Electronic – the red/orange coating underneath a CRT anode suction cup, and is it important

The power transformer in the power supply is producing stray flux lines, which are causing the scope electron beam to deflect.

The magnetic flux in a transformer is never entirely constrained to the transformer core, and as transformers get bigger, it becomes progressively more involved to minimize the stray flux as much as possible. Also, the standard EI core used in power transformers is not the best type when it comes to low stray flux (that would probably be a pot-core).

Basically, the transformer is acting like an electromagnet, modulated at the frequency of the mains.

The effect is akin to waving a magnet around your TV, since the monitor in your DSO is effectively a little TV.

Even scopes which use electrostatic deflection to steer the electron beam are magnetic fields. since it's a beam of electrons in either case, and electrons are charged particles, rendering them influenced by magnetic fields.

I have the same problem with a bit 100 V 10A power supply I have a work. Whenever I turn it on, it drives my (analog) scope nuts.

The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.

So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.

Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.

If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).

But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.

To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.

(still WIP)