I'm Officially Addicted: Oscilloscope Considerations / Precautions

Discussion in 'Amp Tech Center' started by Pick_n_Strum, Dec 4, 2019 at 12:25 PM.

  1. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    So just finished my first build (5e3) - some of you will have undoubtedly seen my thread in the shock brothers forum. Throughout the build I was consuming as much info as I could on amp building, mods, troubleshooting, etc. I've watched a ton of youtube videos (D-Lab and Uncle Doug) and ended up picking a scope and audio generator for good prices off my local Craigslist.

    I totally get that a signal trace / audio probe and DMM can be plenty to analyze amps. I will be making a viable BillM-style audio probe (there is no way I'd put my pedal audio probe anywhere near an amp). But, I really have a desire to check them out through an oscilloscope as well. With that being said, there is some conflicting info on safe methods to do so - obviously many of the same rules apply as checking voltages and working on amps:

    -One hand in pocket
    -Never short B+ to ground (and/or never short anything to anything)
    -Always drain filter caps if working on amp and confirm they are drained
    -In my case: keep chassis on 100% rubber mat and stand on 100% rubber mat
    -Turn amp off when moving probes to new and/or tight locations

    In addition to the above I'm curious what other safety precautions should be considered when hooking an amp to an oscilloscope. I've come across various info and can't seem to keep it straight:

    1. Do you guys hook up your amps to isolation transformers when scoping them? I never plan to work on a "widowmaker" but it sounds like some people will hook up non-widowmakers to isolate ground from neutral so that you and your scope don't become a ground path for mains voltage. However, there is still obviously high voltage present via DC and if you happen to short hot to neutral anyways.

    2. If you don't do number 1, do you use isolated probes on your scopes?


    I have been familiarizing myself with just the Oscilloscope and Audio Generator - and having a blast with it. I will be using a high-wattage dummy load and plan to start by just hooking the scope up to the output jack dummy load. Eventually, I'd like to have the flexibility to trace through the amp with the scope.

    Any considerations, precautions or answers to my questions are appreciated. Thanks!
     
  2. Peegoo

    Peegoo Tele-Meister

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    Other safety precautions for your first list:

    -No kids allowed in the area
    -No pets allowed in the area, e.g., a cat on the bench can be zapped

    An isolation transformer is good insurance. However, if you're plugging it into a GFCI as a safety measure, that GFCI will never see a short to ground on the secondary out of the isolation transformer because there's no direct connection between the primary and secondary. The GFCI cannot save you.

    So always always have the chassis you're working on separately grounded, and always test for current leakage between that ground and the chassis before getting your hand in there.
     
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  3. LightningPhil

    LightningPhil TDPRI Member

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    Always check there’s no voltage present before sticking in hands. You’re probably doing it anyhow.

    Don’t have drinks anywhere near live stuff.

    immobilise all live components.
     
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  4. Nickfl

    Nickfl Tele-Afflicted

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    Unplug the mains power when you are done probing and you turn the amp off. That is the one safety thing I have a consistent problem with, I have repeatedly fired up the soldering iron to make some changes after testing something and realized the amp power cord was still plugged in. It hasn't ever bit me, but it would certainly be unpleasant to brush up against the power switch terminals in that situation... especially with a hot soldering iron in your hand.

    The one hand in the pocket rubs me the wrong way. Only one hand in the chassis? Sure. But if you are such a monkey that you have to actually keep your other hand in your pocket you probably shouldn't be working with electricity anyway. I am halfway kidding, but it always sounds dumb to me. I'm sure someone will chime in about how wrong I am about it though, people get really salty about the safety stuff sometimes...

    Also, that rubber mat might be doing more to make you feel safe than actually keeping you safe.

    Wear safety glasses when you are testing a live chassis. I've never had a cap blow up or anything like that but its an easy precaution and well worth taking.

    Someone already said it, but don't just discharge your caps, you should actually verify that they have actually drained as well before you assume your chassis is safe to work on.
     
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  5. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Great stuff guys - thank you! I guess in my head, standing on the rubber mat will keep me from grounding myself (???). Probably a piece of mind thing.

