No doubt disturbing for the user, but pretty much the norm for an EMC engineer during design. In a nutshell, the PDA is emitting signals such that the Oven is receptive to them, and as such is turning the broiler on high.
Whats interesting about this case, is the two devices worked well for 3 years, and short of any recent repair or modifications, something is starting to go bad. It could be a component level problem, or possibly a system problem.
Its difficult enough to build in suppression and immunity during the design stage, its another deal entirely to deal with failures during the latter part of a products life. Being the PDA is 3 years old, the time sounds about right for solder wisker formation, especially so if the user lives in a northern climate. By the same token, 3 years of realistic use can cause any number of solder fractures to occur… Of course, it could also be shifting values, thus detuning filter circuits, especially if manufacturing processes were not controlled.
Otoh, it could also be the start of a failure in the ovens wiring. Certainly corrosion of a ground lead internally could cause this, yet issues with house wiring are not all that common either. Some places still provide for grounding via water pipes, and galvanic corrosion can take a serious toll. Its not the sort of deal which affects safety, well not until its very severe and pipes are nearly leaking, but by all means, it could seriously affect EMC susceptibility.
Sadly, the consumer is pretty much left holding the bag. Short of bringing in an EMC guy and a spectrum analyzer, only obvious things can be checked. Ie, oven connections, ground connections in the house, fuse box, water pipes etc, yet it could take unless one knows what to look for, it could be a massive exercise in frustration.
Had this been an iphone rather than a PDA, then I’d really be wondering. Ie, Iphones, just like all devices are tested to comply with FCC regulations. Ideally such tests are done with a worst case scenario, yet there in lies the problem. Defining a worse case scenario is a pig in a poke.
Do you run the phone in max transmit power? Do you put the code in a loop? If so, what period, what duty cycle, what about ringing, what about charging, what about different states… All best guesses, well other than charging, being most phone manufacturers prohibit the use of their phone from being used, while attached to a wall outlet.
The remaining elephant in the room is the iphone app. Certainly Apple runs through a bunch of tests for compliance, yet they cant expect every possible permutation a developer might come up with. Its certainly possible, sooner or later an app is going to result in an Iphone going far out of compliance. Hopefully should such happen, it ends up being a minor inconvenience, but again, near impossible to predict when, or what would be affected. Short of pacers which must operate with high margins, I see a lot of potential for trouble, even more so as devices age. No matter what, it will be interesting.