Scope Died. Anyone Use the PC-Based Ones?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by schmidlin, Sep 20, 2015.

  1. schmidlin

    schmidlin Member

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    The old vacuum tube Tektronix finally gave up the ghost so I'm looking at going with the PC-based version. For around $60 you get a hardware interface, software, probes and USB cable. I wouldn't think about it if it didn't have the hardware interface to protect the computer. Most are AC-in max is 60 volts peak to peak, so that would handle most amps. I could easily apply a voltage divider if needed. 20 mhz.

    Anyone use these and are they any good? Here's an example of one: http://www.ebay.com/itm/Hantek-6022...-USA-Seller-/291053997898?hash=item43c42b8f4a

    TIA.
     
  2. Kyle B

    Kyle B Supporting Member

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    Problem with many PC based scopes is the latency. Meaning there's noticable lag between when an event happens and you actually see it on the screen. Annoying at best.
    I bought a bunch of USB scopes a couple years back...we were considering putting one on every EE's desk. They all disappointed. The latency probably hasn't improved much as USB was the bottleneck in all cases
     
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  3. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    That one (the Hantek) looks kind of bad for amp work. Without attenuating probes, the input voltage is limited to +/- 5V! Also the input coupling is DC only, so you'd need to whip up your own capacitive (AC) coupling to let you look at anything.
    I'd be shopping for a nice used Tek on eBay...
     
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  4. J M Fahey

    J M Fahey Member

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    When I'm "on location" or on Tour and need a scope, NOW, I make an attenuator/protector and plug it straight on my Laptop , running a Software scope.

    This is the "Deluxe" one, switchable to see anything, from a bare guitar to tube plates , EXCEPT POWER TUBE PLATES ... where not even a regular scope is advisable unless equipped with a special HV probe.

    [​IMG]
    The back to back 1N400x diodes protect the PC audio card Mic/Line input by clamping anything above 700mV peak; the attenuated signal is way lower than that of course, so it's an accurate copy of what's being fed at the input.

    Caveat 1: it's good to "see" (which is the most important function anyway), not too accurate measuring but you can compare to some known AC volotage, say some winding of the PT.

    Caveat 2: it does not see DC since audio cards don't "hear" it either and highest frequency is 20kHz by design (again audio card limit) but for on the road or non-regular Tech use it's a God send.

    Build the attenuator into a shielded and insulated box, it must be *impossible* to touch any internal part, switch contacts or resistors.

    It's meant for the Soundcard "Line" input, with around 100mV sensitivity, not the "Microphone" one.

    Some useful software scope simulators:
    http://www.zelscope.com/

    https://www.zeitnitz.eu/scope_en

    They do not show DC but are great to see weird stuff happening: oscillations, distortion, noise, hum, ripple, crackling, etc.

    If you NEED to see power tube plates in a conventional scope , you can build this attenuator:
    [​IMG]
    a) it attenuates 100:1
    b) it shows DC (if the scope does, of course)
    c) if you want to stop DC, add a .01uF x 1600V (no kidding) in series with R1
    d) don't replace the 4 x 470K resistors by a single 1M8 or 2M2 one, because high voltage will arc across its ends, it simply has not enough voltage rating, these are 4 470k 1/2W resistors, in series (cut leads 1/4" long, and put them inside a plastic tube (crystal PVC hose or BIC ball point pen body) filled with Epoxy or RTV Silicone
    e) BE CAREFUL, you can have as much as 1500V peaks on plates.
     
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  5. Kyle B

    Kyle B Supporting Member

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    Thanks for sharing :)

    One question/thought:
    1N400x are incredibly "slow" diodes. By the time they clamp, your input is probably kaput already. Why not use something fast like a rapid recovery Schottky???
     
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  6. xtian

    xtian Member

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    JM, that is kick-ass, MacGyver voodoo ****.
     
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  7. J M Fahey

    J M Fahey Member

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    Be my guest, use anything that makes you comfortable. :)
    That said, at the frequencies we are working the humble ones work fast enough, it's guitar amps after all.

