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How to use an oscilloscope ?


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#1 stevenrob

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Posted 16 February 2017 - 07:43 AM

Hi,

 

I have found a low price oscilloscope. I would like to use it to adjust azimut of tape deck. How can do ?  Do yo know ?

 

Best regards

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#2 Superduper

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Posted 16 February 2017 - 03:33 PM

Wow, "how to use oscilloscope."  Question seems so simple and innocent.  Yet, to expect somebody to re-type all the information here really wouldn't be fair because, in truth, lots of information is already out there on the internet where someone has already taken the time to explain in great detail all the features and and controls that might be available to you.  You can just google and there are many tutorials out there to explain all the features.  

 

However, in GENERAL:  You can adjust azimuth in one of two ways:  Reading outputs (L, R) and measure for greatest amplitude on both channels.  Using this method, you can measure using either a mv meter (ac), or you can use your scope and check for greatest amplitude (after all, a scope is in essence, merely a voltmeter that is able to display volt/time relationships).  However it appears that the scope shown is a single channel scope so you would have to switch the probe from L to R to get the reading and compare.  Frankly, I'm not sure how much is 20E but for not much more money, I'm sure you can probably get a dual trace scope.  If that is the case, you can then read amplitudes simultaneously from both channels at once and compare.  Since you want both channels to display the same amplitude (max), it is very easy to do this when both channels are displayed at the same time.

 

The other method is by connecting the outputs to the X-Y inputs of the scope where one channel is X (ampliftude) and the other channel is Y (horizontal) and make sure the scope is in XY mode.  In this method, you will be able to display both outputs simultaneously, only you will be wanting to get a 45 degree line as much as possible.  This is from bottom left to top right.  A perfect 45 degree line indicates that both the X and Y amplitudes are the same.

 

In both cases, you should be performing the adjustment using a test calibration tape.  That might be the hardest thing to obtain and I don't suggest you record your own since you don't know if the tape tracks has fringing or other issues that would not be present on a calibration tape specially designed for azimuth adjustment.  If you can get a test tape of 10khz @ -20db, that is probably fairly standard although I have heard of 8khz, etc.  Whatever you can get.  Also, in many tapes, the test tone is recorded in more than one tone.  It is often recommended to perform adjustment using one tone, then retest/readjust using another frequency (8khz, then 16khz).  However as many boombox decks aren't even rated at that high frequency, you might have to go with tapes recorded at lower frequencies.  Also, when you do the test, it is preferable to use the line-out jacks or pre-out jacks.  Speaker output jacks can be used but sometimes, circuitry differences due to unbalanced resistance in the balance controls due to oxidation, wear, or whatever and/or amplifier circuits which also often traverse headphone jacks and speaker/out jacks can introduce resistance which results in slight imbalances.  Adjusting for equal outs when you can't trust that the speaker outs are identical with the same signal is a problem since that is exactly what you are calibrating for.  Additionally, when you use a scope, the scope ground is shared between all channels monitored.  Some boomboxes use BTL amp outputs which are floating ground and the ground should not be bridged between them.  This is why the pre-outs (which are almost always common ground) is best.  

 

BTW, you can not adjust azimuth using either of these methods by playing music, since music is random and amplitude and frequency is constantly changing and therefore, at any point is time will always be different, although it might make for some pretty screen display though.



#3 Lasonic TRC-920

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Posted 16 February 2017 - 04:17 PM

It always seems so easy, "Oh, just get the equipment, a test tape, somehow magically figure out how to hook it all up and PRESTO!"

 

But this is complicated and made even more so since every stereo system was manufactured to different standards.

 

BUT DAMN IT, this is something I would really like to learn how to do....some day! Thanks as always Norm  :superduper:

 

Interesting thread.

 

:popcorn:  :gathering:  :popcorn:



#4 caution

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Posted 16 February 2017 - 05:14 PM

Seems like overkill for azimuth, just pop in a decent pre-recorded tape and center on the crispest spot, Loctite into place.



#5 Superduper

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Posted 16 February 2017 - 05:37 PM

It's not really overkill Eric.  Especially if you have a scope or a millivolt meter (which almost everyone has).  And if you have 2 meters (to simultaneously monitor both channels or a dual trace scope, I mean, how hard can it be to adjust to max or equal amplitude on both channels?  Takes hardly any time at all.  Anyhow, virtually all service manuals prescribes either one or the other of those two methods.  I have never seen any service or maintenance manual to suggest to just adjust until it sounds good.  The hard part of the equation is that not everyone has a calibration tape available.  Especially factory original test tapes, are very expensive (everything is relative) and most boombox folks won't want to invest the $30 or $40 to acquire one, although sometimes, you can get a deal if you are patient.

 

Anyhow, the OP asked a question, and I think it's a fair one, so I tried my best to answer in general terms.  It's up to him (or anyone else) whether they think it's too much trouble or overkill to follow the usual procedure prescribed by service material to perform this adjustment.  Regardless, the procedure is spelled out for anyone that is interested to know.

 

Although we all know that you can simply rotate the MPX adjustment pot to a location to ignite a stereo indicator light, I'm old school and always do it with a frequency counter and get it as close to 19khz as possible on a weak or non-broadcast location of tuner dial.  It's not that you can't do it the other way, but I have experimented before and getting it close by feel, and then re-doing the adjustment with the counter to verify doesn't always coincide with set point.  On one, you adjust on a strong station, whereas on the other, you adjust with no or weak broadcast.  On a strong broadcast, the tuner has no difficulty locking on and has a wide range on the adjustment pot where stereo remains locked but on a weak station, it's more difficult to capture stereo and the pot adjustment range of center is very narrow.  For this reason, I'd prefer to have it spot on.  After all, if I have the tool, why not use it?  I guess that's my point.  As to the OP, he apparently got himself a scope and wants to play with it.  It can sometimes be fun to participate and end up with the feeling that one has done his "best" to tune his machine correctly.  For me, I probably spend more time doing adjustments to and fixing decks than I ever play them for music.



#6 Fatdog

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Posted 16 February 2017 - 06:40 PM

That's great information, Norm.  If I ever find a good deal on a scope, I will jump on it and then learn how to use it.



#7 caution

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Posted 16 February 2017 - 09:02 PM

Ahh, very cool! Now I have a legit reason to buy a (slow) scope :-D



#8 Superduper

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Posted 16 February 2017 - 10:07 PM

Eric, you really don't need a fast scope for most boombox work.  A good ol analog scope will work just fine.  BTW, a dual trace scope in the 20~30mhz range can be purchased literally for the cost of a cheapo starter boombox.  If you can afford a better one, you can do so much more.  Our analog audio signals top off at around 20khz.  Even on tuners, if you get into oscillators and and IF, remember that AM IF is only 455khz and FM IF (intermediate frequency) is 10.7mHz.

 

Remember that a scope is really just a visual voltmeter.  Imagine this.  To adjust azimuth, the goal is to make sure both channels are in phase alignment and you'll want to adjust the azimuth screw so that the L and R channel amplitudes are equal and to max.  With a dual trace scope, you can simultaneously view the output signal in terms of volts (amplitude on the waveform).  By toggling the azimuth screw, you can see at a glance the relationship between the two traces.  If they were both equal in height, then you've got it made.  OR if you were to put scope into XY mode (if it has that feature), then you can plot one channel to the X axis and the other channel to the Y axis and a perfect 45 degree upward trajectory would indicate that both X and Y are in proper alignment.  In this case, you don't really need to know how many mv is the amplitude (although you can computer it based on the graticle and setting) because the goal is just to equalize or maximize the L and R.

 

I'm not going to recommend any particular unit or brand but if you do some due diligence in your search, you'll find lots of examples for anywhere from $30 to $100 that should work fine.  Just don't buy any where they say "untested" because that literally means it doesn't work and they don't want to say it.  If you can afford something a bit more, you'll be much happier with it.  Please don't say you can't afford it -- because all you guys need to do is look at your boombox "wall" and you'll know that's not true.  It's merely a matter of what you value and prioritize.  Anyhow modern scopes not only have automatic waveform capture (which takes a lot of the guesswork out of the settings) but it can also do math.  Getting the settings exactly right is the hardest thing for a newbie to learn so automatic waveform capture is a really  nice feature.  Also, the new digital scopes also will display the voltage, frequency, and you can also compute parts of a waveform by manipulating on screen cursors to map out a portion of a waveform to calculate from.  It seems so much more complicated with all these extra "features" but I suspect that you'll find the newer scope actually easier to use once you get familiar with them since you'll probably spend less time playing with the settings like you would with an old school tube.  At some point in the (probably near) future, I may offer up the scopes in my tool cache.  I think I have a BK 30mhz, and then (3) Tektronix scopes.  A Tek 465 100mhz, a 2465 300 (or 400mhz) 4-trace unit, and also one of their digital ones, a dual trace TDS1002B (iirc) 60mhz & 1ghz sampling rate.  Due to ease of use, light weight, small footprint, etc, nowadays I use the digital one a lot, even though the 2465 is a whole lot more scope than the digital one and the fast display of the tube vs lcd is much preferable.  Still, quick and easy is often preferable and the digital one does that quite well.  While researching scopes to buy, you should also research to see if you can find an operator's manual too.  Any complicated instrument will be far more valuable to you if you have documentation to go with it, without which no matter how may zillions of features it has, if you can't figure out how to use it, those features will be entirely useless to you.  If you are going with a newer scope and it's made by Tek, you can get them far cheaper (although again, cheap is relative) online but just be aware that frequently, used scopes also mean problem scopes.  On the other hand, if you get a new one, you'll pay a lot but I think my Tek scope advertised lifetime warranty if you register it with them, don't know if they still offer lifetime warranty or not anymore.  For you guys that reside near a technical college that offers electronics engineering programs, you guys might luck out -- lots of students join those programs and pay for the equipments package which is part of the program but then drop out.  At that point, they may have like new, or still new instruments but don't need them anymore and you might get them cheap(er) from CL or something.  



#9 stevenrob

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Posted 17 February 2017 - 01:46 PM

Thanks a lot !!!!. I finish to repair my telefunken studio :jawdrop:  and after I will study the way to use this tool 



#10 docs

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Posted 18 February 2017 - 07:42 AM

Great info Norm, ive had my oscillator in a cupboard for years because I simply dont know how to use it. Not only what to set it to on the front but where to connect it to also.

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