Oscilloscope: DC & AC, Signal Tracing in Amp Circuit, RMS Output Measurement

greetings budding oscilloscope experts and welcome to this video in which we will explore several aspects of the mighty oscilloscope first off will be how does it react to DC what does DC look like on the screen and how can we measure it and what better test subject than a 9 volt DC battery connected here to the DC voltmeter and showing an output of exactly nine point seven volts DC now that we know the voltage from the battery I have attached the probe from channel 1 to the positive terminal and the ground clip to the negative terminal I've set the oscilloscope at ground five volts per division channel 1 and turned it on then when I get the horizontal scan I've adjusted it with the position knob so that it exactly coincides with the horizontal x-axis and actually reach an air and switch the bottom switch to DC which is what we're seeing from the battery and you see that it jumps up almost to complete squares if each square is worth 5 volts that means it's almost 2 times 5 is 10 volts which is exactly where you'd expect 9 point 7 volts to show up just below the temple threshold as you can see DC tracings are not all that exciting they're just a horizontal line the vertical deflection from the x-axis shows the voltage of DC it can be above if it's positive or below if it's negative now let's hook up the probe for channel 2 over here to our audio signal generator we connect the probe itself to the red output and the ground clip goes to the negative shield set it to 10 times 100 which is 1,000 cycles per second and I've turned on the signal generator now to see the sine wave from the audio frequency generator we'll come over here go to channel 2 and we see it on the scope we can use the amplitude adjustment here to set it to about the right height which I'm gonna say is a total of 4 squares high and we can arrange its position vertically so that the midline of the sine wave the zero point is right along the x-axis my channel two bowls per division are 0.5 I'm in alt mode and down below here I am in AC mode and the seconds per division setting is 1 milli second per square that way I have one complete waveform per square on the x-axis which since there are 1000 milliseconds in a second means there will be 1000 waveforms in a full second so this coincides to the 1000 cycle per second output from my audio frequency generator okay so now I've got the AC sine wave on channel 2 I've got the DC signal trace here on channel 1 so let's show them both now when I switch to both you can see it you can see the DC tracing here you can see the AC tracing but they're strobing at you okay here is why it's doing it and how you can stop that when you're asking the scope to draw two different tracings at the same time it's the same as if I you to draw two different pictures on a piece of paper with one pencil at the same time it's very difficult the scope is fast though and it can jump around and actually present to separate those scope tracings at one time if they are of high frequency and if they are of high frequency the alt setting will freeze them 1000 cycles per second is very low frequency and the DC has no frequency so to stabilize this scope image we go to chop chop is for low frequency simultaneous display of two different waveforms in fact a good rule of thumb is if your seconds per division control is that one millisecond or greater you will use chop and if it's at 0.5 or shorter then you'll use alt now we're in a position to actually witness something that a lot of people do not realize or doubt that can occur and that is that AC and DC can coexist in the same wire the same circuit at the same time they don't interfere with each other they don't mix and get all muddled up instead they coexist sort of like oil and water now at this moment they're not mixed one probe is showing us the DC the other probe is showing us the AC okay but if we did mix them what would happen is this the AC would act like oil and float on top of the DC water in other words when they're combined the AC is going to be elevated by an amount that exactly represents the amount of DC voltage the being simultaneously presented with it now if you want to see this to convince yourself it's true reach over here and flip this to ad now we put the two together they're mixed and look what happened the AC tracing jumped up so that the center line of the AC tracing was elevated by the amount of DC that was added to it okay let's see it one more time we're going to go from chopped over here to ad and we watch as the AC waveform is buoyed up by the DC which is beneath it now this is very important when you're tracing signals in amps circuits because you know very well at AC and DC coexist in amplifier circuits when you're going down the line of amplification if DC is present can you imagine a plate voltage of 350 volts or so if what nine point seven volts lifted the waveform that hi can you imagine what 400 DC volts are going it's going to lift it up to the ceiling okay so you might be going along tracing your signal through the amp and the signal disappears now you could think oh well now I've found the breach and the signal change so this must be what's wrong no all what's happened is is the DC has elevated your AC signal out of the range of your scope screen so this phenomenon is something you have to be aware of now there's only one more setting that we need to discuss before we're ready to start analyzing amplifier circuits and that's this one right here the X 1 and X 10 now that should be familiar to you because we've seen the X 1 and X 10 setting on our probes this is for the voltage deflection null which is the vertical displacement this X 1 X 10 is for the hole until deflection now what happens when I shift from X 1 to X 10 is this instead of 10 waveforms on the screen I now see just 1 so it's very helpful if you want to magnify one waveform or if you want to enhance the total width of the x-axis but you better remember that you did this because otherwise you're gonna look at this and say let's see I've got one waveform per screen width and I've got one milli second per square