Oscilloscope 101

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Oscilloscope 101

Postby SDS-PAGE » Sat Nov 10, 2007 10:23 am

I bought a HP 1740A 'scope on craigslist for 20 bucks a week ago. Don't know how capable this scope is, but it has a dual-channel dc to 100 MHz vertical deflection system. Anyhoo, I have never used one before and I have been learning mostly out of the manual. So, I am familiar with operating it. I have also ready a couple of posts here, but I was wondering if anyone would be patient enough to give step-wise instructions on how to properly take measurements for tube amps using a 'scope. What measurements can be taken? I would really appreciate it. Thanks!

P.S. As far as generating signals, do I just use a CD that has different frequencies? If so, where could I get one.
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Postby SDS-PAGE » Sun Nov 11, 2007 10:07 am

Hi Eric,
Apparently the "1V" calibration tap on the front of my 'scope was actually 0.490V. I measured it using a multimeter. So, I guess my 'scope was calibrated, except when I measure the "absolute voltage" across a 1.5V dry cell, I get around 0.75 V. So, I tried "voltage difference measurements" according to the manual, where scale factor (ref volts/(volts per div*div on display) is used and I get an accurate measurement. My question is why did HP put this here if getting an absolute voltage was possible? Thanks. -Min
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Postby 1audio » Tue Nov 13, 2007 12:51 pm

The calibrator is a squarewave of 1V peak amplitude and approx 50% duty cycle. It will measure around .5V on a multimeter, a number that means very little. You should get a 0V-1V peak on the screen and no overshoot if the probe is properly calibrated. I have very rarely seen scopes way out of cal that weren't really broken. First try it with a BNC to wire connection to check the scopes voltage sensitivity.
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Postby Sal Brisindi » Wed Nov 14, 2007 9:51 pm

Min,
Since you are building audio amplifiers on a weekly basis you should purchase a audio generator. They are not expensive at all. Here are 2 audio generators I purchased a few weeks ago (I need more test equipment like I need a hole in my head). The B&K cost me $15.00 (its really a function generator but does sine waves) and the HP cost me $5.00 plus shipping. They both work great, I should know, I did a hearing test and my right ear goes silent at about 13,500 cps.... You can download a nice tutorial on oscilloscope from my website at http://www.tuberadios.com/docs/XYZscopes.pdf

Regards,
Sal Brisindi

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Postby SDS-PAGE » Wed Nov 14, 2007 10:22 pm

Hi Sal, thanks for the suggestion and great find on those generators! I actually did buy a function generator on eBay for $26 + shipping (item 230188643834)
I was originally going to get a CD with sine and square waves, but I figure I could pay 10 bucks more and get a real thing.
So, it will show 0.5 VAC on a multimeter? I need to get the scope checked out.
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Postby Shannon Parks » Thu Nov 15, 2007 6:11 am

I'm sorry if I misunderstand, Min, but did you get 1audio's post? Do you see a 1kHz squarewave if you clip your probe on the calibration post? Does it go from 0V to 1V peak? I think your scope is probably fine.

Scopes are one of the more intimidating tools, but man are they handy once you learn the basics. I love them. Is it wrong to love test gear?
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Postby Sal Brisindi » Thu Nov 15, 2007 7:11 am

separks wrote:Scopes are one of the more intimidating tools, but man are they handy once you learn the basics. I love them. Is it wrong to love test gear?


Shannon,
Well, love is a strong word...., OK, I love my test equipment, there, I said it!

Its really disgusting the amount of test equipment I have, at least 10 oscilloscopes, all Tektronix, 15 or so tube testers, signal generators, signal tracers, audio generators, function generators, meters, vtvm's etc. The sad part is 1 of each would suffice.. Maybe I should trade you a oscilloscope for a few of your boards... Yellow_Light_Colorz_PDT_16

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Postby SDS-PAGE » Thu Nov 15, 2007 8:46 am

Shannon,
Yep, scope set at .1V/div, I get 5 divisions from peak to peak for the square wave using 1V cal. I tried other settings and it all worked out to be .5V. This is why it led me to believe that ths scope is out of calibration. It seems to me like all the voltages I measure works out to be exactly half of what it ought to be. Maybe the probes have some attenuation factor, like of 2? It says 10:1 on them. Since it's consistent, I could just acount that in the final calculation. I can also always send it over to a workshop on campus to get it calibrated, but they are usually slow. -Min
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Postby 1audio » Thu Nov 22, 2007 12:13 pm

t wo quick tests to dig deeper-
1) a 1.5V alkaline battery should measure a little greater than 1.5V when new, check it with the scope on DC.
2) Check without the probe- just a BNC cable (or wires if you must)

Also check that the variable control is at the cal position, and there is a front panel gain adjust somewhere for cal adjust. On the Tek stuff its usually engaged by pushing in on the variable control but different models are different. There will be a front panel label somewhere. The idea is to connect the cal directly to the scope input and adjust the gain so its precisely correct.

Scopes have become quite complex in order to get meaningful info and require a lot of understanding. Digital scopes are even more challenging and really can lie if you aren't aware of their nature. But they are the most useful troubleshooting tool on the bench.
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Thread revival...

Postby jar240 » Tue Jan 22, 2008 3:36 pm

How do I know what the maximum voltage, AC and DC, an oscilloscope can handle?

Are the probes used to attenuate voltage? AC or DC?

For audio work...troubleshooting and the like...what kind of oscilloscope and probes do I need?

Do probes have DC-blocking capabilities, if I want to check an AC signal (voltage) on the B+ line?

I'm really quite a novice when it comes to oscilloscopes, and would like more information before I go out and buy one.

Thanks,
Chris
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Re: Thread revival...

Postby Shannon Parks » Wed Jan 23, 2008 5:00 am

jar240 wrote:How do I know what the maximum voltage, AC and DC, an oscilloscope can handle?


It will usually be listed right at the BNC connector.

Are the probes used to attenuate voltage? AC or DC?


Yes, to a point. 10:1 are the most common probes, but 300V-600V could still be the max voltage you can work with. Use 100:1 for high voltages and 1:1 for noise testing.
For audio work...troubleshooting and the like...what kind of oscilloscope and probes do I need?


20MHz scopes are fine, but look for one that can store triggered shots.

Do probes have DC-blocking capabilities, if I want to check an AC signal (voltage) on the B+ line?


Observe the max voltage rating of the scope and probes, and set the scope for AC coupling.

I'm really quite a novice when it comes to oscilloscopes, and would like more information before I go out and buy one.


There must be some nice guides on the net. Indeed, I'm planning to downgrade my scope to a $100 Hitachi model instead of my $2k Tektronix DPO.
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Postby Mackortoyota » Thu Feb 05, 2009 6:34 pm

so let me get this straight, $120 for a signal gen and scope?
*thinks really hard* do i want the ps2 or the nice test equipment >_<"
I believe solid state is the future, but only tubes should be entrusted with the task of audio amplification.
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Postby Spraiski » Thu Feb 05, 2009 8:32 pm

Can anyone suggest $100-ish scope models to watch for on ebay?
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