    And besides, shoot me :D, I didn't compensate my resistor stick with a parallel trimmer, guess built in parasite capacitance will kill very fast transients ... or you can add 100pF in parallel with the 18k resistor.

    Anyway the PC soundcard barely reaches 20kHz .

    I noticed some complain about USB scopes latency.
    Maybe, never used one, but they are doing some serious number crunching; PC soundcards are very simple and in practice, say I touch the tip of a plug and I see the waveform peak at the same time, no noticeable delay .
    Their job is way simpler.
     
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  8. schmidlin

    schmidlin Member

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    Thanks for the responses, guys. I already have an old desktop on the bench running XP and I really don't use it, so I guess that will change. Fahey, I've never heard of a Line-In for a computer, nor have I ever looked for one. Should I look for something off the soundcard? Why couldn't I step it WAY down and use the mic input? TIA.
     
  9. J M Fahey

    J M Fahey Member

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    Oh, you do, just never used it :)
    Specially on a benchtop computer, doubly so if it's old.
    It's some Laptops and most (all?) Netbooks which have only the Mic one, to save space, although you can often select the mic input to behave like a Line one through Control Panel or something.
    I usually don't suggest using Mic inputs because they are too sensitive and low impedance (around 1mV and 600 ohms) while typical line ins run around 100mV and 10kohms, what my attenuators expect.
    That said, we are McGyvering here, aren't we? :) so probably an extra 10:1 attenuator after the diodes (say 10k/1k) will probably do the trick even on a Mic input.
    Try it, after the diodes you can't really kill anything.

    Sound card panels are very small (duh !!!) , so they use **tiny** labels or mystary symbols , but in general are color coded : pink for the weaklings (mics) and blue for the Macho Men (line) :
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    don't worry, now I'll make some space on my incredibly cluttered bench, kludge one and post the picture here, being used to repair one of my SS amps on warranty servicing, instead of the regular bench scope :)

    EDIT: almost forgot (ok, I DID ) , in the schematic I suggested wiring attenuator out to both ring and tip of a mini 1/8" stereo plug, because *most* inputs work that way : "stereo" Line in and I join Right and Left (tip and ring) ... but some expect a Mic, which is always Mono, and some others send +5V to ring, to power an Electret mic , as some cheesy clone of Phantom Power, in this case use a stereo plug but don't connect ring (in fact, tape its terminal).

    Simpler to check than what I just wrote:
    1) download and install your Scope Software (there's many besides the two I suggested, you can also use Audacity and similar which often have a Scope function)
    2) just plug a stereo (TRS) bare plug, or s chopped earphone plug , start the free scope software and touch the free terminals with your bare finger, same as if testing a guitar cord, to see which wire/terminal shows hum on the screen .... now you found where to connect the attenuator.
     
    Last edited: Sep 21, 2015
  10. schmidlin

    schmidlin Member

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    Spot-on. Your second image is exactly what I have on my desktop. I'm thinking plug in a raw stereo plug and check for 5 volts to ground. Anyway, you've been a big help. Thank you.
     
  11. J M Fahey

    J M Fahey Member

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    Ok, here's the full deal, much more than I expected.
    Thank our horrible September 21 "Spring/Student Day" storm; cold, grey, windy, drizzling all day, kept everybody inside.
    Oh well.

    1) I downloaded and installed http://www.zelscope.com/
    It shows a very complete set of controls, I just set them in the most basic way possible, just to see a waveform, period

    2) I posted the full "Deluxe" adapter on post #4 , it can be housed in a plastic soapbox or something similar, on purpose for this test I built the very simplest one, and by the way what I often improvise (and then discard) in 5 minutes onstage/backstage, when there is some problem which is best found with a scope and it's too late to get a real one.

    The *example* I show here has practically no attenuation, because to show it here quick I just drove it with my Phone earphone out, I just used 470 ohms in parallel with the Laptop Mic input (and the protection diodes) and only used a 1k resistor in series, because Phone/MP3 player earphone level is usually a meager 100mV to 200mV .

