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Unclejed, I have never ever heard of this being done, it sounds very workable in theory though. Are you able to get meaningful readings, checking the drive to a chopper transistor for example with all the noise and hash flying around. You are correct in saying a 1000v input rating would be required, most scopes seem to be in the 400 to 600 vdc range, even most divider probes are only 400vdc or so rated. It is non the less a very interesting proposal with the right kit.
Very interesting indeed, Karl
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oscilloscope in audio tutorial
. You can't beat experience. For audio use a 20Mhz or better dual trace scope is fine. Use with proper probes e.g. switchable 1/10 1/1 . It's the details when measuring to watch, the scope measures peak voltages not R.M.S. values e.g. 680volts peak/peak is mains voltage. DONT TRY AND MEASURE THIS. Please remember that the scope is earthed (or should be) and this means that the earth wire of the probe is also earthed. If you are working on say an amplifier that is also earthed, I am sure you will see what would happen if you connected the probe ground lead to anything other than earth in the amplifier. If there is anything specific you want to know please do ask.
. The primary side of the PSU is out of bounds, to work on this you need a mains isolation transformer and all earths disconnected, with NO earthed equipment within reach. All test equipment, soldering irons etc run of this "floating supply". This is normal practice in repair workshops, not very practical at home. So you are limited to working on the output or secondary side. To measure the ripple on a S.M.P.S. you connect the earth lead of scope to the common earth point on the secondary side. There may be several different rails to measure but they will, 99% of time have a common earth or ground. Then just measure ripple with scope on AC coupling so you can see ripple superimposed on DC voltage. Again experience is the key. An unstabilised supply might have a 1 volt or so pk/pk . The noise or ripple output of a regulator eg 7805 should be as low as say 5 millivolts. The correct point on the common ground makes a huge difference to result. Our 7805 reg would require you to connect directly to centre pin and output pins in order to get accurate result, move the ground lead a few centimeters along the PCB track and the reading could rise a hundredfold due to the resistance of the track. If you have a specific circuit you want to work on post the diagram and will tell you where and what to measure and what to expect. Remember to increase the timebase speed on scope as ripple on SMPS is high frequency, not 50hz. Have fun and stay safe. 
