What oscilloscope Should I buy first time? - Page 1
Forum section: electronics / testgear

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 07:19:07 pm
content: I need help on getting my first oscilloscope and my budget is $200ish I was thinking about a handheld oscilloscope but what Mhz do I need to get? I'm planning on using a oscilloscope for repairing and restoring av receivers and amplifiers any help would be great appreciated.

author: kripton2035
author_rank: Super Contributor
timestamp: July 25, 2016, 07:35:14 pm
content: save a little more and buy a rigol ds1054z

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 07:40:36 pm
content: That would be a great idea I will think about it, but what would be a good handheld would you know of around my price range?

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 07:58:44 pm
content: Quote from: kripton2035 on July 25, 2016, 07:35:14 pmWhat about this one? Siglent SDS1052DLhttps://www.amazon.com/dp/B00GQNN70A?psc=1

author: kripton2035
author_rank: Super Contributor
timestamp: July 25, 2016, 08:16:58 pm
content: Quote from: CodyRepairShop on July 25, 2016, 07:58:44 pmthe best bang for buck today (and for some times now) is still the ds1054z

author: tautech
author_rank: Super Contributor
timestamp: July 25, 2016, 08:19:35 pm
content: Quote from: CodyRepairShop on July 25, 2016, 07:58:44 pmNah, memory depth is a fail.Any of the SDS1000CML series offers 2 Mpts memory and that should be a minimum requirement.

author: kripton2035
author_rank: Super Contributor
timestamp: July 25, 2016, 08:19:46 pm
content: Quote from: CodyRepairShop on July 25, 2016, 07:40:36 pmyou won't get any "good" handheld in your price range, only bad quality you will regret.

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2016, 08:35:25 pm
content: Quote from: CodyRepairShop on July 25, 2016, 07:19:07 pmWhy do you want a handheld? Do you need it to be portable? IMHO a (modern) bench oscilloscope usually has more features / memory and is light enough to move around. If you could stretch your budget a little then $352 buys you a GWInstek GDS1072 (2 channels 70MHz with individual controls for each channel) from Tequipment.net; other webshops may have similar pricing. For R&D work I'd strongly recommend 4 channels but for repair work 2 channels might be enough (but feel free to disagree).

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 08:53:04 pm
content: GWInstek GDS1072 looks like a good one this is what will work probably but is this meant for signal only or is it both power and signal since I still need to learn on how to use it?

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 08:55:10 pm
content: Quote from: nctnico on July 25, 2016, 08:35:25 pmI wasn't sure how good they would be but i'm in a learning process of what is good and what I need so one of the others said GWInstek GDS1072, would be a good one to get.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 08:56:28 pm
content: Quote from: kripton2035 on July 25, 2016, 08:19:46 pmJust to be clear is the memory keep that signal showing only for a limited time?

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 08:59:35 pm
content: Quote from: CodyRepairShop on July 25, 2016, 08:53:04 pmAnd I tried to look on ebay and amazon they both don't have the GWInstek GDS1072, where would I buy it at?

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2016, 09:02:34 pm
content: Quote from: CodyRepairShop on July 25, 2016, 08:59:35 pmPerhaps searching for GDS-1072B gets more results. But this is the link I found and EEVblog member can get an extra discount if you ask for it:http://www.tequipment.net/Instek/GDS-1072B/Digital-Oscilloscopes/?search=trueI have no affiliation with Tequipment.net. It's just an easy website to compare prices.

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2016, 09:04:09 pm
content: Quote from: CodyRepairShop on July 25, 2016, 08:53:04 pmWhat do you means by signal and power? Do you want to look at signals in the live (connected to mains) parts of power supplies?

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 09:07:53 pm
content: What I mean by this is look to see if there is any high distortion if it shows that and see if there is any square waves coming in before and after the restore of the vintage av receiver.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 09:11:34 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:07:53 pmYes it would be connected to the mains what what about connecting the oscilloscope onto the power connections in the back do or would I do that or do I only connect it to the signal of something in the back of the receiver I have no clue of how to connect it to the av receiver?