    If you want to test (trace Audio or see waveforms: distortion/oscillation/buzz/hum/etc.) inside an SS amp or mixer use 100:1 attenuation, so a series 47K resistor and a DC isolating .1uF capacitor; to test inside a tube amp use 1000:1 attenuation, so series 470k , and a .01 * 600V cap *at least* , if you find 1000V or 1600V ones, even better.

    Also on Tube amps it is VERY IMPORTANT that GROUND IS PRESENT ALL THE TIME, because you depend on a contact to ground to attenuate, IF YOU LOSE GROUND (say a crocodile clip snaps off when you pull or move the cable) YOU CAN GET 300/500V AT THE ATTENUATOR so in general I suggest SOLDERING THE GROUND WIRE TO AMP GROUND .
    Not to the main chassis ground but at least to the preamp ground or where you are testing.

    By the way this applies also to a store bought USB scope and, in fact, to "real" scopes too.

    3) so here it is in its full glory ???? , the "proper" schematic is
    [​IMG]
    but here I built a fixed one and omitted the DC isolating capacitor because I tested with the Phone out.

    Almost forgot: download MP3 test tones, here there are 30 second high quality MP3 at: 100Hz , 250Hz , 440Hz , 1000 Hz , and 10000 Hz http://www.mediacollege.com/audio/tone/download/
    Would have loved a 2500 or 3000Hz one, which is a practical high limit in gueetar amps.
    NOTE: set your MP3 playerv to "LOOP" or "Repeat one file" so you have a continuous tone, otherwise you'll have tompush "Play" every 30 sec

    Photo session starts:
    Zelscope welcome screen.
    [​IMG]

    Don't worry too much at the initial controls setup, just inject some audio and then tweak them until you see some image in a height/width you like.
    We are basically trying to see wave *shapes* here.
    Bare 1/8"stereo plug, into Laptop Mic input, trying to see which is the active terminal, by touching it.
    NOTE: Zelscope is NOT running all the time, it has tiny Start and Pause/Stop buttons , when you want to see something you start, when enough you stopit.
    You can do it as many times as you wish.
    I *guess* that it's "recording a movie" of what you see on screen (cool) for later review or post it somewhere.
    Never used that.
    [​IMG]

    I will start just *touching* the bare plug to see which is the active terninal *in my laptop*
    The always popular Finger-o-Generator in all its Technological awesomeness :
    [​IMG]

    this is what ambient 50Hz hum/buzz looks like.
    The picture shows 2 superimposed waveforms because the camera (remember, no flash) took too long, you actually see a cleaner single image:
    [​IMG]
    remember to "start" to see waveforms, then to "stop" afterwards.
    Pretty clean image, huh?

    I tweaked control panel until I saw an image I liked.
    Too slow sweep you see everything too narrow and squashed, too fast and maybe the waveform is wider than the screen and is unrecognizable.
    Usually what our poor wet brains like is see 3 or 4 waves covering the screen, maybe 2 if it's a single sinewave and , say, 30 or 40 if it's "music".
    Beauty is you adjust it until you like what you see.
    [​IMG]

    as Arnold said: ... "I'll be back ..... "
     
  12. J M Fahey

    J M Fahey Member

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    And I came back.

    he Hi-Tech Nasa Quality FAHEY Simple Attenuator
    In this case just 1k>470 ohms attenuation because i'm "looking" at a tiny Headphone input, for real work I suggest:
    1) For Pedal/SS work: 100:1 attenuation or 47k>470 ohms , plus a .1 uF cap in series with tip to block DC.
    2) for tube work: 1000:1 attenuation, say .01uF x 600V (or more) + 470km > 470 ohms.

    ALWAYS back to back 1N4002 to 1N4007 or 1N4148 or 1N914 or your favorite fast diodes (if you wish) to clip anything at 700mV TOPS to protect your soundcard input.
    [​IMG]

    In this case I soldered it to a regular Jack, suggested is some 1 or 2 ft long red/black wires with crocodile clips, twist them to get some shielding.
    If probing low signal high impedance points (think tube grids or pedal inputs) they might introduce their own hum, if possible use shielded wire for the hot one.

