Dear internet traveler, you have found an old-fashioned Web Log that will tell you.
If your DeLonghi KG79 or KG89 grinder (both are the same model inside) constantly stops within a second of grinding, it’s probably not a mechanical issue, but an electronic issue.
How to open your grinder is already well documented online, so there is no need to recap.
Or rather, there is a need to recap, but in the second sense of the word: to change capacitors.
The timing circuit is controlled by two capacitors. Many people reported that changing the big yellow one fixed their issues. It is crucial to use a capacitor with the same specs, rated for mains voltage: no need to roast your coffee beans a second time by setting the grinder on fire.
Double-check that your big cap has the same specs, but mine were X2 275VAC 0.47µF. The ones i purchased from AliExpress were larger than the original, but there is a lot of wiggle room inside the grinder.
For most people, this will fix the issue immediately, but for me, it didn’t, i also had to change this electrolytic capacitor in the center:
Mine was rated 25V 100µF, but again, double-check your circuit is identical, and before you desolder it, take note of the polarity: which side is positive and which is negative. The replacement must be installed in the same direction as the original.
There is an additional electrolytic capacitor (rated the same as the previous one) on the board: i dunno what it does, but while you’re at it, you might as well replace it, electrolytics go bad over time.
The PCB is of mediocre quality: beware of lifting pads and traces when desoldering. I had to bodge a fix by following the traces and soldering one capacitor leg to another component.
Be careful fixing appliances that get plugged into the mains. Or don’t, i’m not your mom.