    I know what you're saying on the one hand literally being placed in the pocket. The idea obviously being don't short anything to yourself and definitely don't make a circuit through yourself.

    I have been using safety glasses (and I also used hearing protection when I first fired it up).
     
  6. peteb

    peteb Friend of Leo's

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    I’ve gotten accustomed to reading the signal from the amp on the oscilloscope recently.

    It takes some practice to get comfortable.



    I hadn’t thought of either of your questions. Using a scope should mostly be hands off, so shocking yourself should not be a problem.


    Shorting out through the scope plugged into the wall is a possibility but I think they design the scope so this won’t happen.



    How do you do it ‘hands off’?



    I would not recommend poking around the amp in different places with the probe. To me, it’s just bad practice. I’m sure there are people who are successful doing it but I don’t care for pops crackles or surprises.


    I recommend clipping in where you want to measure with the amp volume turned down and then turning the amp up.



    I much prefer to clip onto a circuit element on the board instead of a tube pin, although tube pins work too. Power tubes better than pre amp tubes because there is a little more space between the pins.


    An example of clipping onto the board instead of clipping onto the grid pin is to clip onto the coupling cap. They are usually fairly large with lots of room around them.


    And also, it’s ussually small voltages that you are going to scope. Only the plates are high, but you my not need to scope the plates.
     
    Last edited: Dec 4, 2019 at 8:57 PM
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  7. scooteraz

    scooteraz Tele-Afflicted

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    Interesting, my best friend and I were starting building amplifiers from tube TV and radio chassis where we were about 12 or 13. Our dads gave us most of those rules, then left us alone. Guess I’m lucky to still be alive.

    Nonetheless, all of the ideas given so far in this thread are great. I still just, very carefully, attach leads to the proper test points after making sure the chassis ground is truly grounded and that there is no leakage.
     
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  8. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Great point and I will not be sticking my hands in the amp at all. If I were to even move the probes, the amp will be off and voltage on the caps will be rechecked.

    I guess I'm just trying to be fully aware of the risks and wanted to pick the brains of the TDPRI experts.

    I really appreciate all the input from everyone
     
  9. drneilmb

    drneilmb TDPRI Member

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    I've blown a couple fuses on amps trying to see the transformer primary on the scope. The scope has always survived and it's easy to change a fuse, so no danger there if you're a dope like me. If you're not a dope then just stay away from the power transformer primary.

    The scope probe has very high impedance so it is fairly safe from being blown up, or blowing anything up. I would only used the clip-on probes inside an amp for fear of shorting two things together with just the metal tip.

    And you won't need a dummy load unless you want to use the scope on the amp and stay silent. I just clip the probes across the output jack while it's plugged into the speaker cab. Do you need to be doing this silently? Hey, maybe I SHOULD get some 8R power resistors for amp work while my kids are sleeping.

    I have been having a blast with my scope and my little amp, including measuring power supply ripple.

    1204191617.jpg
     
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  10. kbold

    kbold Tele-Holic

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    1
    Important: most CRO's don't have isolation between the common (0V) references, and all the commons are at ground reference.

    If probing with more than one channel, remove the 0V clips from all inputs except one. This one you ground, unless you have isolated with a transformer.
    All channel measurements are then referencing to the one 0V clip.

    This why some use an isolation transformer for the CRO (to remove the commons from ground reference). A problem with isolating is that now the common is floating, BUT all those shiny metal sockets on the CRO are at that common clip potential. You still cannot connect different common clips to different potentials (you still use only one common clip).

    It was a common practice to "float" the product under test, to protect the CRO from damage, but this also comes with safety risks. You won't find it recommended in user manuals, I think.

    The only exception is if using differential input probes, or if the (expensive) CRO comes standard with isolated inputs (commons isolated from each other, and from ground).

    You can use 2 inputs as a single differential input: One input becomes the reference (a floating reference) while the other input is measurement. Both common clips would be removed. What you are measuring is the difference between the 2 inputs. This would be my preferred way, avoiding the need for an isolation transformer, and being inherently safer.

