Completed Ditting KE640 electrics repair report

Equipment doesn't work? Troubleshooting? If you're handy, members can help.

#1: Post by Sw1ssdude »

Well, off to another repair thread.

This one isn't about descaling and polishing chrome and drilling out broken boiler studs. This repair is done on a modern machine, and modern problems call for modern solutions. But I try to make it worth your time.

It's been a while, since I fixed up all my three Olympia clubs. They are all in decent shape, and they are all put to use in kitchens of friends of mine. You can read the rebuild storys here Olympia Express Club rebuild (+ introduction)
And here: Ongoing Olympia Express Club Rebuild

I am especially fond of one of these clubs: the one with the funky bulged shag-waggon-manometer-glass, and the short lever, which is missing about 25mm ore one inch of leverage, since my dad broke the lever thread off around 25 years ago. Yes, this machine was in my family's home, as a 'permanent loan' from a friend of my parents. The machine eventually found its way back to said friend, was handed over to me after decades in storage, got fixed up, was gifted to a friend of mine, and now, it came back to me. And I'm looking forward to good espresso in my kitchen. I promised myself to go about these machines as the previous owner, who generously gave them away to friends and family for as long as they had use for an Olympia club, and before they would sell them or throw them out, he'd take them back for the next person in need of a decent machine. So a friend of mine returned my favourite amongst these three machines (he settled for a '67 Cremina), and now its back, and now I'll never give that one away ever again. Maybe the other two. But not this one.

Anyhow, I've been looking for a decent grinder for some time now, and recently I got lucky for the first time, as I found a very, very cheap, but equally dirty Elektra with a doser. That thing was massive, and waaayy too tall for my kitchen. Also, this Elektra was made to grind through tons of coffee like it aint no thang, so after a thorough cleaning, and fixing the doser counter, I sold it without ever using it, but for some decent profit.

I reinvested the profit in my current grinder, about which this whole thread is about. I found a 'broken' Ditting K640 (also known as Mahlkönig K30) on the internet, put up for auction with an option to buy it for 280 francs, and I went for it immediately. I knew this machine from when I worked for ZURIGA, a manufacturer of high end espresso machines here in Zurich. This company has the same grinder, for quality control (and also for coffee breaks), and it is an absolute tank, with grind-on-demand and big 65mm burrs and nifty timers and whatnot. So I knew these grinders will not just 'break down', and I assumed (or hoped) for an easy fix.
I met the seller on a parking lot of a countryside train station, as is mandatory for every sketchy deal, and had a quick chat with him. He said he used it in his private home, and was apparently the second owner, until one day the fuse tripped on the machine, and since then, it had no vital signs of any kind. He also said that he'd be interested in knowing what the problem was, if I ever manage to fix it, so for this guy (and the home barista community) here is the summary of what happened next.

I took the grinder home, convinced that the trip fuse was just stuck. So I plugged it in, gave the fuse some taps with a screwdriver handle and ...nothing.
Okay, there really is a problem somewhere. I removed the pre-chopped hopper and the dust tray, opened the machine up, and gave everything a good cleaning. The machine looked decent inside. I searched all the PCB's for (possibly) blown fuses, and found none.

So I turned to the multimeter and started probing: the motor windings were good, all the connections were good, the fuse wasn't stuck, and all the solder joints on the power side on the PCB were good. Next to the transformer were two resistors which looked a bit toasty, but they still had the right resistance (you can tell by the color code), so I started to suspect the transformer.
Of course, there was absolutely no useful information on the transformer, so I tied the systematic approach: I hooked a DC power supply to the rectifier on the low voltage side, and started cranking up the voltage. Around 5 Volts, the Display lit up, and read out 'TEMP!'

Allright, this is due to the unplugged grinder motor, since the cable for the temperature switch was unplugged as well. But at least the control side of the machine works. I feared of frying the control chip (an ATmega chip) so I went online and checked its operating voltage, which lies between 4.5 and 5.5 volts. I then measured directly across the power pins of the chip with my multimeter, and cranked up the voltage until I got a healthy 4.9 volts at the chip. Then I hooked up the rest of the electrics, bridged the safety hopper switch, and pushed the grinder button.

With a quiet 'bzzzzzz', the grinder motor spun smoothly for the set 3.2 seconds for the single dose. It's alive! ...more precise: the transformer on the power board is dead.

Figuring out this unknown and undocumented transformer seemed like a big hassle, so I tried to go the easy way and looked online for a spare power board. I quickly found one, directly from the factory, for a hefty 250 swiss francs.

I immediately decided to go the hard way.

I drew me some schematics from the PCB routing, and calculated the necessary transformer output: since the rectifier consumes half a volt per rectifier diode, and the controller receives 4.9 volts on an input of 6.9 volts, an 8 volt transformer would do.

With my drawn schematics I tried to understand the absolute black box of the transformer layout. The board is designed to be hardwired for multi voltage use, either 230 or 115 volts, so there must be some sort of dual primary coils in there. But since this didn't look like an off-the-shelf-transformer and some (possibly dual purpose) components for the 115 volt use were missing on the board, I didn't want to rely on the 'usual' pin setting of a PCB transformer. I also couldn't determine the coils from the resistance between the pins, I was just guesstimating. I finally found a matching PCB transformer, at least matching enough to get it running witch some tinkering and duct tape, and started to prepare the board for the coming swap.

What happened next is a bit an anti-climax to the story: after desoldering the burnt transformer, I found all the necessary information about the transformer printed on the PCB, hidden underneath the transformer. It was an American ERA-transformer, dual coils, with a 12 volt output:

I learned all that after days of pondering, assuming, probing and calculating. Thanks, Ditting! Ah well, before you know, you never can tell. I was also relieved to spare this grinder from receiving some backyard engineering. Fortunately, I got the 8 volt transformer delivered with all four legs kinked, returned it and even got one in the right 12v range for free.

All the schematics proved to be useful again, since this new 12 volt transformer failed to bridge two necessary PCB pins. No problem, a snip off a jumper wire soldered in place fixed that. After Days of planning and scheming, the soldering was done in a jiffy.

I also changed out the two crispy resistors with fresh ones, and started reassembling, which took me around 5 minutes. I plugged it in, flipped the main switch, and the Ditting K640 was back in business. According to the grind counter, it hasn't been in business for a long time: built in 2009-ish, there were only 4000 single doses and 6000 double doses produced, so I guess I will not have to replace the 100-dollar-burrs anytime soon. This grinder is barely broken in!

So this concludes of how I got my Ditting back on track. All it took was a 3$ PCB transformer. And some hand tools. And free access to a very well equipped electronics/soldering lab. And a couple of coworkers with a vast knowledge in electric engineering. And some involuntary spare time in my homeoffice due to a worldwide pandemic* lockdown. Very basic stuff, all in all... But I am quite proud with the outcome, and I hope you enjoyed the read.

I guess I see you guys in the lever and/or grinder section of the HB forum anytime soon, to get the best out of my Ditting K640 and my Olympia Club.

Stay healthy, stay caffeinated.

(*Corona 2020)
Lean Mean Caffeine Machine


#2: Post by johnX »

And it is another great repair post.
Thank you for taking the time to document and post.
Best Regards,