Small inexpensive Oscilloscope

Discussion in 'Hangar Talk' started by coloradobluesky, May 13, 2018.

  1. coloradobluesky

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    What are the good ones? It's for audio applications in the performing music industry. One thing I want to be able to do is detect audio signal yes or no coming out of an audio cable. I think they are 20khz and below. I need floating ground because some of the signals are differential etc.
     
  2. Cap'n Jack

    Cap'n Jack Final Approach

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  3. deonb

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    You mentioned "one thing you want to do". What else would you like to use it for? For the case above, a simple oscilloscope multimeter would work - like a Signstek UNI-T.

    Do you need to be able to capture? Will a USB scope work (no display - just connect to a laptop)? Do you want a logic analyzer function on it?
     
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  4. coloradobluesky

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    Well I also have a model railroad. Nothing fancy, just a portable test instrument. Easy to use. Able to measure differential signals w/o ground. It has to have its own display and power.
     
    Last edited: May 13, 2018
  5. Cap'n Jack

    Cap'n Jack Final Approach

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    Sounds like the Signstek UNI-T mentioned above would fit your needs.
     
  6. airdale

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  7. denverpilot

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    Wouldn’t a speaker/headphones and a set of alligator clips do that job? A scope seems overkill unless you need to see the pattern for some reason. Heck, a $3 VU meter chip and a couple of jelly bean components would be the “fancy” version.
     
  8. murphey

    murphey Final Approach

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  9. 3393RP

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    Prices sure have gone down since the days I worked on UPS systems with TTL logic. I used a four channel 150 mHz Tektronix scope, think I paid $3,300 for it in 1988.

    In 1998 I gave it to a friend when I went into building data centers.
     
  10. coloradobluesky

    coloradobluesky En-Route

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    Right now what I have is an acoustic electric guitar with a preamp powered by a 9V battery. The battery is supposed to last 200 hours of playing with the preamp on. The preamp is supposed to only come on when a 1/4" jack is plugged into it. The battery only lasts a couple of weeks with only a few hours (like 10) of being plugged in. It acts like its on all the time, but there aren't any lights or anything to indicate when its on. The factory Martin repair shop in Boulder doesn't know how to tell if its on all the time or not. Its under warranty (brand new Martin guitar) so I want to show them how to tell if it is on. I suppose I could measure current draw from the battery. Anyway, what test instrument do sound audio technicians use to test their audio pre amps, amps and instruments? I dunno. Be cool to have a little portable scope.
     
  11. idahoflier

    idahoflier Pre-takeoff checklist

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    Last edited: May 14, 2018
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  12. denverpilot

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    Yeah Rigol has been the cheap and works standard for a while now.

    I’m still in shock a guitar repair place can’t properly test and repair a guitar though. I’ll revisit the thread after I stop banging my head on this desk.
     
  13. denverpilot

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    By the way since you’re in Boulder I could give you a working but very unhealthy old scope that needs things done to it. But I would look this gift horse hard in the mouth. It’s a good scope but it needs restoration. Fuzzy CRT, dirty, just needs love I don’t have time to give it.
     
  14. denverpilot

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    And when I say “works” I mean powers up and the traces react to voltages. That’s about as far as I got into it.
     
  15. coloradobluesky

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    The guitar shop replaced one part and now they are replacing another. Martin is standing behind their warranty. It will get fixed. Martin shop,Woodsongs in boulder as good as it gets. At least Martin HAS certified repair shops.
     
    Last edited: May 14, 2018
  16. denverpilot

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    We call that shotgunning it. Throwing parts at it until it works. But yeah, if they’re willing to do that until they hit the right part replacement, they’re at lest taking care of you.
     
  17. coloradobluesky

    coloradobluesky En-Route

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    The days of replacing discreet broken parts are long gone. Welcome to the 21st century!

    "When faced with a real problem, the self-appointed expert usually becomes the fool"
     
  18. 3393RP

    3393RP Pattern Altitude

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    That was a TTL tradition with some of the "techs" I ran across back then.

    :p
     
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  19. LongRoadBob

    LongRoadBob Line Up and Wait

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    Yeah, if it was me (unless you see a need or desire to just have an oscope) I would use a cheapo ammeter and put it in line with the battery...either there is current or not. I'm not sure how you were thinking of using the o-scope, because if you put the leads on the cable, it's plugged in and from your description would then be engaged?

    Can you take the preamp unit out of the guitar easily? Usually (like with fuzz pedals) it can be the jack itself. There there is an extra "tab" on the jack, so when you plug in the 1/4" phone cable to the jack it moves the metal, completes the circuit and turns on the unit. If that tab is a little bent it may be making contact even when no cable is plugged in, and should be easy to fix as it would be obvious which way to just bend it a little so it breaks the circuit when no cable plugged in.
     
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  20. denverpilot

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    Common in telecom, too. In the field anyway.

    If you studied and knew exactly what cards did what things in the overall chassis or larger system design, you’d be a better tech. You could take a list of symptoms and rule out multiple cards or modules right off the bat. Which was one of the techniques I taught in classes when I was doing Product Support.

