I was recently gifted a problem MES 30 with rear controls & window, model #20070411. It has the random shutdown, shutoff problem
which was never able to be fixed even after multiple control unit changes. Wanting to get to the bottom of the problem, I took it completely
apart and reverse engineered the electronics. I discovered that the internal power supply is being overloaded by design. This and poor
power distribution push it over the edge.
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I worked up a fix that seems to work, add a large capacitor to the control unit. The capacitor holds the voltage up enough during
brown outs to avoid reset.
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[edit]see my later post below before you do anything
I realize this is an old thread, but I just wanted to thank James. I could have posted exactly the same thing he did (including the fact that this is a gifted, used unit). I searched the web and found that this model has dozens (perhaps hundreds) of identical complaints, including several YouTube videos that demonstrate exactly what James described.
That is not my video, although it is the identical MES and the problem is precisely the same thing I am experiencing.
I too am an EE and after taking apart the controller on top of the smoker, it was clear that it was fine and very unlikely to be the culprit. In addition, every single description of this problem has the same key piece of information:
sometimes it works. This puzzled me at first: why would it work sometimes and not others? Why would it sometimes fail, without even turning on the heating element? For instance, I could sometimes get it to fail simply by turning on the light!
I didn't want to open up the power supply at the bottom of the smoker, since it is riveted shut, but found a good photo on this site. It looked pretty basic. I at first thought perhaps there was some sort of ground-sensing circuitry (since there have been so many ground issues with MES units), but that didn't seem likely, and the failure mode didn't seem to correlate with anything having to do with ground.
I then hooked up a volt meter to the power leads on the controller board and got only 4.8 volts (5.0 volts is a common standard). As I did various things, I saw the voltage go up and down a little. I probably should have put my scope on it to see how much AC ripple was present.
Bottom line: I was starting to come to the same conclusion, namely that the power supply was pretty small and very remote and that the combination of these two things meant that whenever something happened to cause a voltage drop (pretty much any button press), the local voltage on the board dipped enough to shut off power from the digital chips, causing them to reset.
Thus, a large capacitor might solve the problem by providing a larger amount of reserve energy, for a few milliseconds, whenever a pulse of current is needed.
Thanks to James, I have confidence that this might actually do the trick. I'll write back tomorrow and let you know if this fixed the problem.
[edit] The fix appears to be working. I used a 470 uF 25V cap. I desoldered and then took out the old cap because the extra 100uF isn't really needed. I then used its mounting holes to install the new capacitor. This avoided having to add wires on top of other components. I placed the cap over to the right of the circuit board and hot-glued it to the plastic chassis. I'm a little concerned about the hot glue melting, so I'm going to add a little epoxy or RTV (depending on what I have on hand).
[edit2]Here's a photo of the installed fix. I used a little epoxy to supplement the hot glue. I covered both the leads and the cap itself in heat shrink tubing, just in case it were to break free (unlikely, especially since it is trapped above by the bottom cover, once it is put back on). You'll see two wires coming from each lead of the cap because the only wire I had on hand was 30 gauge, which is a little small to handle the current. So, I used two wires instead of one.
