzel' to clean the foil contact-points. Carefull!! Clean after brushing with alcohol.
Clean underside of speaker terminal lugs the same way.
Re-assemble the speaker and tighten both screws firmly without cracking the plastic. Gently squeeze the cable clamps with a flatnose plier to make better contact when they are pushed onto the speaker lugs.

Sorry, I can barely explain this in English (not my native language) and absolutely not in German.
Hope you see what I mean. Good luck,

mfg,

/tri-comp