OT: Wanting to buy Oscilloscope

Hey guys,

Needin' to get a scope for diagnosing the cnc machines here at the shop. Don't want to spend a fortune, but want to get a good, reliable, capable scope.

Man, Ebay has boatloads of em...

TIA,

Greg

cnctoolcat said:

Hey guys,

Needin' to get a scope for diagnosing the cnc machines here at the shop. Don't want to spend a fortune, but want to get a good, reliable, capable scope.

Man, Ebay has boatloads of em...

TIA,

Greg

Click to expand...

I wouldn't waste the time trying buy a used one on ebay. Just go the the Tektronix website and look at their lower end 100mhz scopes. With a black and white display it should set you back around $1000.
Some of the older stuff on ebay is redicuoulsly overpriced, and even the good deals to be had, you don't know how much life is left on the CRT, how long it has been since calibration etc. The new scopes are smaller, lighter, more portable,and you get a manual explaining features and scenarios and how to test.

What kind of scope do you need? (There are many many kinds) DSO?, digital?, analog?, what frequency?, sample rate?, number of channels?, size?, what processing capabilities? (FFT, signal, lock-in (and combinations))
The array of available options is rather staggering.

You can get one heckuvalot of scope for not much money buying a brand new one from HP/Agilent. Generally HP/Agilent and Tektronix are the best US makes. There are also many good furrin brands. How much are you prepared to spend? You can get a lot of used scope for $500.

One last question... do you know how to use one?

-DU-

Basically I need a scope to test the Mitsubishi servo drives and spindle drives on my Mazak machines.

Mitsubishi test procedures require using a scope to check the firing circuits of the drives. For example, the ac spindle drives have 12 firing circuits, 6 each for the inverter function, and 6 for the converter function. These firing circuits control the high voltage IGBT's that send the ac current to the motor. These firing circuits control the speed of the ac motors by varying the frequency from about 1 hz to about 400 hz.

These firing circuits consist of a hybrid transistor driver with several capacitors, some diodes and resistors, and the firing transistors themselves. Failures can be caused by the hybrids, capacitors, or transistors going bad.

It's a fairly basic use of a scope, I think. By checking all 12 firing circuits, the bad ones should stand out quite easily.

Thanks again!

Greg

How many of those 12 circuits do you want to check simultaneously? A 16 channel or even a 4 channel scope gets to be MUCH more expensive than a 2 channel.

With a 2 channel scope with storage, a DSO, most should have the capability to store signal traces so you can compare. If the scope has a way to connect to a PC (many modern ones do) then you can compare many signals at once... but not in real time.

I have no experience to the test procedures for your type controller. Have you asked Mazak techs what they use?

-DU-

Greg,
If you can get by with a 20mhz 2 ch, FLUKE makes a good portable battery operated industrial scope. It can capture, store and transfer wave forms.
The scope can also preform a full set of tests (FFT, etc) and has several trigger options.

Best feature for you is, it is designed to be used in an industrial environment.

PaulF

cnctoolcat said:

Mitsubishi test procedures require using a scope to check the firing circuits of the drives. For example, the ac spindle drives have 12 firing circuits, 6 each for the inverter function, and 6 for the converter function. These firing circuits control the high voltage IGBT's that send the ac current to the motor. These firing circuits control the speed of the ac motors by varying the frequency from about 1 hz to about 400 hz.

Click to expand...

Hi Greg,

Yes, this is a basic test function for a 'scope.

BUT, there's one huge red flag...
Check out the voltage rating of the probes. You would want ratings of at least 1000 volts if you're working on 240-volt machines, and 1500 volts or more if you work on 480-volt systems. These will probably be special options, not standard equipment.

You will want to use 10x probes, which divide the input voltage by 10 before applying it to the 'scope inputs. But modern test instruments are seldom designed for high-voltage operation. Typical voltage specs for probes may be 300 volts or even less.

Another consideration is the low frequency (1 Hz) being observed. This will require a modern digital 'scope or a storage 'scope. The digital type is MUCH easier to use, and requires to setup or adjustment on the part of the operator. A storage 'scope could be used, but setup can be tricky.

With a digital 'scope, the information at any given point remains visible on the screen until it's over-written by the next sweep. This makes it easy to see what's going on.

- Leigh

Mitsubishi procedures checks each firing circuit individually.

These firing circuits are low voltage (either 8v or 15v) dc controlled, and are part of the drives main controlling circuit board. The firing circuits send the low voltage "chopped" signal, to the high voltage base IGBT's. Only the base IGBT's handle the high current power to/from the motor. The low voltage control signals control the "switching" of the base IGBT.

Thus, you only need to scope the low voltage control circuitry.

Also, I think there wouldn't be a need to scope the circuits at the low end of the frequency range. It would be easy enough to command the drive to output a higher rpm, thus a higher frequency. I assume if the firing circuit is bad or weak, it would be at all frequency ranges.

Greg

This was mentioned by Forrest Addy on the HSM bbs:

http://cgi.ebay.com/150MSa-s-PC-Bas...yZ104247QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Might be all you need.

Greg,

Here are a couple of points you should look into before spending ANY money :

- max operating freq you really need ( then make it times 2 )
- min pulse with you might have to work ( then divide by 2 )
- autocalibration needed or not
- fancy triggering functions
- storage
- dual / quad channel

One must be very very cautious when buying a fully digital unit. The cheapish ones tend to show what they please if the waveforms are complicated. They can not be trusted. Period.
The more expensive ones are very good but not quite as good a CRT one. A CRT should outlive an LCD by an order of magnitude. Though my digital Tektronix LOOKS spectacular
my HAMEG ( avionics scope ) beats it hands down when it matters.
Look around for a 2nd hand HAMEG. They're both analog and digital (200 Mhz), 2 channel and the build quality is ...avionics ( all switches are optical encoders etc...). We never had a fault on those in 11 years, nor any discernable wear of the CRT.
A problem with digitals is that they tend to ignore narrow pulses of high voltage because either they simply don't look at them or the triggering mechanism gets confused. I found often times such spikes to be the reason of repeat failure of power electronics. An analog WILL show them - you might just have to increase brightness. The "loupe" on a HAMEG is absolutely brilliant : a push of a button and a narrow ( ns) glitch fills the screen albeit at reduced contrast.

