Basic Oscilloscope question

[Froumy]

Greetings all-
The wife was kind enough to pick me up a scope this weekend. I've used "virtual" scopes on the PC, but never a real one. It's a B&K 1541c 40MHz dual trace. Before I blow it up, I figured I ask a couple of questions. It has 10x/1x switchs on the probes. I'm assuming that if I use the 10X and measure 500 volts the scope will only get 50V. Is that correct? Or is the scope still going to get the 500v? It says its rated at 250v and I don't want to fry it. I'm assuming that if I leave the probe on 1X, the full 500v will toast it.
Satanmax posted that he used the pc based sweepgen program for a signal generator. What's the downside of using that as opposed to an actual generator? Can I use it coming straight out of my PC? Thanks in advance.

[bob p]

Greetings all-
The wife was kind enough to pick me up a scope this weekend. I've used "virtual" scopes on the PC, but never a real one. It's a B&K 1541c 40MHz dual trace. Before I blow it up, I figured I ask a couple of questions. It has 10x/1x switchs on the probes. I'm assuming that if I use the 10X and measure 500 volts the scope will only get 50V. Is that correct? Or is the scope still going to get the 500v? It says its rated at 250v and I don't want to fry it. I'm assuming that if I leave the probe on 1X, the full 500v will toast it.

a 10:1 probe has a built-in voltage divider, so the scope actually sees 1/10 of what the probe sees.

a selectable switch could be dangerous for use with high voltage tube gear. i would set those probes aside and use them only for low voltage applications, and replace them with non-switchable 10x probes. having the switch accidentally set to 1x could spell certain disaster.

some people could make good arguments for using a 100x probe.

[bob p]

Satanmax posted that he used the pc based sweepgen program for a signal generator. What's the downside of using that as opposed to an actual generator? Can I use it coming straight out of my PC? Thanks in advance.

I have an old Hitachi 60 MHz dual channel delayed trace scope. i prefer a real portable signal generator. i bought two, an older HP 3311A and a Tenma sweep generator. both are SS and are rated to 1-3 MHz.

i prefer these units for a couple of reasons. perhaps the most important is that they are portable units, so that you are not confined to working on something next to your PC. i can't tell you how many times i've taken advantage of the portability feature of my gear. when i'm doing diagnostic work, a lot of it gets done in places other than on the testbench.

[Froumy]

Thanks for the Reply

I thought that's what was going on. I just wanted to be sure before I blew it up. Thanks Bob! Good advice on the probes, too. I only occasionally tinker with my amps, so I'm trying to get away with this cheaply. I'll probably just tape them on 10x.
As far as the sweepgen program goes, I'd keep it on my laptop , so portability wouldn't be a problem. I'm sure an actual generator is the best fit, and I'll get one if I have to. I'm just wondering what are the issues using a program as opposed to the real thing. Will going from my soundcard into the amp be useable, or am I just begging for headaches?

[bob p]

you might get lower distortion figures with one or the other. not really a big deal unless you have a distortion analyzer. by the time that you have one of those, i'd bet you'd already have a real signal generator.

[Froumy]

Sounds like I'll be OK for starters, thanks again. Now I just have figure out how to use the thing...... Plenty of posts around here that should lead me in the right direction. Should be fun.
I'm guessing that if I want to track down a noise issue, I just put a tone at the input, and check out the wave form as it moves along the signal path?

[Enzo]

Well...
Apply a signal at the input and trace it along to look for distortion of the waveform, or in some cases look where it goes away completely.

Noise is unwanted signal other than the signal you apply to the input. Like hiss, hum, or pops and crackles. Having a test tone in the circuit while looking for those would only get in the way. If you were listening for a rattle in your car, you would not be playing the radio at the time - it's like that.

There are some things like parasitic oscillations, that are noises i suppose but are only present with signal. For those you would apply a signal.

[bob p]

i have to admit, i bought a 'scope thinking that it would be the end-all to my diagnostic problems. now that i have a 'scope, one of its greatest benefits is that it has taught me to learn how to use my DMM more effectively. having the scope is really helpful; as they say a picture is worth 1000 words. but sometimes you can get helpful information more quickly with an auto-ranging DMM. i was surprised that the owning a scope taught me how to use my DMM more effectively. i hadn't expected that. of course, you may know all of this already, and i might have just been a slow learner.

