The 50 ohm thing is OK where the place you're connecting to is intended to be 50 Ohm compatible (Input and output clearly) - but am I to infer that other places in circuits where the impedance isn't 50 Ohms that these points are just mysterious places that no-one ever sees?
Often you dont NEED to look at these points.
Surely if you plonk your 50 Ohm test gear at a point where the impedance are not so low then you are just going to short things out?
True. When I have needed to look at one of these odd points I have used a scope probe set to X10 and connected it to the 50ohm input of the spectrum analyser.
Yes the will be a lot of undefined attenuation, but you can get an idea of what is there. Now think if what you see is reasonable, it it what you expect, if not what could be wrong. All comes down to understanding the circuit.
That leads to another question - Does a scope with a 50 Ohm input impedance present 50 Ohms to the system under test?
Provided that it is connected with 50ohm cable - yes.
If so and you connect to a 200W amplifier with a 50 ohm output - will the scope have to dissipate 200W?
Yes.
No, it would just quietly smoke!
Also, what is an RF probe? Is that an AC coupled rectifier-diode arrangement that only rectifies high frequency AC?
Basically, yes.
If you wanted to examine the output of the 200 watt amplifier using the 'scope or spec ana, you would connect the output of the amplifier to a suitably rated 50ohm load.
The load would correctly terminate to amplifier and dissipate the 200 watts.
To get a low power sample of the amplifier output to feed to the test equipment, you could use a "coupler" of some kind in the line from the amp to the load. I have a home made 20dB coupler which I often use for this job (but not more than 10 or 20 watts).
Or you could use a high power attenuator as the load.
I have a 50watt 20dB attenuator which is useful for this job.
Depending on the power you are trying to measure, you may need a coupler with more attenuation, or a separate attenuator after the coupler.
There are several ways to achieve the result, usually knocked up using whatever kit you have got.
JimB