therefore this is a hundred cycles per second no it's not because you shifted over here two times ten it's giving you a the idea that it is but it's not go back two times one you see there's ten and you know that one cycle per square is at one milli second per square is going to be a thousand cycles per second so don't be fooled okay if you do go up two times town be sure you remember to correct your frequency calculations Jack are you hiding somewhere here in the bathroom are you I I can't see you no I guess not there's a lump over in the tunnel oh well they're a it it's not easy for a black cat to hide on a white sink but the master of concealment finds ways to do the impossible well the time has come we've all been waiting for we're going to trace the flow of the signal through the circuit using our oscilloscope first let's briefly discuss the preparation number one you might consider buying something for your workbench and that is a dummy load this is a big heat dissipating 8 ohm resistor that I'm going to use instead of connecting the circuit to a speaker now the reason for that is if you've ever to a 1000 or 2500 cycle tone at fairly high volume for any like the time you'll understand why this is a real good idea okay you won't have any noise the amp will be dead silent yet this resistor will be absorbing all of the output power these are generally available on eBay at reasonable prices this is a single 8 ohm you could get to 8 ohms and then you could cover all the basis series for 16 parallel for 4 single for 8 next step I'm going to plug the amplifier circuit into a current limiter and then plug the current limiter into my isolation transformer using a three-to-two adapter so that the current limiter and the amp circuit will not be grounded okay the next step and this one is optional but it's very helpful I use a green magic marker to designate the grids of each of the tubes that I'm going to be monitoring with the oscilloscope this saves time and it's also a little safety feature so that you know exactly what points in the chassis you're connecting your probes to now why you may ask do we use the grids for our probe attachment rather than the plates well if you remember in the first portion of this video we discussed how high DC can elevate the AC signal perhaps even out of the scope range so we're going to use the coupling capacitor to block the high DC so that will have AC only on our grid to visualize on our oscilloscope also if we did see any DC elevation of the AC signal after the coupling cap that's a great indication that the coupling cap is leaky and it's time to replace it next step either make or buy a cable which will connect to your signal generator using a shielded cable and input a signal with a quarter-inch jack so you can plug it in to the input jacks on the amp next we'll install a probe and channel one of our oscilloscope ran over here and connect it to one of the two grids of the six sc7 the probe is set to x1 the ground clip I'm going to use the same ground that the input Jack used next I'm going to set my signal generator to ten times a hundred or thousand cycles per second and turn it on now you can set yours to 2500 3000 wherever you want to set it the end result is going to be about the same but this is strictly an arbitrary number that I've picked okay now we switch our channel 1 to AC and we get a readout here which we can adjust the vertical deflection using our bolts per division knob I'm going to keep this fairly short for reasons that you will see I'm at one volt per square so that's a plus one minus one AC sine wave that I'm sending into the amplifier you can adjust your amplitude down here so that it is exactly plus a minus one square you can use your vertical position to Center it along the x axis now we set channel 2 to exactly the same settings as channel 1 AC 1 volt per division okay then we connect the probe it's set to X 1/2 the grid of the next tube in the amplification chain which is going to be pin 5 of the 6 J 5 right here I've connected the ground clip to exactly the same wire as the first ground clip so I have no foot tential between my ground clips now take a look at your sculpt tracing you'll see it looks exactly like the input now reach over and turn up the volume all the way so you're not attenuating the strength of the output signal now compare that to the input signal you see a tremendous increase in amplitude that is the gain of your first amplification stage now if you'd like a direct comparison between the amplitude of the input signal and the amplitude of the signal after one stage of amplification just go to both and you can superimpose them the puny little input signal the gigantic signal it's being put out by the first triode of the six sc7 you see what gain is all about look at the tremendous increase in amplitude now besides letting us see this which is a wonderful benefit of the oscilloscope we can also go in and actually measure how much gain took place so let's do that remember that our initial gain right here is about one volt per square so we're plus or minus one volt that was the input signal okay let's switch just to channel two and adjust the amplitude of our one stage of amplification signal down to where we can actually see the top and bottom of the sine wave also notice that clipping is taking place here which is typical when you turn any amp up to full volume you're gonna see some clipping okay but let's measure the voltage that's present in the amplitude of this amplified sit to do that we'll adjust the position to where the wave is centered and we look at our volts per division setting which is five volts per division so I'm gonna say that it is about eight volts would you say five plus three is about plus or minus eight volts so after one stage of amplification our signal is eight times greater than the input signal okay that's the result of one stage of amplification an increase from plus or minus 1 volt to an increase of plus or minus 8 volts now I turn off the amp and with one hand I set the channel to probe on the grid of the 6v6 tube so we'll see now what is the strength of the signal after 1/2 stages of amplification ok I turn on