Oscilloscopes For Audio 101 - Part 2 - Safety and Precautions 

Really appreciate you doing these videos, Mark. Can't wait for the next one.

I never felt the need to comment on one of your videos before, but this time was different. I have been working on vacuum tube circuits for at least five years now and am a EE by trade. I think there is a HUGE disconnect between what people think about measuring inside an amplifier as it relates to solid state devices and you captured that really well. You follow a very similar approach that I do, by doing tests at the DC blocking caps, and I know a lot of engineers who fail to get that concept so I wanted to thank you for speaking about that very clearly and in a way that people without a classical background might understand. I think a lot of people get tripped up on how at DC a capacitor is an open and the inductor winding of the transformers are a dead short with some resistance. Thank you very very much for laying out in a graphic the points you would recommend scope testing, because I think this is the hangup a lot of people have when it comes to using a scope: where can I probe within range of my probe rating or my Vpp input on the scope front end. Anyways, always have enjoyed your videos and thank you for putting a different spin on oscope use, because sure we can turn prospective hobbyists to EEVBlog or others, but you summed it up short and sweet. Keep up the good work!

Another awesome instructional video, Mark...Many thanks!

Fantastic! Thank you for taking time to put these together.

Thank you very much for this video series. Exactly what I need to know. Looking forward to the next episodes. Greetings from the Netherlands 😃👍

For what it is worth I have watched Dave's video on oscilloscopes 2 or 3 times now. I appreciate a different approach on the subject because we all don't learn the same way. :)

Much enjoyed Mark, so clear and easy to follow.

Absolutely amazing I'm just starting out and feeling overwhelmed but you simply explained so much please keep the videos coming thanks

A masterful example of simplicity and thoroughness, and another thoughtful introduction to your course on scopes. Many thanks for your time and professional spirit to teach others what you have learned.

Thank you so much for this! I've been tinkering with electronics for decades and never knew some of these basic safety rules. I think I've just been lucky so far! Much better informed now.

Thank you for the lesson. Actually I learned a lot of troubleshooting techniques with this lesson.

This awesome video brings me back to sixth grade electronics class having to learn safety before we were allowed to use the scopes on high voltage. The difference between then and now six graders working with high voltage in class.

Thanks very much for this series im learning lots and explains very well how ground works.

I'm really enjoying this series, and look forward to the coming installments. This is all great review and every time I watch your videos, I pick up some new tips. Would you please say and show a little more about that 'amp clamp' on your bench? That looks like an awesome piece of kit.

Great video. Thanks for showing how to actually practically probe with an oscilloscope and what differential probes are and how they work.Thank you😁

Mark, thank you for s great video. I've been using a scope for many years, I wish I would have had this type of instruction as to how to use the scope correctly and safely. 👍👍 Understanding the barrel of the BNC is at chassis ground was one of the most important things I learned in "A" school. Aligning synchros transmitters and receivers required the use of a scope or a VTVM to measure the 120 volt synchro voltage. Again great video and thank you.

Great video, love this series!

Thanks.. After years of experiences you always have time to learn more.

Great video as usual. I am not an electrical engineer but somehow when you explain it, I get it.

Another great learning video, THANK YOU!

That was a nice explanation. Congrats!

That wood test bench setup is awesome, lots of adjustment to accept many units, thanks great video.

Great video. Looking forward to the trouble shooting section. Mainly finding causes of distortion.

THANK YOU exactly what i was looking for !!!!!!!!!

Looking forward to this! Just picked up a couple of dual trace scopes from alocal radio club to play along at home

Mark, I am enjoying this series. I hope the next episode is coming soon. Dwayne

Great tutorial! I mainly test a lot of rf. Transceivers, etc.. I really hope you can show a little of that testing in one of your upcoming vids. Thanks!

I have watched several videos on isolation transformers. I always left with questions. Not this time. Excellent presentation!

Thx I worked in a repair shop but seldom did anyone use a scope. I kinda taught myself and have some audio equipment to fix so your video is going to be very he!Paul.

Thank you for another great video !

Great video, thank you for sharing your expert knowledge Mark..I'm learning all the time..Ed..U.K....😊

Great lesson. Real keeper, thanks.

Very glad the first video in this series popped up as a suggestion since I'm looking into getting my first scope soon to work on some old radios. Enjoying the series a lot so far. One thing to watch out is some isolation transformers have a continuous ground which more or less defeats the isolation part. I had to modify mine to make it truly isolated. I'm tempted to put a toggle switch in so I can lift the ground when needed and also have it there when needed.

Omg thank you so much! You finally answered a question I had when your chassis ground is not the same as circuit ground

Great info to keep folks safe. I do work on vacuum tube equipment but I'm used to working with high voltages and current. I worked in and around Ion Implanters, which can produce 150K volts at up to 40mA. Needless to say, they can kill you before you hit the ground. I worked mostly on semiconductor manufacturing equipment with power supplies rated for up to 1500V @ 25A. Those could be quite painful too. At least for an instant.

You are a very talented teacher. Thank you.

Very good informations. Want to add, that there are also scope probes available 100:1 with max 2kV. They can be used to measure any B+ directly without differential probes - of course with all safety measures.

Fantastic Video! Thank you.

Brilliant - thanks Mark

This series on videos on oscilliscopes is gripping. Thanks you so much for making and sharing them. One question though; The reference to ground is understood, but what is meant by the signal ground, and where does it exist in DUT. Thank you.

I learned somethings during my professional career when working around electronic hardware. First, we were required to wear anti-static labcoats,, which is a good idea anyway. Second, whenever I was working with high voltage circuitry, I kept my non-working hand inside my labcoat to eliminate a current path through my body.

Great advice !

Thank you for that video its very helpfull for beginners.

Very nice video. One minor point at time: 46.48 you mentioned there are two ground connection lugs. The Rigol Oscopes have the chassis and earth grounds connected together, they are not separate. On my Rigol the top lug is for probe compensation and the bottom one is ground (chassis/earth).

Wonderful set of videos so far! Will most of these instructional videos apply to my early 1970s Bell and Howell model 34 scope?

Gary has it right. But I also mentioned the recepticel, they have a mounting point for the green wire but that is also connected to the mounting strap so the 2screws attaching it to the case provide a ground connection. So out of the box the green wire is to the case and the recepticel so removing the green wire from the outlet isn't enough, the recepticel has to be isolated too or swapped out for a hospital grade to which the strap is not internally connected. The typical iso made for general use is basically for transients and noise, not for electrical repair, you have to make those mods yourself.

Great series. A video idea, vintage resistor wattage. I have been trying to find a video on identifying wattages of vintage resistors. There isn't much. I have a resistor that is 1 1/4" long by 1/4" diameter 500 ohm. There isn't much out there to help identify the wattage by physical size that I could find.

At 11:30, you sound very passionate describing the ac signal!

Man that amplifier holder looks awesome!!

Great video! What’s that neat contraption you have that you’re mounting your DUT on? I’ve been needing something like that to relieve neck strain caused by leaning over all the gear I work on.

Nice 101 on scope use with tube gear.

thanks for the lesson. bought a siglent scope to work on electronics but since i never used one i am searching a way how not to blow it. i understanded more your video than the eevblog. i have one question how to know that your power supply is isolated? i bought a SIGLENT SPD1168X

Great information. You covered the scenario of connecting scope to a DUT connected via an isolation transformer. If the scope ground clip is connected to the chassis in such case, wouldn't it still provide path to the earth ground? Can you please clarify this, especially how the scope ground clip should be connected in a case where DUT has an isolated chassis ground provided by the negative of the bridge rectifier on secondary side of the power transfer. Thanks.

I've got a question about "isolation transformers." Santa brought me a new (off the shelf) isolation transformer, but when I checked, it's output grounds (and neutrals? maybe) were all directly connected to its ground pin of the plug. When I looked inside, I saw that it has _five_ internal connections directly to its chassis ground lug. An EE where I used to work recommended not changing anything, but I've seen some RU-vid videos recommending disconnecting some of the internal "ground" wires in order to convert a "commercial" isolation transformer (IT) into a "Technician's" isolation transformer. What do you recommend possibly changing in order to convert a commercial IT into a Technician's IT? Mine appears to be more complicated than the ones online where only one wire was disconnected in order to make the change. My IT is a Tripp-Lite 500W IS500 unit.

Can you go over the grounding procedure for chassis probing in rf frequencies at some point? That is what I am interested in working on.

Thank you for the great videos. Q: if I have a hand-held battery-powered digital oscilloscope, that one is safe to use in situation where a differential probe is otherwise required, e. g. for probing a bridged amp output, right? Or not?

Thx for your vids. Why are we creating so much voltage from the plate and B+ if we'e just going to lower it at the output transformer?

For use with solid state amplifiers, what size (in VA) of isolation transformer would you consider sufficient?

I see you have a Mojo Tone adjustable project stand. Nice!

Good videos! How does that amp mount work? Where to buy one?

Anyone have thoughts on the proper construction of a isolating transformer? Also great vid, thank you.

Great video. I been nervous to use my good rigol scope in my amps. Not a big scope user but I got one in case I need it. Was just using it for simple audio circuits with 2n3904 and the like. I have a question. Is a scope a good device to find leaky coupling caps? Should I just use a DMM? I have a amp that is troublesome. I am wondering if I have leaking DC that's being put on the grid effecting bias. Red plating and screen glow. I banged my head on this amp years ago and gave up. I took it to a tech and he said it was just biased wrong. Well checking his work he biased my EL34's to 23ma which is 40% or so. I should be somewhere in the 35ma range. I have switch the transformer to a 460v model instead of 500v in hopes it would help and it did but the tubes grids would glow still. I think thats why he biased so low and just sent it out the door. Didn't want to really troubleshoot it.

I have an old Tectonic 2225 with the ground wire input. I can’t find where to purchase the 3’ ground wire that you show. I would like an alligator clip on the other end. I intend to use this setup with an isolation transformer.

I do not understand @41.30 did you just ground the unit under test defeating the purpose of the isolation transformer? Thanks great video.

Where can i find the stand to hold that amp for the bench test?

Thanks, does anyone have a link to some rubber gloves for electrical work? I've always wanted that extra layer of safety when troubleshooting 450V tube amps. All I can find is lineman stuff for HV work on power lines.

I'm used to working with electricity, but I'm new to oscilloscopes. My old house consists of ungrounded outlets so I use a lot of g.f.c.i.'s and I use an isolation transformer for DUT. Can you comment on if there are issues that have to be taken into account when performing tests with the scope under this scenario (before I start using this for testing). Thanks!

Do you place your isolation transformer before or after your variac? Does it matter? Thanks

Mark suggests to watch the other videos about isolation trans. His is 2 wire but new will be 3wire so typically you disconnect the green from the output side. Bear in mind the mounting strap of the outlet is tied to the ground prong which also connects to the case (or frame) which will still be connected to earth.

One detail not mentioned yet in relation to the voltage limits in scope inputs and on probes... The video implies that the peak voltage on the anode of an output tube will be close to the HT voltage. However, for non-resistive loads, that is only true under no-signal conditions. The anode of an output valve/tube may see peak voltages far higher than the HT rail. This is may be obvious in a push-pull output stage where one valve may be non-conducting and the anode goes positive as part of the transformer "swing". It is less obvious in the single-ended case, where an inductive load may drive the anode positive above the HT rail as the magnetic flux collapses due to reduced drive from the valve. In this case the transformer or choke will attempt to maintain the previous current level. Thus an amplifier with a 400V HT may see peak swings on the output valve from (say) 100V to 700V. Under no-signal conditions the anode will be at a DC voltage of just under the HT voltage as a result of the DC resistance of the winding (which is much less than the reflected load from the loudspeaker coil). Simply put, with inductive or transformer loads, everything may be fine until a large signal goes through the amplifier and takes out a probe or the scope's front end.

If I had to test one on the go when I’m out and about can I just bring a phone and the appropriate cable and just run some simple sound waves off RU-vid through it to see if it’s working ?

Getting more confused...I am looking for a video that shows me how to get an audio signal (some sound....whether generated by an electronic device or just an un-amplified sound such as dropping a book on the floor) into an oscilloscope so that I can see the sonic waveform (say, the composite shape of the wave created using two oscillators on a synthesizer). Is this video suggesting that I have to get into an amplifier and probe around to do that? I am about 2 hours into these lessons and still don't know what the end product that you are showing us will look like, so I don't even know if this is the right set of videos for me.

You refer to a chassis ground and earth ground on the lower right hand side of that Rigol. On my Rigol one is earth ground and the other is the source of the 1KHz signal for your probe compensation setups. I bet they aren't both grounds.

So the test stand, made from plywood that is holding the amplifier? Do you have plans anywhere for this test stand??

so now I'm wondering where you think that 15khz noise could be coming from. just in the air since the bottom of the amp is off and its inerds are exposed ??

If I was to use (differential scope probes) would this act the same as a isolation transformer.

Will a handheld oscilloscope be safe and avoid grounding dangers and use of differential probes ?

Where did you get the Amplifier mount from???

Okay im a bit confused. If both the oscilloscope and the device are earth grounded, dont you have to isolate the device (or the scope)? For audio gear you can just not use the grounding clip in this situation and it’s fine? What is the signal being compared to in this case?

