Repairing a faulty Inventum NBM-20D Automatic Breadmaker.
The fault Machine will not start. It indicates it is not yet cooled down sufficiently, although in reality it is cold enough. On the internet I cannot find an electrical diagram or any technical information about this machine. Also my local parts shop replied that he cannot get any parts from Inventum at all..
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Safety Warning: Always check that electrical items are fully disconnected from the mains supply before opening enclosures, and be aware that some items retain charge even when physically disconnected from the mains supply. Always read and comply with safety warnings on the product/appliance and within the manufacturer's documentation; on repair substances, replacement parts, materials and tools used. Always take appropriate precautions to ensure your safety and that of those around you when undertaking fault diagnosis, repair and subsequent testing of any product.
Difficulty level & repair time
|Repair Cafe Delft - Nederland - Date 07:2016|
Tools & parts used
- Various screwdrivers & pliers
- Soldering iron
- Shrink tubes
Inspection After opening, no obvious damaged parts were found. There seem to be two components connected to the hot part to control temperature. One is a Thermostat, probably an overall heat protector, and one seems to be a Thermistor (NTC). At room temperature the value of this resistor is about 4kΩ. Replacing it by a wire or leaving it open results in loud beeps and warnings that something is wrong, so the electronics and processor are probably still ok. Replacing the NTC with a variable resistor results in a correctly starting bread machine at a value of 25kΩ..
My local electronic parts shop only has NTCs of 15kΩ or 100kΩ available, so I buy two 15kΩ NTCs and connect them in series. Starting the machine empty, with a thermometer stuck under the lid shows a baking temperature of over 240°C, at which temperature I stopped the process. Repeating this measurement on a breadmachine I own myself showed a baking temperature of about 220°C. Apparently my 30kΩ NTC has a value that is higher than required, so I solder a 220kΩ resistor parallel. Now the maximum temperature reached is more like my reference machine. The day after returning the machine to the owners they surprised me by bringing a perfect and still warm bread. Nice!.
What if ...
The NTCs I used hopefully are good for these temperatures, my supplier had no additional information. On the internet one can find different NTCs with a guaranteed temperature range of over 250°C, and maybe even the correct resistance value. If the owners ever come back because my repair fails, I will try and find a better NTC. In my solution the final baking temperature can be slightly adjusted by connecting different values of parallel resistor. Some experimentation was required.
Replacing the original NTC with two NTC's with a totally different shape requieres some ingenuity and knowledge of how to do this mechanical end electrically save, keeping in mind that it must withstand temperatures of up to at least 220°C. Do not attempt this if you have not done something like this before or if you are not familiar with electrical insulation at high temperatures.
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