Test Equipment: Oscillocopes

[Brian516]

There is a simple test:  Hook your laptop to an AC powered stereo.  If you get a bunch of 60 cycle hum then you have a bad ground connection.  If not, then there is something else going on with your oscilloscope set-up.

When I have the laptop hooked up to the AC powered stereo unit (with the unit on only stereo mode, no enhancements such as surround, dolby, etc) It works perfectly.  no hum at all.  It plays music without any hum at any volume, without any input from the laptop with the stereo volume up, and with a 1khz sine wave output.

[picowatt]

It works perfectly when plugged into an AC powered stereo.  no hum at all.

Brian,

When you say "it" works perfectly, do you mean that when the laptop HP jack is connected to the input of a stereo audio amplifier, you can produce, for example, a 1KHz sine wave output from your FG software and HEAR a nice steady 1K tone from both the left and right channel of your stereo's speakers (or headphones)?

PW

[Brian516]

Brian,

When you say "it" works perfectly, do you mean that when the laptop HP jack is connected to a stereo audio amplifier, you can produce, for example, a 1KHz sine wave output from your FG software and HEAR a nice steady 1K tone from both the left and right channel of your stereo's speakers (or headphones)?

PW

1khz sine wave played from PC FG into AC sound system set on stereo-
https://www.youtube.com/watch?v=zfmSXja5Iz0

I also noticed that when I have it on X-Y with no input and on 5mv/div, it's not just a dot in the center.  If I turn the intensity up a little, I can see what looks like half of the infinite symbol emerging from the dot towards the top of the screen, so more or less a teardrop shape with the point being at the dot.

(where's the 'smashing my head into the wall' emoticon thing??      )
Brian

[picowatt]

1khz sine wave played from PC FG into AC sound system set on stereo-
https://www.youtube.com/watch?v=zfmSXja5Iz0

Brian

Brian,

OK, the FG software appears to be working fine. 

From your previous video:

Regarding your "dot" on the far left.  It's probably just the "beam off" blanking level for your trace while waiting for a trigger.  What does it matter if you have to crank the H position way over and crank up the brightness to see it?

I don't see evidence of what I would consider an open ground in the video you posted.

Personally, I think you should for now just focus on getting decent displays using just one channel.  Use only channel one for a while, set the H position so that the trace is centered left right and leave that knob alone for now.  Same with the trigger holdoff.  Set it to norm and leave it there.

Select to display only channel one with trigger source set to channel one.  Once you get good at displaying various waveforms with channel one, then you can move on to dual channel use.  Before doing so, practice getting decent channel one displays so that they are centered in the upper half of the display (using vertical position) with the vertical sensitivity set so that the displayed waveforms do not use any more than the upper half of the display.  This clears the lower half of your display for use with channel two.

I am not sure what your calibrator's frequency is, they are typically 1KHz, but I believe its amplitude is 300mv pk to pk.  Using just one probe and channel one, get good at displaying the calibrator signal.  I would select AC coupling on the channel input and AC coupling on the trigger source (just in case any source has DC offset).

Once you are confident with using channel one to produce stable views of 3 or 4 cycles of the calibrator waveform, set your PC based FG so that it produces a square wave of the same frequency (1K?) and of similar amplitude as your calibrator.  Connect your scope probe's ground lead to the HP jack's sleeve (gnd) connection.  You should then be able to move your probe tip connection between the calibrator and one channel of the laptop's HP output, and without flipping any scope switches, see the same waveform displayed in a stable fashion.  Adjust the PC based frequency and amplitude as necessary so that when it is displayed, it closely matches the calibrator display.  The point of this is, that if you can get a stable display of the calibrator signal, and if the calibrator and PC based FG are producing the same signal, you should be able to see either one without changing any scope settings.

Now, once you can do that without issues, and if you have the calibrator or FG display confined to the upper half of the scope's display, select ALT for a two channel display.  Set the CH2 vertical sensitivity the same as CH1's sensitivity (and with AC input coupling).  Connect a probe similar to CH1's probe (X1 or X10) to the CH2 input and connect its tip to the same point you have the CH1 probe tip connected.  Adjust the CH2 vertical position control so that the CH2 waveform is centered in the lower half of the display.  You should now have two identical looking waveforms displayed, CH1 in the upper half, CH2 in the lower half, and the displayed waveforms should not be overlapping vertically.  You should also be able to move the probe tips between the calibrator and the PC based FG output with the displays remaining stable and looking similarly.