    So ..... care and awareness when playing with tube amp voltages. Make sure the insulation on your CRO leads and clips is intact.

    2
    Practice with low voltages (maybe a pedal) until you are familiar with the CRO. If connected incorrectly, the CRO becomes a somewhat expensive fuse.

    3
    Read the CRO manual. Download one if you don't have a copy.
     
    Last edited: Dec 5, 2019 at 3:02 AM
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  11. Steve 78

    Steve 78 Friend of Leo's

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    I have a question. We were learning to build computers at trade school, the instructor said to keep the power chord plugged into the wall with the switch off, so that the chassis still has an earth connection. Above, it was mentioned to unplug the power chord from the wall, but this will disconnect the chassis from the earth, or are we relying on the probe chassis connection for the earth ground? What is the best practice here?
     
  12. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    If you're referring to Nickfl's comment, I think he was saying to unplug the amp so one can go back in with one's hands to actually work on the amp; there would still be power running through the primary wires if it were still plugged in. This would be after you are done working with the oscilloscope. If you leave the amp plugged in, you could accidentally short something (like a finger or a soldering iron) from hot wire to the chassis - turning the amp off would only cut power after the on/off toggle. There is also potentially still DC voltage on the filter caps.

    I may be misinterpreting what you are referring to and/or getting my wires crossed on something (pun intended).
     
  13. kbold

    kbold Tele-Holic

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    If you're assembling a computer, for a start you don't want power, so I would say don't plug in the mains. Secondly, any built up of electrostatic charge you hold would have a nice solid pathway to ground via the mains plug. Electrostatic charge is the one most damaging things (to components) when assembling/repairing any electronic device. Electrostatic voltages can be very high.

    Best to connect yourself to the computer chassis with an anti-static wrist strap. If you don't have a wrist strap, be sure to touch the chassis before touching any electronic components.
    Better still to work on a dissipative workbench (anti-static matting on your workbench, with the computer and any unattached PCB's on the mat) and you connect to the mat with a wrist strap. Most computer circuit boards are shipped inside plastic bags that dissipate, so you leave the bag on the matting and allow it to dissipate any potential before unpacking or handling.
     
  14. RottenTheCat

    RottenTheCat Tele-Holic

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    Mostly, scope leads "clip on", and you'll want both hands to manipulate the scope controls, the amp controls, and the input (audio gen) controls.

    That said... I always used to somehow add a bleeder to the end of the B+ chain of things. Used a 1 meg 1 watt. Power supply never saw it, but it would bleed down the circuit when powered off in just a few seconds. Maybe 10 at most. Just in case I got lazy and forgot to keep things safe.

    One thing I liked to do is use both scope traces at once with a 1 ohm resistor between cathode and ground. The voltage drop across the resistor gave a perfect indication of what the output was doing, channel 1 on the top half, and channel 2 on the bottom half. VERY easy to see crossover distortion, how much each side of the push pull was conducting at any output level. See which side was clipping how much, and with my scope (and I'm sure most of 'em today) you can apply math functions to get percentages and such off the cursor's positions.
     
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  15. Steve 78

    Steve 78 Friend of Leo's

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    I mean plugged in but turned off at the wall socket, so there would be no power to the amp, but there would still be a wired connection to the earth ground of the house (assuming a 3 prong lead, which is standard here in Aus).

    Again, I'm not saying plug in for power, but so the earth is connected. Yes the best option is to use a wrist-strap, but if that wasn't available we were told to use the chassis to dissipate static build-up and it would go through to the earth. So it seems you guys are saying it is better to unplug completely.
     
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  16. kbold

    kbold Tele-Holic

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    I don't see any advantage to having the power lead in just to earth the chassis. No real disadvantages either, except that the chassis is tethered, so less mobile. Plugged in/no power does stop any electrostatic buildup of the chassis, but not you. With no anti-static strap the best option is to touch the chassis every time before handling a circuit board, whether plugged in or not.

    The main (or only) issue is the potential to zap components with an electrostatic charge. So the priority is to neutralize yourself to the same potential as the computer chassis. Whether you go down to chassis potential or vice versa is irrelevant so long as you are both at the same potential.
     
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