Testing Bad Filter Cap with a scope for 120HZ hum

[wsscott]

I think that 60hz fuzzy signal is environmental, since it shows up on the scope when the scope isn't connected to the Output jack or even if it is connected to the Output level jack and the amp is unplugged.

So that's probably a red herring!

[wsscott]

I should add that the guitar cable I'm using has the shield connected at both of its plugs.

In this case, since the Output jack is not presently connected to a ground, I assume that both ends of the guitar cable need to have their shields connected to the plugs or there couldn't be anyway for the signal to return, and thus no sound out of the amp?

[shooter]

I think we're getting close.

optimism is a good quality to have 

[wsscott]

I unsoldered the cable shield from the plug leaving the Output jack going to the amp, and it didn't make any difference.  There was still the lower level hum, and it still carried the guitar signal to the amp.  I then swapped the cable end for end and that had no effect.

Would there be any risk of a shock hazard if I disconnected the shield on both ends of the cable that connects the Output Jack of the ReVibe (which no longer has its ground lug connected to any ground) with the Input Jack of the Amp?

[shooter]

if I disconnected the shield on both ends

that could cause the same problem you're now chasing, shielded cables are typically grounded at ONE end only, typically the "source end"


think of that shield as a giant antenna collecting all the bad things around it, if it's not grounded at either end, all that "junk" builds up, then inductively couples the junk onto the signal (crazy math not included for simplicity:). grounding one end only, all that junk has a "direct pathway" to ground, stopping the junk from coupling into a signal source.  grounding BOTH ends causes what I had to check by moving the scope ground to various points looking for a "ground loop" condition

[wsscott]

I thought I would upload an image of the Output signal from the ReVibe.  I injected a 1khz/150mVRMS signal into the input, and the image is the output of that signal with the Output Level being at "parity" (I know that's not the correct term, but I can't remember what the proper term is) with the Input signal.

It's pretty smooth, but you can see some "blurring" on the edges of the sine wave.  I'm assuming that's the 120hz hum that I've been dealing with.

Is there some way that I can use the scope to identify the reason for the blurring, ie. if it is the hum how can I get the scope to "dissect" the signal into its components?

[shooter]

adjust the Fq per div so that the sinewave is "stretched" out till you start to see something coherent