Quick overview. Are you running the isolation transformer off the variac ? In other words. 1. Variac off house mains. 2. Isolation transformer is then plugged into variac. 3. Device under test is plugged into isolation transformer. Just so everyone is clear.

Would it be worthwhile to use a battery operated Oscilloscope when working on tube equipment?

Did I miss something? Is there a "Oscilloscopes For Audio 101 - Part 1"? Recently subscribed, love the content and presentation. TIA!

Nicely explained, but since the oscilloscope is grounded, again the chassis from the DUT is connected to the ground via a green cable with an alligator clip, so again there is a risk of electric shock if it is accidentally touched by hand even though an isolation transformer is used 😉🤫

One dumb thing that I did when biasing a guitar amp: i was sure to use one hand in the amp but i was holding the plugged in guitar in the other hand. Yeah, the strings are grounded and I took a small shock. Don't wear a guitar while working on your amp even if you have to hear it play. It's too natural to grab the guitar neck and ground yourself.

why not the oscilloscope use an isolation transformer , you said its a bad idea? could you please explain.......

Is that a diy stand/holder for equipement? I mean the one holding the amplifier

Some variable transformers DO have isolation.

It may sound a little stupid but; if you're just isolating the DUT from earth or ground by cutting the green wire in an isolation transformer (3rd outlet port) why not just use a 3 to 2 prong adapter into your house outlet? With a 2 prong outlet there is no wired earth ground (just HOT & NEUTRAL). This has never been shown or listed as a possible scenario in any of the videos I've watched. They all seem to be 'pushing' expensive isolation transformers.🤔

I have a humming toroidal transformer in a DAC. I suspect there’s DC on my 120V AC line voltage. I have an oscilloscope, a DMM, and an audio signal generator. How can I measure DC voltage coming out of my wall outlet? Seems to me it’s garbage in, garbage out in audio. I really want to analyze my AC voltage and any DC riding on the AC voltage.

For a first scope for testing audio, would you get a analog or digital scope?

Please leave your warning message in the beginning up for at least 5 seconds so we can actually pause the video. ;-)

2:15 Voltage is the hypodermic, Amperage is the toxin, but the path is what's critical.

You started to talk about the reason for using a VARIAC - but then ended up discussing it was not an ISOLATION TRANSFORMER. Why would you need to use a Variac instead of your normal Mains Voltage Input?

I've gotten shocked several times in my life with 230v, not fun. There's also 400V three phase around here, but fortunately not for any audio equipment!

High voltage I was standing next to my teacher hen he touched the h.v. in a tv set 25,000 volts. He jumped around and yelled but wasn't hurt. Anyway I'm not saying its safe to play with.

I was badly shocked when I was a stupid kid and didn't know better. I tried to separate the hot and nuetral running a electric motor sitting on a concrete. floor As I tried to stop the short , I got between it, BANG and it through be back about three feet

The noise at the end of the video could be WiFi and /or POE …

Please explain why it is relatively safe to ground your scope to a chassis under test that is isolated but the scope is grounded to earth. Does this action of grounding your scope not defeat the purpose of an isolation transformer? That question is answered here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-vz70GSBhYjA.html

I recall a video on YT noting for audio work, a non-induction dummy load was not really an issue and the cheaper wire wound resistors were fine. There was a bit of math involved to prove the point. If I find the video, I’ll post a link, but if this is true, it can save a bit of cash. Anyone else come across this? Found the link ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5hRwIRpxc0Y.html

Good info, indeed, and should be watched several times... I have the same oscope and did the chip reprogram, nice kit, indeed.. " Green Earth Ground" is part & parcel of the 'Green New Deal', is it not..?? LOL! Earlier and later in the video you are reading the grids (low vac), but, at 30.37 into the video, you are clipped to plate (high vdc) at the pt.. Why..?? The one fisted bite travels from the hand through the body to ground, while two the fisted bite travels across the chest and through the heart and is "Defibrillation" at it's best..! LOL! Talk about 'jarring your preserves' and putting lead in your pencil..! I've seen boob toob techs drape their arm across the chassis while taking vdc measurements w/the other.. Yeah, baby..!! Some wear rings, watches, etc...!! In the past I have asked them ('the guitologist' comes to mind, LOL!) if he/they post the videos of their being shocked and electrocuted..?? "Shockingly" , I never receive a reply from the 'electrocuted', but DO receive replies from boob toobers defending the amateur tech. boob toobers, ya' don't have to luv them. LOL!