Used Oscilloscope Buyer's Guide?

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GoingFastTurningLeft
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Used Oscilloscope Buyer's Guide?

Post by GoingFastTurningLeft » Mon Feb 11, 2008 7:28 am

I decided now's the time to invest in an oscilloscope. I plan on saving a bunch of money and getting a used one, unless what I could use is relatively inexpensive and I'd be better off getting a new one.

I don't do any high frequency stuff, the fastest signals I have are the clock signals for my AVR microcontrollers. Fastest clock rate mine support is around 16Mhz to 20Mhz, though I don't work with any signals near that fast.

What should I look for in an oscilloscope??? I'm guessing I should look for something with a minimum of 20Mhz bandwidth? Anything to watch out for when buying from somewhere like eBay? About how much should I be looking to spend?

Thanks in advance!

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Post by Robert Reed » Mon Feb 11, 2008 7:53 am

Goin Fast
I have a great scope for sale that will serve all your needs and more. PM me for details. Also I can give you some valuable info on test equipment purchases on E-Bay.

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jwax
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Post by jwax » Mon Feb 11, 2008 3:38 pm

Bought my HP 1740A (100 MHz) on ebay for $70+$25 shipping. Great deal, but hey, you never know. Check the feedback of any seller and take the chance.

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Post by Dean Huster » Tue Feb 12, 2008 4:29 pm

In 1980, a 20MHz bandwidth saved you a pile of money. These days, 100MHz scopes are reasonable. Just don't insist upon new scopes. There're lots and lots of Tektronix (and Hewlett-Packard, Philips, Gould) scopes out there that are used, but originally were intended for the high end market and today sell for less than a new 20MHz scope. If you have the bench space and don't need super-portability, I recommend the Tektronix 7000-series, specifically the 7603 (100 MHz), 7704A (250 MHz) or 7904 (500 MHz) mainframe with at least a pair of plugins, one vertical (7A26 recommended for all), one timebase (7B53A for the 7603, 7B92A for the others recommended). Because of their plug-in capability, these scopes have versatility like you'd never imagine, with curve tracer, DMM, frequency counter, spectrum analyzer and logic analyzer capabilities.

The portable Tektronix scopes recommended would be the 465 or 465B (100 MHz), 475 or 475A (200 and 250 MHz respectively), 485 (350 MHz) or one of the newer portables, although those won't be as reasonable in price. You can expect to find anything in the lists above in the $250 to $500 catagory depending upon when you hit the auction on ebay.

Unless you like lots of repairs and unobtainable parts, stay away from the older 500-series (tube-based scopes) and the older portables such as the 453 or 454. Although all are/were good scopes, their age can bite you in the rear.

Oh, and I might add .... if you find one of the recommended scopes in fantastic shape for $200, don't gripe about having to pay $100 for a service manual. Overall, you have quite the deal. Recently had a guy get a cherry 475 for around $50 (what a "fluke"!) and then turn right around and gripe about the cost of a manual. Getting what was originally a $3500 scope and gripes about a manual. Geez. What an idiot.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Re: Used Oscilloscope Buyer's Guide?

Post by Bigglez » Tue Feb 12, 2008 6:53 pm

Greetings David H.,
GoingFastTurningLeft wrote:I decided now's the time to invest in an oscilloscope. I plan on saving a bunch of money and getting a used one, unless what I could use is relatively inexpensive and I'd be better off getting a new one.
A lot depends upon what you expect to do with a 'scope.
If you currently don't have one, but have access to one
(work, school, etc.) you should try to get the same
brand and model - it's familiarity that counts.

For the smallest bench footprint and ability to print or
save screen shots consider a PC/USB accessory DSO type.

I'd stay away from the old workhorses of the industry,
such as Tektronix and hp, their products are expensive
to maintain and served a completely different era.

During the 1980s there was a breakthrough in solid
state 100MHz instruments and many of these can be
had for under $100 on eBay.

My personal choice is the Japanese made Leader LBO
series. The LBO-518 is top of the line, but any of the
500 series would be welcome on my bench!

Comments Welcome!

Dean Huster
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Post by Dean Huster » Wed Feb 13, 2008 6:13 am

I'd stay away from the old workhorses of the industry,
such as Tektronix and hp, their products are expensive
to maintain and served a completely different era.
I have a loaded Tektronix 7904, a Tektronix 465DM44 and a Tektronix 213 and have never had to put a dime into them. In addition, I have several Tektronix TM500-series T&M products -- same deal. Just with the 7904, you're talking about buying EVERYTHING it has for $500 when the original selling price was over $12,000.

Served a COMPLETELY DIFFERENT era? Pray tell, what the heck has changed? Voltage, current and resistance are the same. Waveforms are the same. Is it the fact that it isn't DIGITAL? I've used digital scopes for a long time, too. The only time I ever really found much of a use for them is capturing waveforms for inclusion in electronics curriculum. Waveform storage is in the same category as 384-function calculators, high-end cell phones and all the other trappings of the modern world -- you use it because you happen to have it, not because you really need it. Overall, I've found digital scopes a LOT more difficult to use than analog scopes, but then I haven't sacrificed my ability to learn how to read a graticule or an analog meter scale for laziness and slavery to the readout on a digital scope.

If you want a digital scope that's comparable to an analog storage scope, you're going to have to pay nearly 5 digits for the same performance, because you're going to need a 10GS/s sampling rate just to get close to the single-shot resolution of a 100MHz analog storage scope.

Scope cards or USB add-ons for your computer are toys. You might as well download a free program to turn your sound card into a simple scope. Unless you have a laptop, they aren't very portable and none of them have what it takes to be a true oscilloscope.

Ya know why there was a "breakthrough" in the 1980s for a 100MHz solid state scope? Because Tektronix designed the 2200-series to counter the Japanese invasion of scopes. In 1982, Japanese manufacturers were having to sell their poorly-constructed scopes below cost just to unload them in the U.S. and then go through a whole new design process to compete.

Get on ebay and do a decent search for a good scope.

You did say "Comments Welcome", right?

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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philba
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Post by philba » Wed Feb 13, 2008 8:31 am

It's interesting that no one mentioned the number of channels. Get one with 2 at minimum but I've had a number of cases where more would have been useful. (like debugging comm stuff) If I were buying today, I'd consider a 4 channel scope.

I have a tek 2225 - nice and portable, great scope for what I use it for though it only goes to 50 MHz.

Other features that I couldn't do without - delayed sweep and external trigger. Ext Trigger is really nice - I've built specialized circuits to generate a trigger on some complex event. A logic analyzer would have been better but $.50 worth of parts can't be beat...

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GoingFastTurningLeft
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Post by GoingFastTurningLeft » Wed Feb 13, 2008 8:56 am

I definetly need at least 2 channels (need to compare input and output!), and I need to be able to take a "capture" on the scope of whats being displayed to analyze it. I don't need any other fancy functions.

I used 2 different scopes back in college, i beleive they were both Tektronix. One was a portable model with a Green CRT display (pretty sure). It had the functions where you could scroll on the screen with your cursors and read off what the voltage and time from Xo was.

The other one was a new portable model. It had a color LCD screen, a bunch of signal processing functions, and a floppy disk drive you could save a captured display onto. That one is def way out of my price range! :)

I guess my biggest concern is if it hasn't been Calibrated recently. If it hasn't been calibrated, how far off will it be? Considering i'm not doing anything with high precision near the limits of the scope, will I be Ok if it hasn't been Calibrated???

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Post by muntron » Wed Feb 13, 2008 10:47 am

I found This Article interesting. It is written by a manufacturer and thus a little self serving (examples reference their device), but at the same time relatively unbiased.

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Post by Bigglez » Wed Feb 13, 2008 10:51 am

Greetings Dean,
Dean Huster wrote:
Bigglez wrote:I'd stay away from the old workhorses of the industry,
such as Tektronix and hp, their products are expensive
to maintain and served a completely different era.
Served a COMPLETELY DIFFERENT era? Pray tell, what the heck has changed?
Dean
I'm wondering if you're paying attention to the OP?
GoingFastTurningLeft wrote:I definetly need at least 2 channels (need to compare input and output!), and I need to be able to take a "capture" on the scope of whats being displayed to analyze it. I don't need any other fancy functions.

I used 2 different scopes back in college, i beleive they were both Tektronix. One was a portable model with a Green CRT display (pretty sure). It had the functions where you could scroll on the screen with your cursors and read off what the voltage and time from Xo was.

The other one was a new portable model. It had a color LCD screen, a bunch of signal processing functions, and a floppy disk drive you could save a captured display onto. That one is def way out of my price range! :)
In a nutshell that is the answer to all of your reactionary
comments to my earlier post.

The OP is a product of the current technology and education.
The expectation is that a "scope" is a capture device to
record data for later anaysis. A digital scope is being
considered.

