One day, my son turned on the electric oven to bake something for lunch. I only found out when I got home that the oven had stopped working. It seemed that he turned on the oven and after a short time, there was a sound, and it went dark, with only the clock showing. This oven is a Kleenmaid TO500X which was quite expensive when we bought it back in 2007, and a few years later, Kleenmaid went out of business only to be resurrected after that.
With the digital clock working, it meant that it was getting power, but none of the other controls, such as oven light, fan, grill – even the thermostat light was dark. When I got around to it, I turned off the oven power at the switchboard, and pulled out the oven – removed a few metal panels so that I could inspect the inside of it.
Typically what usually happens is that the thermostat fails, so I had been checking on prices of thermostats. Anyway, with the covers off, I could check that the thermostat was – surprisingly ok, by turning the knob to any temperature, the contacts show a connection – which it should if it is working.
I decided to check the heating elements anyway, and each element had a measurable resistance meaning it should be functional. I couldn’t see anything else that might be wrong, so closed it up and went to do more research. After some further time, as in days, I came back to the oven, to check if the thermal overload had triggered. I found the device screwed onto the rear fan mount, but it showed continuity – and anyway, it would only be a problem if the thermostat had failed in the on position and caused overheating – which it didn’t have time to do.
Back to the drawing board – so anyway, I woke up one morning and realised something that had been staring me in the face – this multi function over has a timer switch that can cut the power after a set time – I use it all the time when cooking frozen pies, so that I don’t overcook them, as in – burn them to a crisp. Sure enough, after opening the oven again, the clock timer module has a board on the back with a relay, where the relay contacts control power to the thermostat – now we are getting somewhere.
Removing the clock timer module is complicated, by first removing the thermostat and the control switch – but the hard part was removing the front knobs which I worked out, just need some brute force. After that the assembly could be removed, then the clock timer module removed from the metal frame.
Removing the circuit boards from the module was also a bit of a job, would be handy to a lot more hands, but eventually it came out. I checked the components and worked out that the relay was driven by a signal going to a PNP transistor, and eventually after applying some power (albeit carefully) confirmed to my satisfaction that there was no power going to the relay. I had earlier confirmed that putting 12V onto the relay allowed it to switch and I confirmed that the contacts were closing correctly, hence the relay is good – therefore it was not getting a signal to turn on.
Debugging it further would require removing the display module in order to work out what was wrong with the timer. The display has about 20 pins, being a vacuum fluorescent display – which is not an easy job, as I found that my desoldering station wasn’t heating correctly. So, to fix this, I decided to just bypass the relay – effectively by connecting the contacts to make it think that the relay was on. The relay contacts are Faston connectors and I remembered having a piggyback adapter in my stock of parts, so after checking a few boxes, found my little adapter. It plugs into one terminal and allows two cables to plug in – the ones that originally went to each relay contact, now go to this adapter.
After doing this, I started reassembling the control panel, putting wires back on – in the correct place which is why I usually take photographs of anything that has lots of wires. To my piggyback adapter, the red and orange wires are connected – which originally was to the relay contacts.
Then the final test, was to turn the oven power back on, and voila – the oven now works. I checked that the internal light came on, that I could choose heating modes and more importantly, if I set a temperature, the thermostat light shows that it is heating, so all good, except that we have lost the timer function – not a big problem. A replacement timer module would cost almost $500 which is already close to the cost of a new electric oven. The moral of the story, is that sometimes a repair only has to make the device work again, and if we accept that some functionality is lost, then that is ok.
Of course, I could have spent more time to actually determine the cause of the failure – but we needed a working oven, and adding this $2 part made it work.