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2016, 09:13:49 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:07:53 pmYou can use FFT (sometimes called power spectrum) for looking at distortion if that is what you mean.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 09:15:48 pm
content: Ya something like that is there clipping it shows too? if a receiver was clipping it would show that?

author: pascal_sweden
author_rank: Super Contributor
timestamp: July 25, 2016, 09:17:15 pm
content: Isn't the GDS-1054B a better choice if the BW can be upgraded? This way the OP has 4 channels

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2016, 09:23:56 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:15:48 pmYes. Severe clipping can be seen in the waveform. Slight clipping should turn up in the FFT spectrum display as extra harmonics.

author: madires
author_rank: Super Contributor
timestamp: July 25, 2016, 09:42:58 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:11:34 pmYou don't want to connect your scope to mains or a mains referenced high voltage unless you also got a differential probe for 400 bucks.

author: tautech
author_rank: Super Contributor
timestamp: July 25, 2016, 09:48:12 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:11:34 pmFor your personal and equipments protection and based on your apparent lack of knowledge of normal oscilloscope use I would strongly suggest selection of a scope with 400V rated inputs.

author: bitseeker
author_rank: Super Contributor
timestamp: July 25, 2016, 10:27:18 pm
content: CodyRepairShop,Since oscilloscopes are new to you and budget is a major factor in your purchase, you may want to consider a used analog scope. A 2-channel 20 or 50 MHz model can be had for around $50-75. That way, if you make an error while probing a high-powered amplifier, the loss is less catastrophic than blowing up a $400+ scope. With your stated budget, you could have a couple of scopes and funds left over for accessories (connectors, adapters, cables, etc.).If you plan to use the scope with high voltages, be sure to get appropriate probes for such uses.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 10:41:54 pm
content: Well it's hard to find a working one without any issues when I look on ebay all I see is for part or not working for anything that is not new.

author: timb
author_rank: Super Contributor
timestamp: July 25, 2016, 11:05:30 pm
content: Quote from: CodyRepairShop on July 25, 2016, 10:41:54 pmThey say "Parts or Not Working" because the seller either doesn't know how to properly test the unit, or doesn't want to provide a warranty. Through a little smart shopping and research, 100% of the "For Parts" units I've purchased have arrived fully functional. Stick to auctions that show the unit powered on, with a trace on the display.That said, do you really even need a scope right now? For what you want to do, your money might be better invested in a decent multimeter, soldering iron and a decent USB sound card. (I'd throw in a USB isolator as well.)There's really great, high end software you can get for free that will let you do distortion measurements, in addition to turning your sound card into an audio range scope.That would be inexpensive and allow you to learn as you go. With your current level of knowledge, spending money on *any* scope would be a waste in my opinion.Start with the basics.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 25, 2016, 11:12:27 pm
content: Timb,I thought I would need one to make sure there isn't any clipping and distortion of too much when testing my units after repair and restore because I don't do this kind of work very offtan and wanted to be sure I would not kill my speakers or my av receiver from something stupid i could have done during the restore.

author: tggzzz
author_rank: Super Contributor
timestamp: July 25, 2016, 11:58:33 pm
content: Quote from: CodyRepairShop on July 25, 2016, 11:12:27 pmA scope will show gross clipping and gross slew-rate limited waveforms.A scope is not the best tool for measuring distortion.A scope will be good for showing you whether the low voltage power supplies are ripple and noise free.A scope will be good for tracing signals through a circuit.If you are budget-limited, then consider aworkingsecond-hand analogue scope. Ignore adverts which don't show traces (until you have more experience). Try to see the scope working before purchase (pay for it and take it with you, which avoids shipping costs and damage due to poor packaging). 10 or 20MHZ bandwidth is adequate for audio. Expect to pay $1-1.5/MHz plus shipping.

author: bitseeker
author_rank: Super Contributor
timestamp: July 26, 2016, 12:04:34 am
content: Quote from: CodyRepairShop on July 25, 2016, 10:41:54 pmThey're out there, but it takes time (sometimes a lot of time) to get good stuff at good prices on eBay. Be sure to also check Craigslist, local ham fests, garage sales, etc.As Tim said, not everything marked "parts, not working" is bad. It's less risk/hassle/effort for the seller to label as such. Of course, shopping locally where you can try out the scope, is best. At least connect it to its probe calibration waveform to ensure it shows a proper trace.

author: tautech
author_rank: Super Contributor
timestamp: July 26, 2016, 12:31:38 am
content: If you're going to get an old CRO, be sure you can find service manuals for it and while you're at it get anothe CRO in order to have the necessary tool to fix the original when it's broke.Don't ask me how I know this.EditOh, and BTW not all CRO's have 400V rated inputs either, be sure to do your homework.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 26, 2016, 12:54:20 am
content: Quote from: tautech on July 26, 2016, 12:31:38 amBut the biggest issue is how would I connect an Oscilloscope to a av receiver to test it would I connect it to the speaker outputs?