Once you get this far, you can connect CH2's probe to the second channel of the PC based FG output for display of independent FG channels.

Regarding your last video:

Does your stereo audio amp have a preamp out or record output?

You might consider using a pair of phono cords to bring the line/record out from the stereo over to the scope and connect your scope to the phono plugs at the end of the phono cords.  In the absence of phono to BNC adapters, you can use alligator clip leads to go from the tip of the phono plugs to the scope probe spring hooks.  Trying to hook the phono plug tips directly might be a bit rough on your probe hooks (alternately, cut and strip the ends of a spare phono cord).  You only need to hook up one probe ground lead to one phono plug's ground (or use another clip lead between your scope's ground jack and one of the phono plug shells.

But, as above, consider using only one channel for a while until you are good at getting a stable display on just channel one.

PW

[TinselKoala]

Made a vid to show the results of this test. From what I gather from your explanation, for some reason I do not have a ground/common connection from the laptop to scope.  Let me know your thoughts on the matter.   Also, I pointed out that there is a little dot to the left, just off screen when centered, when the scope is in standby mode. Is that normal? Watch vid and then you will understand the pic that I linked to.

https://www.youtube.com/watch?v=jkDdHDRAoyQ

http://i1081.photobucket.com/albums/j347/Brian_Bloom/CAM00254_zpsxtm5ctp2.jpg

The dot is normal and usually off the screen to the left when you are displaying a signal.

To check for the improper ground 60Hz hum, you should change the Trigger Source to "LINE" and see if the trace stabilizes. I kept wanting you to do this in the video but you didn't.    You used every other control but the one that would show the 60 Hz noise if it was there. Also at 6:25 or so when the screen blanks... you are in NORM trigger mode and the trigger _level_ is set outside the range of the signal, so the screen blanks! Slow down please... think about what you are doing. If the screen blanks but you know you have a valid signal input, the FIRST thing to check is the Trigger Level setting. Ah...I'm writing this as I'm watching and at 6:35 or so you finally adjust the Trigger Level and get a trace. Good! But you never tried "LINE" on the Trigger Source.... If there is a 60 Hz hum coming from the lack of proper grounding this will show up as an "envelope" for your actual main signal, making it wobble up and down or other weirdnesses. If you select "LINE" on Trigger Source the scope will trigger on the mains input to the power supply and any weirdness caused by the 60Hz hum will stabilize. The main signal will "run free" of course but with the proper timebase setting for 60 Hz you can view the stable "envelope" or other 60Hz weirdness on the main signal when "LINE" is selected as the trigger source.

A 60 Hz waveform has a period of (1 sec/60Hz) = 0.0166 seconds or 16.7 milliseconds. 3 full cycles would then take 50 milliseconds. Your screen is 10 divisions wide, so to display 50 milliseconds worth of signal across the screen you set your timebase to 5 ms/div. Then set Trigger Source to LINE. If you then see a stable "envelope" of three cycles with your main waveform running inside that envelope, you have identified your problem to be a 60 Hz hum most likely coming from a bad signal ground.

You could also do this: With the connections between scope and headphone jack on the PC made as you have tried them, and both the Scope and the PC turned OFF and unplugged from the wall, use a DMM to check the ground connection for continuity and resistance. Touch one DMM probe to the BNC shields at the scope and the other DMM probe to some ground on the PC, like the USB shield or the headphone jack outer part, to see if there is high resistance or open circuit in your ground connection from the scope to the PC.

Don't get so frustrated! You are learning a whole lot about scope operation and etc. that you would never have learned otherwise, and you have the satisfaction of bringing this classic and sensitive instrument back to life! That is nothing to sneeze at. The Oscilloscope is the King of test equipment and is a complex bit of apparatus, as you can see, and your scope isn't exactly a "beginner's" scope, it is considerably more complex than many lesser instruments, with its delayed timebase features etc. You are on a very steep learning curve and you are doing just fine, so don't be so hard on yourself.

ETA: Take PW's advice to heart. I can add this: SLOW DOWN. Every time you change a setting, stop for a few seconds, observe, and _think_ about what you see or don't see. The scope is a very complex instrument, but it is dumb, and it only does what you tell it to do. If you tell it nonsense, you will get nonsense back from it. GIGO applies in full force here.
And lighten up, you are getting such a valuable education here, and I'm quite sure that others are watching along and also benefitting from our explorations here.

And here's your head-banging smiley: