Question about signal tracing with a scope.

[Ethan]

When using my scope to look at a balanced output (pre-amp, compressor, etc.), I attached the ground clip of my probe to pin 3 and the probe to pin 2. What are you "supposed" to do when tracing the signal inside, like at points before/after an amp stage etc? Is the ground probe supposed to be kept on the pin 3 or signal ground? I've been referencing to signal ground, is that ok? Since most audio signal grounds are eventually tied to chassis earth, would it be ok to attach the ground probe to chassis?

Every now and again, I question the way I've been doing things... :roll:

 

[CJ]

Warning:Be careful!

For some reason, and don't ask me why, just about every scope I have used has the grounding alligator clip tied to your eletrical ground in your house. So now you a possibillity of having 120 volts on one side of your chassis under test, and the otherside connected to your scope alligator.

Kabllooie Looouie!

Don't ask me how many times I have seen the blue flames, and I ain't talkin about Jimi's first band.

So, either isolated your scope with an isolation transformer, (why don't they have them already installed inside!!! ) or use a ground lift adaptor.

To be sure the above precaustions have worked, use an ac meter between the scope alligator and the chassis you are scoping out.

One quick way to save you from using an iso xfmr, which you probably do not have, or a ground lift adaptor, which may provide a false sense of security, is to use a 0.047 cap at 470 volts on youtr alligator inbetween the chassis and clip. But wrap some tape or something around the alligator so it does not hit the chassis while you are moving cables around.

Next post please:

 

[Ethan]

Thanks, CJ...
But now I'm more confused...

 

[CJ]

Don't worry, I can hear PRR and Brad and the rest of us who have had shock therapy preparing another gem as we speak.

 

[SSLtech]

In simple terms, the ground clip should go to the signal low.

In a transformer balanced circuit this is pin 3. In an unbalanced circuit, this would be the common.

Common isn't always the same as ground. hence the CJ caution. What CJ might be overlooking is that even with an isolation transformer (all of my scopes have fully isolating transformers) the scope probe common is STILL tied to electrical ground. On amps, this has caused a few thunder clouds to "harsh my mellow" from time to time.

Now I use a ground lift adaptor whenever a quick investigatory probe of the signal common inside the amp (or whatever) reveals an elevated ground. If it's at ground, you can often get away with not using a ground clip (if you're looking for voltages or big signals and not hunting for small noises) and this speeds things up a little too.

As soon as things get floating or transformer isolated however, you have to clip the probe ground to the signal low no matter what.

Keith

 

[Ethan]

In transformer balanced gear the signal "low" usually get's "grounded" to the signal common. So, if I want to look at a signal at any point "between" the input and ouput transformers the ground clip of my probe should go to the signal common right, not pin 3 (and likewise, it would be clipped to pin 3 when looking at the output of pin 2 after the output transformer?)

 

[SSLtech]

Usually yes...

But supposing that the reason for transormers is more than balancing... what if the low leg of the signal -the "common" is not at 0V, but at +100VDC...

...Then if you clip the probe ground on to common, you've got a big flash!

-Not often the case, and yet... tooo often the case! :shock:

Keef

 

[Ethan]

Of course!
Thank you!

[Does the maintainer of the newbie meta feel this thread worthy?]

 

[Butterylicious]

I almost never use those confounded ground clips that hang off of the probes. I tie the 2 chasis together with a lead and run the probes ac coupled (usually). Them damn ground leads are worse than dropping solder in a live circuit.