author: BMack
author_rank: Regular Contributor
timestamp: July 26, 2016, 01:06:42 am
content: Quote from: CodyRepairShop on July 26, 2016, 12:54:20 amYes, you will also need a signal generator.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 26, 2016, 01:32:29 am
content: Yes, you will also need a signal generator.What kind of signal generator would this work?https://www.amazon.com/KKmoon-Precision-Dual-channel-Generator-Arbitrary/dp/B014KT3SVO/ref=sr_1_3?ie=UTF8&qid=1469496631&sr=8-3&keywords=signal+generator

author: BMack
author_rank: Regular Contributor
timestamp: July 26, 2016, 06:46:30 am
content: Quote from: CodyRepairShop on July 26, 2016, 01:32:29 amWhat kind of signal generator would this work?https://www.amazon.com/KKmoon-Precision-Dual-channel-Generator-Arbitrary/dp/B014KT3SVO/ref=sr_1_3?ie=UTF8&qid=1469496631&sr=8-3&keywords=signal+generatorEven though I'm not an older tech, I prefer the older equipment. I didn't even bother looking at the specs(I'm sure others can help recommend a good signal generator brand and model) because I want knobs and simple, quick-to-use interface. This was the first one I found on eBay, I'm sure there are better options but I wanted to show you the difference so you can see why these are easier to use:http://www.ebay.com/itm/Wavetek-VCG-Model-116-Function-Signal-Generator-Works-/142056260568I have a digital portable scope that I bought because I got a great deal on it but I much prefer the shop's analog scopes, so I may not be the best for recommendations. I'm sure some will laugh at me but I really like the old Heathkit equipment, we have quite a bit at the shop, older than me and they still work perfectly.

author: tggzzz
author_rank: Super Contributor
timestamp: July 26, 2016, 10:30:58 am
content: Quote from: tautech on July 26, 2016, 12:31:38 amInthiscase my attitude would be that a working 10/20MHz scope should be so cheap that if it breaks you can just get another. Or buy two.The OP will, one way or another, pay to get experience. There's an argument that the first experiments should be fast and cheap so that the OP can work out what theyreallyneed without wasting too much time/money.QuoteAnd check the probes, especially if higher frequency signals are present.

author: Fungus
author_rank: Super Contributor
timestamp: July 26, 2016, 11:38:35 am
content: Quote from: CodyRepairShop on July 25, 2016, 07:40:36 pmYou'll regret purchasingany"handheld" for $200.TheSiglentmentioned earlier would do the job but it's the minimum you should consider. As pointed out: the memory is quite small (still bigger then most old analog 'scopes though). You might also be able to find a Rigol DS1052E if you shop around. This is a lot 'scope for a teeny bit more money. Still over $300 though.An old analog 'scope will work for audio. If you can get one for $50 then it's an option ... but I have no idea where all the people who say "get one for $50" live. They certainly don't sell $50 'scopes around here. I wouldn't pay more then $50 for one though. They're all old and likely to die at any moment (although repairing them might be good practice for your line of work). As noted by other people: You need a signal generator to provide an input to the amplifier while you're looking at the output.As for safety: If you're going to go anywhere near mains equipment then get a decent,fixedx10 probe. Don't use a probe with a little "x1" switch on mains equipment - it's too easy to move the switch accidentally (or for somebody else to move it).nb. You hardly ever need the "x1" position on a scope probe so you won't constantly be swapping probes or anything like that. "x1" is for very special cases (I don't think I've ever used mine in the "x1" position, go figure).A x10 probe divides the incoming voltage by 10, ie. If you have a 100V input the 'scope will only see 10V. Most 'scopes can handle a few hundred volts on their inputs so it's safe to look at hundreds of volts with a x10 probe.Remember: The ground clip on scope probes is connected to real ground. Youcan'tgo clipping it wherever you like or you'll short-circuit things to ground. Only use the ground clip on isolated circuits (usually everythingafterthe power supply). If in doubt don't use the ground clip, switch the 'scope to "AC" mode instead. Ground clips are usually detachable so they don't accidentally touch things when you're poking around inside power supplies.

