Tommee Tippee Perfect Prep Machine Repair

This machine decided to give up on a young mother, after she had used this making bottles for her previous child. These prep machines cost around £100 new and the machine just cut out on her when she went to use it at 3am. What a nightmare when you are woken by your child and the prep machine gives up the ghost!
The perfect prep machine is designed to dispense water at the perfect temperature, to make baby bottles. This means baby will not get scalded with hot water and makes the production of bottles 'easy and convenient'.
Despite all the spiel it turns out that Tommee Tippee don't like to make their stuff user serviceable. I opened up the back and came across tamper proof screws. Not an issue for myself as I have the right tools, but I dislike any company who tries to keep people from servicing equipment they have paid good money for.
I find this kind of thing very cynical and it is nothing more than an exercise in futility when you have someone determined to take it apart anyway! On first sight, there was a maze of pipes and concealed circuit board. First test was to make sure power was getting to the board.

Nothing.
The customer had replaced the fuse in the plug so I double checked to confirm this was the case and it tested ok. I looked at the incoming mains connections and saw some heat proof tubing. I pulled that apart to check the connections and I discovered in-line spade socket connections.
Not ideal, however, the connections were fine. Since there was no continuity between the live and the board terminal, it appeared that there must be a break in the cable.

Sure enough, I undid the connections around the element to access the cable and I found an in-line thermal fuse. I tested it on the multi-meter and found that it was open line, indicating that it had blown.

Unfortunately, the kind of low quality thermal fuses manufactured in China are ubiquitous in most equipment. They are of very low quality and often fail before they are ever pushed near their limits due to fatigue. Luckily they are easy to replace.
In order to replace the fuse, it could not be soldered in-line, since the heat from the element could potentially melt the solder enough to break the bond, causing an open line in the device, rendering it inoperable. Whilst the amount of heat from this particular element is unlikely to do this, it is best practice to use mechanical fixings near a heat source.

I crimped both ends of the fuse to the cable, and used heat shrink tubing to cover the connection. I tested the continuity and the reading was OK. I performed a function test with mains power and the machine still did not power on. I tested the continuity of the Neutral wire and I found an open circuit between the plug and the circuit board, which indicated another fuse.
The fuse on the neutral (157 C) was a much lower thermal rating than on the live (230 C), which indicated that the temperature may have surged when the machine was first used after being in storage.

The more likely explanation is that the thermal fuses had both fatigued and gave up the ghost once the element began heating. The machine heats the water to 75 C for a sterilisation shot of water, and then the thermostat dispenses it at a safe temperature for baby at 35 C. This rapid heating, after being left in a cupboard for a few years, was probably adequate to blow both the fuses simultaneously.
The thermal fuses on the live and neutral are known as 'one shots'. That is to say that once a certain temperature is reached, they will break in order to stop any current flowing. This is an important safety device and is often found on anything with a heating element.

Unfortunately, the kind of low quality thermal fuses manufactured in China are ubiquitous in most equipment. They are of very low quality and often fail before they are ever pushed near their limits due to fatigue.

If there was an issue with the element overheating, in a runaway fashion, then the lower rated fuse on the neutral would have fried first, leaving the integrity of the fuse on the live intact. The fact that both fuses fried at once tells us that they were likely fatigued. The chances of the element reaching temperatures above 230 C when the machine only heats water to a maximum of 75 C are slim, and it would have also have to mean the one shot on the neutral failed to go open circuit above its 157 C rating, which again is also slim. This tells us that it was either a catastrophic short circuit which caused a massive surge in temperature, or fatigue that caused the issue.

It became apparent the machine could have a potentially dangerous issue with a shorted heating element at this stage, and I had to test the resistance of the element which read a healthy 60 ohms. I then bypassed the fuse to run a 500V insulation resistance check, and found that the heater was not shorted to ground, and this element was good.
As the heater tested perfectly well we can start to eliminate the catastrophic failure theory from our line of enquiry and conclude that it was fatigue that had caused the different temperature rated fuses to blow out simultaneously. Even if it was a short circuit, the issue would have reoccurred immediately after the repair and blown out the current protecting fuse in the plug anyway. The customer had the old fuse in the plug and it had continuity, suggesting overcurrent was not the problem, so that was all the evidence pointing towards fatigued 'one shot' fuses.

Reassembling the machine was a nightmare, thanks to the clip around the heating element. You had to pull a piece of metal around the tube with pliers in one hand and then try to line up a screw with the other behind the tube! This took 20 minutes of frustration to get it connected correctly but it locked the cable in, and the fuses against the element.
After reassembling the unit, I PAT tested it and found no faults, then I filled it with water and I could not get it to work. A trip to the tommee tippee website yielded the manual and it was simply down to operator error. I finally pressed the right sequence of buttons and it came to life and it function tested OK.

Using the laser thermometer I recorded the correct operating temperatures and it was ready to return to operational service. Mother and father were very happy they did not have to buy a new one as funds were tight with a newborn. It is amazing to think that many people would have just thrown this machine out and bought another one, such is the throw away society we live in.
It is worth repairing our technology, it is a shame that manufacturers try to make their stuff so difficult to service. Tommee Tippee get a thumbs down here for poor serviceability and inferior components, but I was glad to bring this device back to life again, that was destined for the scrap heap.