Bosch Gourmet 8400 oven mode selector switch repair (Dreefs 4-DPS/260) (#P31)

A long, tedious blog post for today. This has been in the making for about 9 weeks and I’ve been looking for parts for another year or two. Pictures have been taken on multiple occasions in multiple locations, so it’s a hotchpotch that somewhat reflects the process. This is the use case for the 3D model generation of the WHL #61 post.

Our Bosch Gourmet 8400 microwave and baking oven has been there for as long as I can remember. It’s a huge and darn heavy thing for a microwave oven (27l volume!), and rather small for a baking oven with grill (broiler) functionality. When I partially moved out to my student dorm, I took it with me, as my mom still had a regular microwave oven and a regular-sized oven appliance in the kitchen. I got to keep it after that, and the somewhat non-sticking 4-way switch controls for the oven always bugged me a little. Of course there’s no such thing as replacement parts for those after 30 years (let alone their financial feasibility), so I was looking for scrap units for harvesting. Recently, I found one on eBay small ads (basically craigslist). Which turned out to be two very similar units. With the same defect on the switch. And after taking them home, mine finally failed as well. Great.

Before I get to the unit, I need to add that on my way to the pickup location, I partially avoided a major traffic jam on the three-lane A8 autobahn,


and while doing so, THOSE folks (note the number plate holder) were directly in front of me on some rural road with a lesser traffic jam. While they didn’t notice at first, once the front seat passenger turned around to grab something, they totally lost it. :cool:

Right. Now, those ovens have a control panel on the entire right side of the case (first “sorry for the photos”). This actually is not the Bosch unit, but the equivalent “Privileg Duo 6027e”. I would also suspect there is another unit with Siemens branding, since BSH (Bosch Siemens Hausgeräte, “Bosch Siemens Appliances”) would certainly offer a Siemens unit for those ignorant folks that pay top dollar for Siemens instead of Bosch or even Privileg, a brand name of probably the German mail order business Quelle.

Side note: Quelle didn’t quite understand the importance of the interwebs and therefore didn’t make it into the modern age – that’s a surprise for when you dominate the early mail and telephone order market and even jumped onboard the early online system BTX (Bildschirmtext) in the 1990s. I fucking ordered stuff myself “online” when I was five, sitting on my dad’s lap in front of the 486 with a 1200 baud modem whistling along. Major business fuckup!

That Privileg unit is different to the Bosch units in terms of coloring and labelling of the front, as well as the bottom oven indicator. The Privileg uses print on the front and a status light, whereas the Bosch has a more fancy rotary dial inside that is illuminated from behind. Same switch gear and functionality behind, just more expensive to make.

The four buttons above are the culprit. The leftmost button will wear out in that it does no longer stick in pressed position, which also disables the top and bottom heat mode in case you do not want to be standing next to the unit and holding it yourself all the time. It amazes me that the tiny part that does this has survived 30 years of use…

The entire control panel can be swung out by removing three screws, two on the bottom of the plastic air vents on top of the unit, and a metric screw that holds the top left bracket of the panel in place. Move it to the right a little bit to wiggle out the top catch, and it swings out. It might be necessary to remove two of the case screws on the right, but it can work without.

The quad switch unit is just held in place by two screws, but has a lot of spade connectors plugged in. Those are shown later.

Back of the original switch unit with the 100kΩ and 0Ω jumper links still in place:

Top of the switch after repair: (I did not have a 100kΩ with higher power rating, so I used 180kΩ and 220kΩ 1/2W each in parallel, plus a regular wire on the other side – solder these in to the PCB side before assembly of the switch components!)

Bottom of the switch, with the red cover in front of the neon lamps removed. Note that the lamp wiring is brittle and it might just snap directly at the glass interface…

Disassembly is straight forward – cut or desolder both links on the back side and use a pry tool to remove the PCB from the case clips. Remove the top spring if it’s still on place (about to ping out in the photo), as four more will jump out in a second when fully removing the PCB.