You obviously have strong opinions, wasn't the request
by the OP for help with their particular request?

Comments Welcome!

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GoingFastTurningLeft
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Post by GoingFastTurningLeft » Wed Feb 13, 2008 12:27 pm

The main focus of my post was primarily to get "buyer beware" warnings, and see if anyone had any bad experiences buying a scope.

I understand that I should not buy any scope on ebay that does not have a picture showing sharp traces on the screen, anything that does not pass all startup tests, anything very old or known to be problematic, anything untested, etc.

Ideally, I don't want to spend more than around $150 to $200... I noticed every scope that's been calibrated is over $500... That's why I wonder how important calibration is when you're not doing anything at very high resolution.

The old one was very similar to this TDS420A...

Image

The new one was very similar to a TDS 3012 - def way out of my budget!

Image

But thanks for the idea of what scopes i've used... I never thought to look that up even though it tells me exactly what I'm looking for!

One thing I don't understand is why I need the manual... Isn't using a scope kind of like driving a car... once you know how to use one, you can pretty much use any of them?

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Post by Bigglez » Wed Feb 13, 2008 12:39 pm

Greetings David H.,
GoingFastTurningLeft wrote:Ideally, I don't want to spend more than around $150 to $200... I noticed every scope that's been calibrated is over $500... That's why I wonder how important calibration is when you're not doing anything at very high resolution.
Calibration is the process of comparing an instrument
to a standard, and making adjustments to reduce the
error to a stated spec. This has nothing to do with
resolution, which is the ability of an instrument to
resolve small differences in quantity (voltage or time
for example).

All instruments will have an initial accuracy. Over time
or if they are abused, that accuracy may be lost.
Generally, an instrument with greater initial accuracy
is more costly to build and more costly to maintain.

All scopes drift, older scopes drift more.

Most instruments have a calibration check on the
front panel, to allow the operator to confirm the
instrument is not out of calibration before use.

The process of complete calibration to factory specs
is time consuming and requires access to other
instruments of greater accuracy for testing. The
labour cost is added to instruments for sale.

The fact that an instrument comes with a certification
that it meets or exceeds the factory specs is of great
value to some users.

As you grow familiar with your particular instrument
you will know it's limitations and accuracy. Much
like driving a car, all vehicles are pretty much the
same but in your car you will know details such as
stopping distances and accuracy of the gas gauge
that you can't possibly know about a rental car,
for example.

Comments Welcome!

Bigglez
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Post by Bigglez » Wed Feb 13, 2008 12:43 pm

Greetings David H.,
GoingFastTurningLeft wrote:One thing I don't understand is why I need
the manual... Isn't using a scope kind of like driving a car... once you know how to use one, you can pretty much use any of them?
It's an insurance against the instrument needing
adjustment or repair. It is also useful to understand
features that may exist only on that instrument.

A well written manual will have three sections;
(1) Description of the instrument and specs
(2) Theory of operation
(3) Calibration procedure

There may be bonus sections such as a trouble
shooting guide, and parts lists and diagrams.

Comments Welcome!

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GoingFastTurningLeft
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Post by GoingFastTurningLeft » Wed Feb 13, 2008 12:54 pm

Bigglez wrote:Greetings David H.,


Calibration is the process of comparing an instrument
to a standard, and making adjustments to reduce the
error to a stated spec. This has nothing to do with
resolution, which is the ability of an instrument to
resolve small differences in quantity (voltage or time
for example).
As usual, I used the wrong word. By not doing things at very high resolution, I mean accuracy. If a scope is off by 1 or 2 mV, thats not going to make or break me. I don't work with high frequency circuits.

Bigglez
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Post by Bigglez » Wed Feb 13, 2008 1:22 pm

Greetings David H.,
GoingFastTurningLeft wrote:As usual, I used the wrong word. By not doing things at very high resolution, I mean accuracy. If a scope is off by 1 or 2 mV, thats not going to make or break me. I don't work with high frequency circuits.
1 or 2mV compared to what?

A typical scope such as the TDS3012 that you cited has
an accuracy of 2%. The maximum sensitivity is 1mV/div.

Your expectation that it is good for 1 or 2mV is
100% to 200% of the instrument's basic calibration.

On the other hand if the vertical calibration was off
1 or 2mV on the 100mV/div range it would be in spec.

A hobby grade scope is probably not better than 5%
(of full scale in either X or Y deflection), but for most
hobby activities this is perfectly good enough.

Comments Welcome!

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