If you'll dive deeper into power electronics, you'll need sometimes the scope to do an FFT - a spectrum analyser. That's where a digital one shines if fast enough. I then recommend a PICO brand. I found it very good for that purpose. Hitachi mad also a few VERY good avionics scopes - larger CRT than the HAMEG but no optical switches.

For a PC based scope with spectrum analyser try

http://www.picotech.com/

PicoScope 2200 perhaps?

Relatively cheap and reliable, easy to use also.

Mark

Greg,
This what I've got, and for general troubleshooting, Dc/Ac drives, and most You will find in any of our type shops, will never find It's limitations.

http://cgi.ebay.com/TEKTRONIX-465B-...yZ104247QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Greg,

You're getting some good advice. In addition, there is an ebay review on buying oscilliscopes:

http://reviews.ebay.com/Buying-an-Oscilloscope-on-ebay_W0QQugidZ10000000001568756

Hope this helps, too.

Jim

I know very little about electronics so my choice was a Fluke Scopemeter. I wanted something portable and rugged that could still capture a waveform and upload it to a PC.

Ultimately (for me), being able to have people much more skilled than I take a look at a waveform was my #1 feature.

Whether you are a novice, or expert, being able to upload your waveform (picture) to the net is almost like having a scope that you hook up, press a few buttons, and it just tells you what is wrong and what to adjust (wow, wouldn't that be neat?).

Being able to get your waveform to the net, archive, and paper is invaluable IMHO.

Sean

I purchased a used 100Mhz Fluke Scopemeter 105B on ebay about three years ago. Cost me $700 for a very lightly used unit which would have originally retailed for close to $3000 when new. Its a great tool for troubleshooting communications gear, marine electronics and just about anything else that you might encounter in a commercial environment.

I like my FLUKE, but...

cnctoolcat

I have a FLUKE at work it set "us" back $2g or so. It has MANY nice features, however, as Leigh mentioned it may not always show you want to see. An analog model can be harder to "dial in", but on the FLUKE you push <AUTO>. This is nice, but NOT to be trusted unless you know what yare look'n for.

I do NOT like the NIMH batteries on the our fluke. This was a mistake I made when purchasing, hey boss get the NIMH, the are way better than NICAD oops. They don't hold a charge for crap when setting on the shelf. Remember this if you buy any type of battery scope.

DO get something w/ 10x probes that CAN look at the higher voltage stuff. It is very handy to watch the form on the output side of the IGBT. My experience, MITSI included, is that the big guys like to die when they get hot. If you seen a good signal at the firing circuit, you can then quickly check the drive output, for a corresponding wave form.

All that said, I don't miss, and hope to never have to push a cart around, with a big clunky scope, get to my machine, and say #$^@ I forgot my 100' of cord to get to the nearest 110v outlet. Only to have the 110v to dump a bunch of noise into the scope.

1 last thing, saving a "screen shot" is nice. To E-mail to some one for explanation, or show the boss ... ... ... We've used that a bunch.

Good Luck,

Doug S.

There was a recent discussion on the Monarch Forum you might find useful:

http://www.practicalmachinist.com/vb/showthread.php?t=154693&highlight=oscilloscope

Tektronix makes some 2 channel scopes that have three nice features.....

1) they are a little bigger than a regular hardback book

2) they are battery or line powered.

3) the two channels have 600+ volts isolation, meaning that you can read line voltage with one channel and a logic signal with the other, for instance.

In common with all LCD screen scopes, they can lie, but there are modes that will 'expose the lie". Also due to the 500 M samples/sec there are some areas where they can give an odd display. But in general they are quite handy as general-purpose scopes.

We have them at work, and I am trying to locate one at a reasonable price for my own use. They go for about $700 used. The military recently sold a bunch, and that reduced the price for a while.

I bought a Tektronix 455 scope at auction, with a selection of probes and etc. A large shop used it for tuning their cncs so I hoped it would work for me. What is different between this scope and a 465? Is this adequate for tuning drives?

A nice scope and entirely adequate for your needs. I think that the only fundamental difference from the 465 is bandwidth.

First get some basics on scopes; you will find two documents here;

http://www.valuetronics.com/Details.aspx?ProdID=7018&Model=Tektronix_455

One is about probes; the other is about scopes.

I couldn't find a free-on-the-net operation manual for a 455; my trusty friend BAMA let me down for once.

Google "tektronix 455 manual"; they are for sale.

On edit; the front panel looks pretty similar to a 465M (I have one for old times' sake ); there is a 465M manual on BAMA;

http://bama.edebris.com/manuals/tek/465m/

This will guide you through the sometimes-baffling array of controls on these fine old instruments.

One or more of your probes will almost certainly be a X10 type; this is what you need.

The scope has a "calibrator"; it's under the screen in the approx centre. Hook the probe tip to its output and ground the probe shell via its grounding clip to a nearby bit of bare metal (one of the BNC connector outers).

Play around until you can get a square wave on the screen; once you can do this you are on the way to making measurements.

Someone earlier noted that the voltages in drives can be high; it would pay to check the probe rating but I expect you will find that any respectable probe will be fine at the low frequencies you will be testing.

Have fun!!