The order of the switch buttons is important, from left to right when viewing the spade connector side: No contacts and two humps on the top, four contacts, two contacts, no contacts and one hump.

After careful removal of the four neons from the case, the entire PCB can be removed. Carefully slide all neons out, remove switches afterwards.

The culprit is now accessible – its the black, sometimes dusty, sliding part of the switch that releases the pressed pin when another is pushed in. They always break on that edge, losing the ability to hold the leftmost switch in place. Remove that thing and keep an eye on the spring.

Also be careful with the small plastic sheets that might break as well – I did not have to print one because I had working spares from my third unit, but I guess those are perfectly fine to recreate and print, contrary to the main switching mechanism. They need to be aligned like this, with the print visible on top.

Here’s my replacement unit for the oven that remained at work, printed in Nylon PA12 – more on that in the next blog post. I had to file it down on several edges to make it fit in that groove, to make the spring fit on the inside, and to make it run smoothly when actuated. YMMV greatly depending on materials and printer technology.

Last check before putting everything back together:

I found it easiest to put the springs onto the thorn of the switch gear first, then align with the opposite side of the case, mush together and hope for the best. They need to be checked afterwards, as even functional assemblies can develop a wrong spring alignment before the PCB is put back into place. Do this three more times and try not to swear…

Complete process takes about 30 minutes (when doing this for the third time…), with about half of the time being spent for those fucking four springs. I’d LOVE to see how that was done in the factory, there has to be either a plain, simple and foolproof way, or a very sophisticated machine. I’d quit doing this manually, regardless of pay.

Reconnecting is straight forward. In case you did not take a photo of the original configuration, here’s how it goes:

Top row (closer to VFD when mounted): 2 blanks, White-violet, empty slot, empty slot, yellow, black, blue
Bottom row: brown-white, blank, empty slot, empty slot, blank, white, black, orange

The two black cables can be interchanged, as these just lead to the status indicator below, so it’s just another neon tube without polarity.

Bonus photo from the main capacitor wiring – these explicitly non-PCB filled (thank god!) barrels with the HV diode attached are still perfectly in spec (2100V 1,07µF 3%!) after all those years.

Put everything back together, ran the DGUV V3 tests, and the result was already shown: Left button sticks, other modes enable as well, microwave worked from the get-go. I did this for the unit that will stay at the office, and the very same for my Bosch unit home. The third one was scrapped for parts, e.g. those button caps, the neon indicator with the two black cables, also some screws, the main silicone window seal, the thin sheets of the switch and a few other bits and bobs. We also did a fun experiment with the 30Ω power resistor of unknown purpose on the top of the oven…(spoiler: It does not die at 1000% rated load, but the apprentice was mightily impressed with the glow :mrgreen: )

Second bonus item – the original manual. As I did not spot any copyright “do not fucking copy and release this” note and this is 30 years old already, I’m offering this as a direct download. @Bosch, if you’re not content with this, please drop me a note before handing this over to one of your many lawyers. Also, this might already qualify as museum-grade material, since it’s written for 90s people that are not used to operating microwave ovens – e.g. “do not cook a raw egg in there”. They even tested meal quality according to DIN 44566 – it’s a German product, of course they did…
DIN 44566 is still with us, as it has been transferred into DIN EN 60705, “Verfahren zur Messung der Gebrauchstauglichkeit von Mikrowellengeräten für den Hausgebrauch und ähnliche Zwecke” (Methods for measuring performance of household microwave ovens). Germans… :whistle:

Plus, and I never knew this, it shows how to disable the VFD (which is still really bright at night, so I always unplug the unit) and that there is a timer functionality for the oven, not just the microwave. Some earlier eBay seller also wanted their device gone as it did not work, which is generally the case if the clock is not set. I told her, and she never even said thank you…

Bosch HMG 8400 8420 manual

Well, made two working units out of three defective ones, learned a lot about commercial 3D printing, and now we got a huge ass oven at the office. Worth it!


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