Update: 15 May 2020

Microdrive refurbishment manual

Standard refurbishments

• Replace the rubber roller (sometimes available from SellMyRetro.com, but check out the underneath paragraph as well)
• If there are no capacitors present at C7 and C9 on the ‘ZX Microdrive Head Board Assembly’, place 100pF capacitors on C7 and C9*
• Replace the electrolytic capacitors right next to the 7805 (it’s ok to use e.g. 1uF instead of the 0.22 uF and 0.47uF ones).

*I think I discussed this once, but can’t remember where or with whom.
If I remember correctly, these capacitors C7 and C9 improve reliability – they’re drawn ‘optionally’ on the original schematic:

Rubber rollers

Do not throw away original rubber rollers!
These can be renewed completely by using something like ‘Rubber Renue’, a rubber rejuvenator – this is very aggressive stuff, so be careful using it (gloves etc.).

Johan Engdahl is currently (May 2020) offering new rollers on Facebook.

Still issues?

When the Microdrive reads well but does not format well:

• Solder a 47nF capacitor on the power pins of its ULA.

When a Microdrive keeps spinning and the ZX Spectrum doesn’t respond anymore (to e.g. BREAK):

• Replace the 7805 of the Microdrive

More tips

More information at this World of Spectrum post.
Here is the ZX Spectrum Service Manual describing lots of details about the ZX Microdrive from page 20.