Advice needed probing low voltage supply Tektronix 2440

[adrianni]

Hello,

I would like to probe the LV power supply of my 2440 to see if the waves show the same as the service manual says they should.
But the manual also says that some portions of the LVP are at the AC line potential and advise for using an isolation transformer.
My question: Is this advice given only to prevent somebody getting electrocuted by touching accidentally a live point in the circuit and thus providing a short to ground? Or it has to do with touching accidentally a live point with the ground alligator clip of the oscilloscope probe?
I mean, I would clip the ground lead of the probe to the chassis of the scope and then probe with the tip the points specified in the manual. Am I going to be in trouble if I am careful not to probe all over the place and of course not to touch anything (other than the probe itself...)?
I also have a differential probe but I fail to see how this is going to help in this situation.

Thank you,
Adrian

[David Hess]

If the probe ground is connected to the chassis ground, which is the only safe place in this case, then the probe tip can be applied anywhere safely.

The problem comes when trying to measure the voltages on the primary side because common is *not* at chassis ground potential.  So measuring the primary side voltages to the primary side common requires an isolation transformer, floating input, or a differential probe.

[adrianni]

Thank you David.
This is indeed common sense. But I do see the need to reiterate it now and then.
Luckily, I don't need to measure that. But if I did, I would most certainly use an isolation transformer or at least a differential probe or use the two channel math method.

Cheers

[David Hess]

Hopefully you will not have to make any primary side measurements.  The critical capacitors on the input side are the large line frequency ones and they wear out last.  The inverter operates in current instead of voltage mode so the output capacitors for the preregulator are actually on the secondary side and those are the ones which wear out first.

One thing to watch out for when probing on the primary side (or in any circuit) is that the ground ring at the tip of the probe does not short out on something.  For safety, insulate it with some tape when not using the witch's hat grabber.