    But other times even that would fail you. One on site trip where the symptoms made no sense became very clear when the chassis was opened. Mold had grown from a building leak up from the bottom of the chassis into the card cage shorting various things out. LOL.

    Another site trip yielded the tidbit that the building the system was installed into had two ground rods, and there was a voltage potential between them. Half of the equipment was in one side of the building, half on the other side...

    Hmmmm, ground is apparently not ground, the world around. :) :) :)
     
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  21. flyingron

    flyingron Touchdown! Greaser!

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    For audio, there are a slew of cheap-ass digital scopes for $200 from china these days. Some under $100. For about $200 you can get a used Fluke 97
     
  22. LongRoadBob

    LongRoadBob Line Up and Wait

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    How did they find the ground potential between grounds so far apart? Just wondering, I don't recall the resistance of wire, but with a voltmeter, the long lead would add resistance, so the only (off the top of my head) way I could think to check would be two sets of wires, equal gauge and length from the one to voltmeter, the other to voltmeter....

    I'm sure I'm missing an easier way to do it exact. There often IS potential between seperate ground, I thought they used actual connections between to help get rid of it. My buddy used to live in the New Mexico desert, where it isn't enough moisture so just sinking a ground rod into the ground was not so "grounded".
     
  23. Clark1961

    Clark1961 Touchdown! Greaser!

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    Once ya get into industrial wiring the code gets sort of muddled and accepted installations can be a little off until problems turn up. Bonding everything is generally a good practice but it can also create some ‘interesting’ conditions where a single short could lead to briefly energizing equipment cabinets all over a facility. That possibility should give pause to anyone working around high voltage systems. Ever wonder why there are usually rubber mats in switch rooms? Ever wonder why there aren’t rubber mats anywhere high voltage equipment is operated?

    I’ve only had to send one man to the hospital for treatment/observation after a shock from a 480 volt system. They guy just switched on a 40 horsepower motor. Did I mention I hate hospital visits for accident follow-up.
     
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  24. wsuffa

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    AKA a ground loop. Not as uncommon as it might seem, and sometimes induced when grounding appears to be I n compliance with codes. This can be a significant issue in video and audio facilities.
     
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  25. denverpilot

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    After the theory was formed, just head to the room in the building where receptacles are across the room or hall from each other and measure there.

    But we knew because we had gear in the telecom room that had cabling running from gear in the other half, so we had access to both grounds inside our cabinet. Our chassis / “safety” ground (as @Clark1961 mentioned) was separate from our system / “data” ground.

    They did end up bonding them @Clark1961 ... FWIW. And yeah, that can be entertaining when a “safety” ground becomes energized...

    But in the meantime they said it fixed other things besides our gear. Their massive wave solder machine had been freaking out. It stopped. :)
     
  26. denverpilot

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    Yup. Hummmmmmmmmm. Especially when one of the grounds comes loose in a panel where there’s 300 phone operators working all on switch-mode power supplies in their terminals. Hahaha. BTDT too!
     
  27. Palmpilot

    Palmpilot Touchdown! Greaser!

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    My discovery of ground loops occurred as a teenager, when I was troubleshooting a nasty feedback problem in the sanctuary of my mom's church. I don't remember what led me to think of it, but to this day, I remember interrupting the ground connection on a shielded audio input cable, which immediately got rid of the feedback.
     
  28. kgruber

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    The founder of Tektronix, Jack Murdoch, perished in a Super Cub.
     
  29. jsstevens

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    I'm pretty sure if that fixed it, it wasn't feedback.

    I've chased a bunch of ground loops in audio systems over the years and they are a pain in the neck (though I often have a lower opinion...). Our current church system has a bad one that I'm pretty sure I'm going to have to go digital snake to eliminate. I had everything cleaned up between the platform (where various pieces of gear like keyboard, electronic drums etc. are plugged in) and the sound room. Until they bought a new LED lighting system which was not installed in conduit. :mad: Really annoying 60 cycle square wave hum. I talked to the engineer from the lighting company right after installation and he claimed that it wasn't the lights. I asked him why it went away completely when I turned the lights off and is was most pronounced when the lights were charging levels? Then he stopped answering my phone calls. :mad:

    Anyway, grounding is fun. Especially for good lightning a surge protection... A lighting strike can generate quite a voltage drop even across great big copper conductors...
     
  30. Palmpilot

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    Whatever you want to call it, the symptom was that a tone would begin coming out of the speakers whenever the gain was set above a certain level. When the gain was set below the critical level, the tone would stop. I don't remember the exact details of the setup fifty years later, but I do remember concluding that there was a possibility that some of the current to the speakers was flowing through the shield of a coaxial input cable to the power amp, thereby inductively coupling to the center conductor of the cable. That's what gave me the idea to disconnect the shield at one end of the input cable, and it made the problem go away completely. (This may have been a cable from a preamp to the power amp.)