author: Fungus
author_rank: Super Contributor
timestamp: July 26, 2016, 11:47:15 am
content: Quote from: Fungus on July 26, 2016, 11:38:35 amDoes anybody know why they aren't called "divide by 10" probes?

author: timb
author_rank: Super Contributor
timestamp: July 26, 2016, 11:49:22 am
content: Quote from: CodyRepairShop on July 25, 2016, 11:12:27 pmYou could really do all this with a USB sound card. There's software that will let you use one as an oscilloscope. There's also software to turn one into a distortion analyzer, plus a signal generator!You'll need to create some attenuators to bring the voltage from the speaker outputs on your amp to a level safe for the USB sound card, but that's not hard and can be done with nothing more than a few resistors.

author: Fungus
author_rank: Super Contributor
timestamp: July 26, 2016, 11:53:53 am
content: Quote from: timb on July 26, 2016, 11:49:22 amIf it'sonlyfor looking at amplifier outputs then yes, you do it with a PC and $10 of components (plus a bit of time and knowledge).OTOH owning an oscilloscope opens lots of doors to other things.

author: tautech
author_rank: Super Contributor
timestamp: July 26, 2016, 11:54:52 am
content: Quote from: Fungus on July 26, 2016, 11:47:15 amWork in reverse:Scope input rating x 10.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 26, 2016, 03:32:04 pm
content: Ok so I was thinking about getting me a High Voltage Probe one that is like 100x probe but since I don't know how much power a 10x and a 100x is rated in volts since my av receivers and amps each receiver is plugged into the wall is 120 volts AC @ 60hz and most of my av receiver units are rated 100 watts per channel but I have a amplifier that is rated 300 watts into 4 ohms and 200 watts into 8 ohms all my receiver when @ the 100 watt that's at 8 ohms. I will have to buy me dummy loads and those probes maybe and the signal generator since this onehttps://www.amazon.com/Rigol-DS1054Z-Digital-Oscilloscopes-Bandwidth/dp/B012938E76/ref=sr_1_1?ie=UTF8&qid=1469545654&sr=8-1&keywords=oscilloscopeDoes not have a built in signal generator so I am thinking of getting me this signal generatorhttps://www.amazon.com/KKmoon-Precision-Dual-channel-Generator-Arbitrary/dp/B014KT3SVO/ref=sr_1_2?ie=UTF8&qid=1469545952&sr=8-2&keywords=25+Mhz+signal+generatorsince I don't have money growing on trees I don't do this kind of work all the time.

author: idpromnut
author_rank: Unknown
timestamp: July 26, 2016, 05:01:34 pm
content: Quote from: CodyRepairShop on July 26, 2016, 03:32:04 pmLet's have a quick look then; you'll recall both:V = I * RP = V * Ifrom which you can derive:V = sqrt(P * R)So if we look at the example of a 200W into 8 ohms, you get sqrt( 200 * 8 ) = 40V. This is easily handled by most scopes + probes, but if you want to be safe, a x10 probe will be perfectly fine with this. Note that it is the voltage that is important when looking at an oscilloscope probe's rating, as you will not be passing much (hardly any) current through it (unless you are probing incorrectly and short out the ground clip, for example).

author: nctnico
author_rank: Super Contributor
timestamp: July 26, 2016, 05:02:20 pm
content: Nowadays you can buy lots of cheap DDS signal generators (like the one from the Amazon link) and I think they are great for general purpose use. The output level, frequency, etc are controlled digitally which is a major improvement over the old style generators. I wouldn't go for an oscilloscope with a build-in generator because the cheap DDS signal generators offer much more bang for your buck.

author: Fungus
author_rank: Super Contributor
timestamp: July 26, 2016, 06:28:43 pm
content: Quote from: nctnico on July 26, 2016, 05:02:20 pmYep.You save a bit of bench space but the difference in price simply isn't worth it.

author: bitseeker
author_rank: Super Contributor
timestamp: July 26, 2016, 07:01:38 pm
content: Quote from: timb on July 26, 2016, 11:49:22 amI'm with Tim regarding using your computer's audio output for your signal generator. There's a variety of programs available for generating tones. Alternatively, you can use an old MP3 player, outdated smartphone, etc. to play audio tones that you create on your PC.I'd be a little more wary about using the computer's audio input as an oscilloscope right now, since you're new to scopes, unless you're using it with a very inexpensive computer that is OK to fry (only accidentally, of course).

