How do I take proper voltage measurements on device with multiple voltage sources sharing the same ground?
Let me first say that, while high-voltage electronics may seem interesting and fascinating, it is also very dangerous. A CRT supply can deliver 20kV at up to 20mA, which is twice as much needed to kill a person in a painful, long, and bad way. I strongly suggest that anyone interested in electronics forego experimenting with, measuring, or attempting repair or modification of any kind of high-voltage equipment.  There is still tons to learn without putting yourself in danger because of a fascination.


  I need to know how to take measurements on a complex system like this, not just electrical theory. The whole question I just posted was about testing method, not wanting to know theory.


Jared, you've got this backwards. You need theory to understand how to test.

Now "ground" is just a reference point.

Consider the following circuit. Long story short, a transformer is physically coils of wire sharing magnetic flux. The coils themselves are isolated, unless we connect them together. A transformer works on the principle of changing magnetic flux inducing a voltage in wire. Since they only work on changing magnetic flux, AC must be used, not DC. Put voltage and current change in one coil, and this will appear on all other coils, according to the turns ratio. If the primary (input, driven side) has 10 turns, and the secondary 10 turns, then 1v in will produce just slightly less than 1v out due to losses. If the secondary had instead 10,000 turns, then slightly less than 10kV would be output. Notice where the grounds are placed.







simulate this circuit &ndash; Schematic created using CircuitLab

If you take more than two coils (depicted as four due to limitations of the schematic editor) you should be able to see how it is possible for "ground" to be misleading - the 6.3v output only measures as 6.3v if referenced from the 10kV output.

Note that if using an oscilloscope to measure CRT's:


The probe's ground clip or pin is physically connected to the knurled grip of the connector and chassis of the 'scope. Many people have been shocked (or worse) forgetting this.
The ground clip is also connected to the ground pin of the scope's power plug. If plugged into a grounded outlet, then touching the ground clip to certain points in a circuit will short them to Earth ground, likely causing damage to the circuit, the probe, 'scope, or all the above.
To get around this, often a scope will be powered from an isolation transformer and the Earth ground disconnected. This allows the technician to ground the scope to *anything, without fear of damage. However, now they must remember that the scope body can be "live" and potentially lethal. Note that, an isolation transformer is usually rated for a maximum isolation of about *5kV. Some may be less than this. So if measuring a 20kV node, this can grossly exceed the isolation rating of the transformer and scope. Hence, not a good idea to measure CRT's, even with an isolation transformer. 


The only way to measure a 10kV node is with a high-voltage probe. These have special high-voltage-rated resistors inside to safely drop that voltage down to a sane level and disallow arcing-over inside. Still, the "ground" of the scope cannot be placed at that level. So in the case of the above schematic, subtract the node differences (10,006.3 - 10,000) = 6.3v. Most scopes have a math function built-in; just probe both nodes at the same time (both with HV probes), do a subtraction on them from the scope, and read out the difference.