the amp and I look at the incredible increase that has happened after 2 stages of amplification remember we set this to 5 volts per division and shrank it down to where it was about 1.8 squares now look at it do you see a more gayness taking place as a result of the 6 j52 well exactly how much gain do we have now after 2 stages of amplification you ask and I say well let's find out now we're pretty well maxed out on the x 1 probe so step one here to find out the actual two-stage gain is we're going to switch our probe 2 times 10 and you see what's happened here it's clipping now top and bottom because it's a stronger signal and it's more prone to clip let's adjust the curve so that it is exactly centered on the x-axis and it looks to me like it's 5 times 10 remember its times 10 on my probe 50 volts plus and minus so after 2 stages of amplification my signal is now 50 times stronger than the input signal here's something else that the school can do for you do you want to see when the onset of distortion occurs here in this amp I'm gonna crack the volume down to about half volume a little over half and you see on my curve as a nice the peak and trough that's a smooth curvature then run the volume back up until it starts to square off and look at the volume setting that creates that that is where the preamp distortion is going to begin in your amplifier at that volume setting when you crank it all the way up it doesn't get all that much louder does it but it sure distorts so for about one third of the volume rotation I'm not getting that much more volume but I am getting some pretty significant distortion so it appears that our input signal of plus or minus 1 volt was stepped up to plus or minus 8 volts by the 6s c7 and then by the 6j v stepped way up to 50 volts plus or minus being applied to the grid of the 6v6 now let's use our oscilloscope to measure the rms output power of our mighty true sound ain't first remove all the probes from inside the chassis the amp is turned off we come over here now and connect channel ones probe set to x10 2 with the probe connected to the positive speaker wire and the ground clip connected to the ground speaker wire if you have any trouble figuring out which is which just see which wire has continuity with the chassis that's your ground speaker wire okay we have the volume set to 0 the times 10 probe correctly aligned here on our ballast resistor a dome representing the speaker we turn the amp on let it warm up we see the position on the screen of the zero volume line and now we're going to reach over and start to crank in a little more volume and watch what happens here we're gonna get stronger and stronger signal until distortion starts to kick and see how the peaks are starting to sharpen the side is caving in on it we're getting pretty good distortion above half volume I'm gonna back off to a little over half volume okay that looks pretty good to me the peaks look nice and symmetrical let's position it up to where it is centered about the x-axis and now we're going to need to measure and determine the amount of voltage represented by the amplitude of this undistorted sine way okay I'm set to point two volts per division because microbe is times ten that's two volts per division so it's going to be plus two for plus and minus six volts will be the excursion here of our the AC waveform now if you're wondering how the plus or minus 50 volt input signal to the 66 comes out to the speaker as a measly plus or minus six volts will remember what output transformers do the output from the six v6 which was very high voltage very low current went into the output transformer and came out just the opposite very low voltage in this case plus a minus six volts but very high current so that it could drive the speaker okay let's use the measurements we just obtained callate the rms output power of our a little true sound amp the load resistance is 8 ohms that is not an inductive load it is rated with a DC resistance of 8 ohms the voltage 6 volts DC we got from our scope amplitude RMS output formula is peak voltage squared divided by load resistance times 0.707 why do we do this because I told you the scope gives us peak voltage ratings you're used to having your multimeter convert your voltages to RMS the oscilloscope does not do that for you you have to do it yourself so this is a correction to correct the peak voltage to RMS voltage we end up with 6 squared divided by 8 times 0.707 which is 3.2 watts RMS output power now before you start groaning and moaning too much remember this is what a 65 year old amp operating on a used 6 v6 and also this is pre-distortion output power now if we were willing to crank the volume all the way up wide open we'd get a higher output power but it would be with a great deal of distortion and may not even be usable so this is the pre-distortion rms output power of this little amp and that's 4 well I guess that does it for this two-part video we had ten minutes of theory and then about 16 minutes of hands-on practical use of the oscilloscope for testing amp circuits in the future I'll be posting videos showing other things you can do with the oscilloscope to troubleshoot amps and test them if you found a little true sound amp that we used as our test subject interesting it was a gift from a very generous viewer who sent it to me to see if I could fix it up and use it in a video and I did in fact it's going to be the star of it's own video in the near future in which I show the step-by-step a restoration of it from the way it arrived to the way you see it now I would like to express my heartfelt thanks to all the patreon patrons and PayPal contributors who are keeping this channel on the air and advertising free I'll include links in the video description for those of you who would like to join the numbers of contributors and help us stay afloat meanwhile thanks so much for watching I hope you at least will subscribe and stay tuned for future videos because they are on the way we'll see you then

Channel: Uncle Doug

Views: 49,046

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Id: PmVq7NLv31s

Length: 27min 17sec (1637 seconds)

Published: Wed Jun 27 2018