author: nctnico
author_rank: Super Contributor
timestamp: July 26, 2016, 07:20:32 pm
content: I'm not a fan of the computer & soundcard approach. Software is often cumbersome to use and a soundcard may have sound effects enabled which screw up the frequency response completely. I'd go for the dedicated generator and oscilloscope approach.

author: David Hess
author_rank: Super Contributor
timestamp: July 26, 2016, 09:09:30 pm
content: Quote from: CodyRepairShop on July 26, 2016, 03:32:04 pmDamage from overload of the oscilloscope will not be a problem with audio amplifier output signals unless you are working with 100s to 1000s of watts or more (into 8 ohms) but a x1 probe will only allow 5 volts/div which will not be enough. x10 probes will allow 50 volts/div which is plenty to watch a full power output.QuoteDummy loads are not difficult to make.I agree about an analog oscilloscope. Since you are in the US, it should be relatively easy to find a working one or maybe you have an electronics friend who can help you restore one to working order. You should be able to find something for $50 or less unless you are deep in a rural area.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 26, 2016, 09:43:18 pm
content: How many watts in in that 50 volt since I can't figure out the amps part to convert volts to watts I don't know?

author: timb
author_rank: Super Contributor
timestamp: July 26, 2016, 10:28:10 pm
content: Quote from: bitseeker on July 26, 2016, 07:01:38 pmIn my original post I recommended a USB isolator to protect the computer. They're fairly inexpensive and will galvanicly (or capacitivly) isolate the computer from the sound card.Most of the inexpensive ones use an ADI chip that's good to at least 1kV; the more expensive ones use an LT chip which isolates to a higher voltage, but that would be overkill in this case. (The LT chips are pretty awesome though; they actually etch the isolation transformers into the substrate of the chip!)The other approach would be to hook the sound card to a Raspberry Pi and use some remote USB port software. Basically you install the driver on the Pi and then on your Windows/OS X box; it actually adds a "Virtual" USB root interface. Then, when you plug a USB device into the Pi, all the data is transferred over Ethernet to your PC and enumeration happens there.It's pretty slick and actually works fairly well. I'm not sure how well it would work for a device sensitive to latency like a sound card... But on the other hand I've seen it work with webcams, so maybe it would be OK? I might give it a try this evening, just to see... (I actually ran all my USB test equipment this way; that is I plug them into my Pi and can access them from any PC/Mac on my LAN.)As to the OP and an analog scope, I'm not sure how much that will help him... Sure, he can see clipping, but he can't really do distortion measurements (at least in any meaningful way). Plus there's no FFT...For the $50 + shipping price of an analog scope, he could buy a USB sound device, isolator *and* a multimeter!That would serve him well enough to learn with, while he saves up for a digital scope.

author: CodyRepairShop
author_rank: Contributor
timestamp: July 26, 2016, 10:44:35 pm
content: Quote from: timb on July 26, 2016, 10:28:10 pmI'm not doing anything with my computer it's only a year old and not going to use it for something with this audio stuff not trying to be mean or anything just I don't want to use my computer for any of this stuff.

author: timb
author_rank: Super Contributor
timestamp: July 26, 2016, 11:11:05 pm
content: Quote from: CodyRepairShop on July 26, 2016, 10:44:35 pmThat's not a realistic option, sorry.Keep in mind that professionals who design and repair audio gear almost always use a PC with some sort of external interface (be it an Audio Precision System One, or even a $70 Behringer UM2 USB Audio Interface+TrueRTA software).In the end, to really do what you want, you're going to need to use your PC. You'll obviously need a scope as well, and I didn't mean to imply you didn't.That's why I recommended starting out with just the PC, as you can get some software to turn the USB audio interface into a scope as well. However, if you're not comfortable with the idea of making your own attenuators and such, then by all means just get a cheap analog scope! Like someone else said, having a real scope can open up doors to other areas of electronics.The analog scope will show you clipping in various parts of your amps, and you can use it to get a rough idea of the noise floor, but you won't get any sort of precise numbers out of it.If you want to actually see your THD or THD+N %, or measure actual frequency response, you basically have two options:1) Buy thousands of dollars worth of vintage test gear (spectrum analyzers, distortion analyzers, etc.)Or2) Use your PC and a 24/192 USB Sound Card, plus an analog or digital oscilloscope.