[Froumy]

Bob, I'm just getting my feet wet with ALL of this stuff. If using a scope makes me better with my meter, then I'm all for it! My 7 year old played with the scope for more than an hour to see if he could make the lines change. As for me, well, yeah, it's kind of like that, too. Hopefully I can put it to good use...
Enzo, thanks for the response. I may have been thinking distortion of the wave as opposed to noise. Glad I typed "noise." You wound up answering two more questions that I hadn't thought of yet. Thanks. Very helpful. I would have been looking for noise with a signal on, too.
My current build does have a little hum that I'd like to clean up. (jtm45/1987 (perhaps swichable to a 2203, eventually....)) I'm assuming grounding scheme. But first, I need to get the normal channel up and running. Then I'll try a virtual center tap on the heaters. I should be able to tell if the hum is 60 cycle or 120 with the scope but I have a feeling that it will not be useful in determing a poorly designded grounding scheme. Or will it?

[Mars Amp Repair]

Froumy,
I worked in consumer electronics repair for some 30yrs. I used scopes in VCR alignment as well as camera & camorder signal tracing. Once you learn what your looking at or what to expect in a certain part of the ckt, they are invaluable.

Amps don't have the complex waveforms that you see in video waveforms & video tape path & camera alignment, etc, but to be looking at the speaker output to determine what stage is causing say for instance the waveform to be flattened on one half or signal tracing noise, is where it becomes very important.

You can 'see' that with a DMM, eventhough a voltage you stumble on may tell you that a plate resistor is bad, a quick walk-through the amp with a signal generator on the input & scope going from grid to plate to grid to plate etc. can be quicker & more educational about how that particular amp works.

Another sometimes overlooked piece of test equipment that can also be very valuable esp if you have no lit on the amp is an audio signal tracer. I have made mine out of powered computer speakers ( bought for near to nothing & quite plentiful at thrift stores like Savers & Goodwill) coupled with magnetic phone amp pre-amps. you can also purchase a commercially made one for around $150, but that's wayyymore than I'd pay for one. You do have to reduce the value of the input coupling cap to keep from jolting the 1st stage with a couple of hunderd volts when you put the probe on the plate, but that's easy enough to do.

Play with that scope..you'll find it can be very helpful once you get the hang of it. The audio tracer helps where you really need to hear the quality of the sound & a visual doesn't always do that for you.

Have fun! glen

[Enzo]

Here is something for the 7 year old. COnnect a microphone to it and let him watch what his voice sounds look like. MAybe get him to explore the difference between oooh and aaah, and eeee...

[bob p]

i still remember when i was about 10 years old and my neighbor's big brother hooked a microphone up to an echoplex tape unit and piped it through his stereo. after that, i was hooked!

[Enzo]

One of my favorite pranks I used to do when I was on the road about 35 years ago was to insert the Echoplex in the lead singers mic line, turn it over to pure delay but have the head right at the end so there was almost no delay. Then as he talked into the mic I would slowly increase the delay until it was behind enough to confuse his speaking. Pissed him off every time.

"Enzo! Turn that god damned thing OFF!!"

Try saying the alphabet or something while hearing your own voice delayed a second...

[Froumy]

Enzo, thanks for the fun and games with the scope. Anything that he can have fun with and learn at the same time is great(me too). As far as the echoplex? My father had one when I was kid. I played with that thing for HOURS on end. Sliding those heads back and forth made some of the craziest sounds. That's got to be the most fun delay to play with EVER.....
Mars, I hadn't heard of an audio signal tracer before. To actually hear what's going on instead of having to rely solely on skill and knowledge? Well, that's right up my alley until I'm more confident in what's going on. I'm a little confused in your set up, though. magnetic phone amp pre-amps? Is that a headphone amp going into....? As far as the coupling cap..... Stick say a .02 before the input of the powered speakers? I think that there are others around here that might benefit from a low cost, lowfi solution like this. If you could clarify, that would be greatly appreciated!
Ohh yeah, can a scope help me determine if I have a poorly designed ground scheme?

[Enzo]

A scope is a tool like a meter, it lets you see what voltages there are at any circuit point, but it is up to you to interpret what you see in a useful manner. It won't say, "here's your hum, bud."

Signal tracers are a very old idea, you could buy commercial ones or build a Heathkit or Eico version yourself, or make one from scratch. They were a lot more popular back in the 1950s.

Any amplifier can be a signal tracer, connect a probe to its input and probe the circuit under test. You need to be able to block the DC voltages present, se we add a series cap at the input. On the tracer end of the cap, there need to be a resistor to ground to give the cap a charging path other than the input stage.

A guitar amp wil work - plenty of gain to listen to weak noise signals. A scope probe is a convenient probe, but anything using a shielded cable is fine. Put a 1/4" plug on a scope probe or whatever and mount the iso cap and resistor right in the plug body. That probe alone makes any amp a signal tracer.