    That's what Iron Maiden looks like:
    [​IMG]

    slow sweep:
    [​IMG]
    I played with scope controls until I got an image I liked (adjusted height - sensitivity - volts/div) and width
    faster sweep:
    [​IMG]
    controls set to:
    [​IMG]

    a poor innocent 440 Hz sinewave (ring a bell?) , from http://www.mediacollege.com/audio/tone/files/440Hz_44100Hz_16bit_30sec.mp3 ready to be destroyed by mean guitar distortion:
    [​IMG]

    ok, so now you know what to do when the scope dies ... or simply there is no scope (nor generator) available.

    Hope it helps :)
     
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  13. schmidlin

    schmidlin Member

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  14. J M Fahey

    J M Fahey Member

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    Somehow actual scope attenuator images were lost, here they are again:

    1) fully adjustable one, must be built in a grounded-to-amp-ground box, output is safe (diode limited to 700mV peak and usually passing 100/200mV peak tops) for a standard PC card *LINE* input jack.
    Some small Netbooks (such as my own NB505) have a single jack, but it can be preset for Line or Mic use. Use Line of course.
    It can measure/show most anything in a Tube or SS amplifier, ***EXCEPT POWER TUBE PLATES WHICH CAN REACH 1500V PEAKS EVEN WORKING PROPERLY***
    But you can read up to Phase inverter plates.
    [​IMG]
    Of course, you can monitor output signal, not at power tube plates but at the other end of the OT: the speaker out taps, where we are talking tens of Volts, not Thousands.

    2) IF to solve some complex problem caused by HF/RF ringing, instability, etc. , you MUST look at Power Tube Plates straight in the eye, this other attenuator extends the range of REGULAR scopes (not PC software ones) .
    GThe High Voltage probe I suggest is essential, it lives inside a plastic tube, resistors are fully enclosed in Epoxy or Electrical grade Silicone, etc.
    You are suggested to solder it to tube plate and leave it there while testing or clamp it where approppriate with a good crocodile clip which you will NOT touch while the amp is powered.
    Sounds silly to build a properly separated and insulated probe and then grab its tip trusting the thin crocodile clip PVC sleeve.
    [​IMG]
     
  15. trobbins

    trobbins Member

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    I accidently lost a soundcard or two back when they cost a few dollars. Over the last 5 years I've used the cheap $1 USB soundcards, and a USB isolator, for bench work, with the isolator powered from a small 12V battery to minimise hum loops.

    I've just converted a $7 USB soundcard with 96kHz sampling, to run up to 40kHz bandwidth, but that involves a bit finer smt soldering, as the tech has got smaller and smaller.

    With a bit fancier a clipper, the full-scale of the soundcard chip can be used. Anyway, something for thought for diyers. For the bench, the REW spectrum analayser can't be beat, as has some powerful features, included automated THD, and harmonic numbering, and IM distortion assessment with the in-built generator.

    http://dalmura.com.au/projects/Soundcard mods.pdf

    I have used something like Juan's simple attenuator, but I was given a soundcard preamp many years ago so have just used that, but effectively the same. There are also a few other preamp/attenuator designs about for those wanting a bit more:

    http://dalmura.com.au/projects/K2875 Soundcard Preamp EA Aug 1998.pdf
     
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  16. TimmyP

    TimmyP Member

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    A few years ago I looked at PC scopes. The low input voltage limit scared me, so I ended up with a used Owon SDS6062. It cost more of course, but it's much handier - and it can run off of batteries, which I did not know when I bought it - a nice bonus.
     
  17. mattball826

    mattball826 Member

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    I use a portable unit all in one DMM and Scope. Works great!! Isolated because it is battery. Works about 7 hrs on a charge.

    Primary use is on SMPS circuits, newer powered speakers with SMPS , amps and other lv electronic devices. Works on any guitar amp too. There is some latency, but it's not too bad. Image isn't as good as a good analog scope, but I can live with it.

    I have 2. One is a better model than the other. That obvious means improvement = higher price. That said, I can work the field or bench with either.

    Hantek DSO1060

    Siglent SHS820 is my latest. 200Mhz dual ch DSO.
     

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