author: ez24
author_rank: Super Contributor
timestamp: July 26, 2016, 11:14:17 pm
content: Quote from: CodyRepairShop on July 25, 2016, 10:41:54 pmHave you tried Craigslist ? I got a good one via CL. I went bust on 2 with ebay because of shipping charges. If you like to gamble go ahead and try fleabay.If you want to probe the AC on back of something, make sure you have good ventilation to take out the smoke from burned probes and scopes. When I did it 30 years ago, the clip lead on the probe turned red then "exploded". You can simulate the experience by taking a piece of wire and sticking it in the holes of an AC socket in the wall (and this will save you money). Use 16ga or smaller wire and use rubber gloves. Make sure you have a good circuit breaker so you do not burn down your house. If you do this, it will make you think twice about using a scope on AC mains, or at least do it outside.Nothing beats seeing the arcs and smelling the smoke.It would be rare to have bad AC (I do during the winter). In my case I am going to use a AC wall wart to look the AC. I would just assume the AC on the primary of a transformer is ok (waveform) and use a multimeter only.Since you may be working on an audio amp - here is a link on how to build an electric guitar to go with it, just ignore the "NOT" in the title. I think he was force to put that in.

author: tggzzz
author_rank: Super Contributor
timestamp: July 26, 2016, 11:53:34 pm
content: Quote from: ez24 on July 26, 2016, 11:14:17 pmNowadays most wall warts are SMPSs. Many, e.g. those used for laptop power supplies, have one output connected to mains earth, i.e. not floating. So make sure you connect the scope probe's shield to the "correct" output, unless you like sudden noises, light and smoke.It is unusual to find a wall wart that is a "naked" transformer.

author: ez24
author_rank: Super Contributor
timestamp: July 27, 2016, 12:10:44 am
content: Quote from: tggzzz on July 26, 2016, 11:53:34 pmYou are right - I have one from the 80's. Also remember the wall wart is AC to AC, Not AC to DCOP and I want to look at AC waveforms.Probably easier to find a door bell transformerhttp://www.ebay.com/itm/NuTone-C905-16-volt-10va-120V-AC-door-bell-transformer-/282084152996?hash=item41ad8696a4:g:hKwAAOSwKfVXJPJqBut here is a AC/AC wall warthttp://www.ebay.com/itm/9V-1-11A-AC-AC-WALL-WART-POWER-SUPPLY-ADAPTER-CORD-MOD-AC-091A-/272294336372?hash=item3f6601d774:g:MjwAAOSwHF1XHBz~You can find more by searching on "AC/AC" I am surprised how expensive everything is.Now I need to go and look for mine

author: bitseeker
author_rank: Super Contributor
timestamp: July 27, 2016, 12:14:28 am
content: Quote from: timb on July 26, 2016, 10:28:10 pmI totally missed the USB isolator. Thanks for the clarification, Tim.

author: tggzzz
author_rank: Super Contributor
timestamp: July 27, 2016, 07:51:11 am
content: Quote from: ez24 on July 27, 2016, 12:10:44 amThat's a sensible suggestion.When I did this recently (for the first time in ~40 years) I was so surprised at what I saw that I went looking for explanations. It appears the clipping is due to the large number of electronic devices that only draw current when the input voltage is higher than that on their smoothing capacitors.

author: Fungus
author_rank: Super Contributor
timestamp: July 27, 2016, 08:30:38 am
content: Quote from: timb on July 26, 2016, 11:11:05 pmOr3)https://www.eevblog.com/forum/projects/analog-disovery-as-audio-analyzer/

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 27, 2016, 09:05:53 am
content: Quote from: CodyRepairShop on July 25, 2016, 07:19:07 pmFor that kind of work you are much better off with a decent inexpensive analogue scope and at least a simple cheap function generator + a good True RMS multimeter. One of the good current options is a second-hand Keithley 2015 - a very nice meter, a quality audio signal generator and a good distortion meter all in one for about $350.For example:http://www.ebay.com/itm/262538247874?http://www.ebay.com/itm/371582307445?CheersAlex

author: ez24
author_rank: Super Contributor
timestamp: July 27, 2016, 09:44:16 pm
content: Quote from: CodyRepairShop on July 25, 2016, 09:11:34 pmI bought this to look at my AC, it is about a 10:1 reductionhttp://www.ebay.com/itm/172255252160I think by now you know it is not a good idea to connect onto the power connections on the back with a scope