Background
A friends microwave suddenly stopped working. It was a 'combi' type oven having both a grill and a microwave function. The unit was purchased in late 2009 so was only just over 5 years old. I thought it would be worth having a look at it to see if it could be relatively easily repaired. A comparable replacement unit would cost around £70-£80 so if the problem component or components could be found spending £20-£30 on replacements would be well worth while compared to having to arrange safe disposal of the old one and purchasing a new one and all the aggravation of having to learn all those new functions and controls.
I tested the oven and found that the microwave function did not work but the grill function did. The interior light, fan and turntable motor all still worked. Without a circuit diagram this still made a repair possible by tracing the wiring inside the case and identifying the major components and sub assemblies.
The cover was removed to get access to the components and wiring.
This picture shows the main control boards with the relays which control the power to the complete system, the grill and the microwave generator.
There was a bit of good fortune when i took the case off. There was a schematic diagram on the inside from which it was easy to identify all the major components and the wiring between them. I suspect there is a good chance that this circuit diagram would be pretty 'generic' to this grill/microwave type of device so could be useful for other makes and models.
From this it was easy to see that the most likely components that could be the source of the problem were the 'Power Relay', the 'High Voltage Transformer', 'High Voltage Capacitor', the 'Magnetron' and the 'High Voltage Fuse'.
I disconnected the connectors from the magnetron and tested the unit setting the timer to 10seconds. I could hear the relay clicking which was an indication something was happening, the contacts may not be closing properly but testing with a multi-meter confirmed the relay was working normally.
This picture shows the Magnetron and the High Voltage Transformer below it.
The two white connectors were disconnected during testing while the system cover was removed to avoid cooking myself or anyone else nearby!
This picture shows the output of the High Voltage Transformer on the left feeding the High Voltage Capacitor on the right via the High Voltage Fuse(The long white capsule in the middle which is partially obscured by the red cables.).
There was no mention on the outside of the case that there was an internal fuse but then this is not a user accessible or replaceable part and to be fair to the manufacturer there is a label on the outside making it clear users should not remove the case.
The obvious thing to do here was to check the fuse before going any further. I removed the fuse case from it's holder and checked the fuse. It was blown. That would explain why only the microwave section was not working but the light and grill were. But why did the fuse blow? I searched the internet and found that it is not uncommon for these fuses to fail due to age and to a considerable extent due to the environment in which they are used, i.e. hot, steamy, oily kitchen! I easily found replacements on EBay and ordered a replacement fuse plus a couple of spares in case there was an underlying problem with some other component. While checking out the fuses i also checked what other components were available and to my relief the high voltage transformer and magnetron were available from many sources for approximately £20 each if they were required. I was hopeful that the fuse was the only problem as examining the components and wiring revealed no signs of damage or any smell of burnt components or wiring.
The fuses were delivered within 48hours. The fuses were high voltage types rated at 5000Volts, 0.7Amps.
The cost for the three replacements was £3.
This picture shows a new replacement fuse(top) with a blown fuse(bottom).
The replacement fuse was installed and the cover loosely put in place. The microwave was run for 10seconds as a minimum test to see if the fuse would blow. The fuse survived intact. I put a cup of cold water in the microwave and ran it for 2 minutes on full power and the water was successfully heated. The cover and internal surfaces of the microwave were cleaned to removed dirt and dust and then the cover was secured in place. The microwave was then tested several times using several cups of water.....they were not wasted, they were put into the kettle to make a cup of tea and to wash some dishes!....reduce,reuse,recycle...
Conclusion
In short a £70-£80 microwave has been saved from the recyclers yard all for the the cost of £1 for a fuse and i now have a couple of spares so if anyone out there has a similar problem please make contact and i will post a fuse on to you and you can try to save another piece of perfectly usable equipment from the recycling yard or worse still landfill! In all, the repairs took less than 2 hours and that includes placing the order on line for the fuses.
p.s. Secretly i wanted the microwave to be unrepairable as the control panel with it's timer control and multi-output relay control would make a great basis for a simple countdown timer to control devices in the garden, e.g. water controller, cooling fan etc. The control panel could easily be converted to operation from a 12V DC supply rather than from the current 230V Ac supply.
In short a £70-£80 microwave has been saved from the recyclers yard all for the the cost of £1 for a fuse and i now have a couple of spares so if anyone out there has a similar problem please make contact and i will post a fuse on to you and you can try to save another piece of perfectly usable equipment from the recycling yard or worse still landfill! In all, the repairs took less than 2 hours and that includes placing the order on line for the fuses.
p.s. Secretly i wanted the microwave to be unrepairable as the control panel with it's timer control and multi-output relay control would make a great basis for a simple countdown timer to control devices in the garden, e.g. water controller, cooling fan etc. The control panel could easily be converted to operation from a 12V DC supply rather than from the current 230V Ac supply.