Help me understand an oscilloscope display

I'm not entirely sure this is the right forum, sorry if it's not.

I've been working on some circuit experiments, seeing exactly what I expected on the o-scope out of my breadboarded oscillator, fuzz & filter circuits. Today I hooked the output of my keyboard (Dave Smith Instruments' PEK) to the o-scope to get an idea of what voltage swing I should expect from a synth output.

What I saw was a funky waveform of 60Hz (so clearly related to power supply, right?) that was about 160Vpp. This was with no detectable output level, metered or audible, when connected to a mixer. When I brought the output level of the synth up to some nonzero level, nothing on the o-scope changed. Nor did anything change when I changed the oscillator shape. This remained when I turned the synth off, and only changed when I unplugged the cord from the output or unplugged the synth's (external) power supply.

Obviously I'm missing something I should know. Can anyone enlighten me?
 

infinia

Member
2005-05-15 9:51 am
SoCal
What I saw was a funky waveform of 60Hz (so clearly related to power supply, right?) that was about 160Vpp. Obviously I'm missing something I should know. Can anyone enlighten me?

yikes that's a lot of voltage.
60Hz usually indicates a measurement system ground loop. PS stuff is usually fullwave so 120 Hz is the norm. When looking at power supply stuff change the scope trig. source to <Line>.

Always use a earth / safety 3 prong plug at the scope, never 'float it' or defeat the 3rd wire. When you connect the probe gnd., that DUT point then becomes the only gnd connection.
 
yikes that's a lot of voltage.

Exactly what I thought! But that's what it said.

60Hz usually indicates a measurement system ground loop. PS stuff is usually fullwave so 120 Hz is the norm. When looking at power supply stuff change the scope trig. source to <Line>.

Always use a earth / safety 3 prong plug at the scope, never 'float it' or defeat the 3rd wire. When you connect the probe gnd., that DUT point then becomes the only gnd connection.

I triggered off the channel that was connected. I'll try Line next time. I did, however, try both the tip and sleeve connections of the TS connector, and both were the same.

The scope is grounded, through a grounded power strip. As far as the probe ground, perhaps I'm missing that. The only time I've connected a ground from the probe is when doing the scope's self-calibration, as described in the manual. Should I be connecting the probe ground to the ground connection of the synth output?
 

infinia

Member
2005-05-15 9:51 am
SoCal
The only time I've connected a ground from the probe is when doing the scope's self-calibration, as described in the manual. Should I be connecting the probe ground to the ground connection of the synth output?
Reverse of that

Yes Every connection needs a return, so always use a gnd from the probe. Except maybe at the scope itself, ie at the cal. port.

Think about this for a second
So without using a probe gnd the scopes return path is only thru a long convoluted path > the pwr line.
 
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