What to look for in a scope. How are Proteks?

[bg4533]

I have been playing around a lot with DIY lately and I really want to get an oscilloscope. I can't really afford much and I really don't need much anyway. So far I have built quite a few Mints, Pimetas and a PPA. I plan to build a tube amp and Dynalo soon. Maybe a nice DAC and a Dynahi in the future. The scope is mainly just a toy though. What should I look for in a scope? What sort of bandwidth is good for DIY stuff? What sort of bandwidth is necessary to debug DACs?

I currently have the option to buy a new Protek 6502 scope for ~$160. It is a dual trace scope and has 20MHz bandwidth. Would this be a decent scope for me?

Thanks!

 

[Woody]

Any scope is better than no scope, but I'd personally look for for 50MHz or above. And a digital storage scope is the ultimate. For a decent 50Mhz digital storage scope I reckon you could bank on spending about $350, but as I live in the U.K., that is a very rough guess.

The vertical and time base sensitivity is also pretty important and may be limited on the cheaper scopes.

A 20MHz scope may not be enough if you wanted to track down high frequency noise for example.

Look on Ebay, just dont bid on one though that just says "Not tested, sold as seen" etc.

If you buy on ebay, most come without probes.

One final word of caution before you use a scope for the first time, make sure you understand the significance of using a non-isolating scope probe, in other words take care where you clip the probes earthed croc-clip.

 

[bg4533]

Quote:

Originally Posted by Woody One final word of caution before you use a scope for the first time, make sure you understand the significance of using a non-isolating scope probe, in other words take care where you clip the probes earthed croc-clip.

I have no idea what this means. I have never used a scope before. I will read the manual and stuff before I use it, but does anyone know of any good sites to read about scopes?

$350 is a lot more than I am willing to spend right now. Even $160 is a bit much right now given everything I just bought.

 

[stadams]

Do a search on ebay for Tektronix scopes. You can find them from $200.00 on up. A couple of years ago I bought a 465B/DM44 for $300.00 in good working condition. I then spent another $100.00 on the proper probes--Tek 6105A's. This is a 100 Mhz scope, and will do just about anything that you could want as far as DIY goes. There are other models like the 465, 475, 475A, and 485, that can cost more or cost less however, to get a good scope, you probably will have to spend ~$300.00.

Woody brings up a good point. You should only buy scopes that offer proof of operation and better yet a 30 day return policy--good luck. Just remember that this is a one time investment that will possibly last longer than your hobby. If you can't afford the $300 - $400 at the moment, wait until you can. You don't want to have to buy two.

Later,

 

[tangent]

There are tradeoffs here, as always:

- Noise: Analog tends to have lower system noise, so it's good for getting a qualitative feel for noise measurements and such. You have to step up to a pretty expensive DSO to do serious noise testing. And no, a $1000 TDS1002 doesn't qualify as "serious" here.

- Ease of use: DSOs are easier to use, because they can "find" a signal for you, they can hold onto a one-time event (analogs can only show detail if the signal repeats), they have better signal manipulation features, modern ones are smaller, etc... Analog scope is trickier to use because you really have to understand things like 'microseconds per division' and such in order to find your signal to begin with.

- Repairability: If you buy used and it breaks, you get to keep both pieces. If you buy an off brand, repair might be hard to come by. If you buy a Tek, the only question is whether you can afford the repair cost.

I have a Protek 6506, I believe. It's only about a month old, so I don't have much experience with it. So far, I haven't found anything I don't like about it, but that's not saying much. I can tell you that it is no Tek, but it isn't a steaming pile, either. At the same time, I bought it only because it was cheap, in my view, so I won't be hurting if there's a fatal flaw. You, with your tight budget, might view the same situation very differently.

 

[tangent]

Some good resources for learning about scopes before you buy one:

Tektronix' XYZs of Oscilloscopes. Also look up their ABCs of Probes, once you've gotten through that.

The Bob Pease Show just did one on oscilloscopes. A little advanced, but entertaining anyway.

 

[Frenchman]

I definatly agree with previous posters in that you should wait until you have the money for a good one. My buddy got a 4 channel 100Mhz Tek scope on ebay for about 250$. That's probably what I'd shoot for if you're looking for home use. I personally have gotten way too used to these expensive college peices. I'm not sure I could possibly live with a scope with less then 1GHz bandwidth

Speaking of which, since I believe in good investments, you might want to consider whether you want a CRT display or a Digital display. If you really prefer one over the other, it might actually come into play when you decide to drop some good money on a scope. Essentially, Tek will mostly use CRTs and Agilant will mostly use LCD type screens.

 

[bg4533]

Thanks for the responses. I have been browsing Ebay the past few days for scopes. There are tons for sale, but I haven't really known what to look for. Most say they power on, have not been tested and are sold as is. I am not considering these. A lot seem to have physical damage and be missing knobs and stuff. The ones that are guaranteed and are in good condition seem to be going for a lot of money. More than I want to spend...

Most of the things I found online recommend Tektronix scopes and mention little else. It seems most people just go that route. I haven't found any complaints about Protek stuff and the few things I have read about Protek are positive. I just got a Protek 608 multimeter and I think it is great for the money. My opinion on this might change if it breaks or needs calibrated though.

Anyway...I think I am just going to buy this scope. It should be adequate for what I need it to do and it is about the max I am willing to spend right now. If I am still into DIY after I graduate college I will consider upgrading.

When I get the scope and learn how to use it I will post my opinions of it. I have nothing to compare it to, but I can at least determine build quality, ease of use and basic functionality.

Thanks for the help.

 

[Pars]

If buying in unknown condition off of eBay bothers you (and it would me with some of the idiots up there), check out Tom Gootee's site, http://www.fullnet.com/u/tomg/gooteesu.htm for scopes that have been tested and repaired/reconditioned. Of course you would pay more here, but some of the sellers on eBay whose business appears to be selling used test equipment, will often take lots of items purchased from business failures, etc., and hide behind a claim of "don't know how to test" or some other BS, and sell it as is (but "powers up"). I would guess its usually more like they know how to test it, and know it doesn't work, so they sell it like it "should" work.

 

[Frenchman]

The battle between Tek and Agilant is like that of Intel and AMD. They're at each others throats and everyone picks a side. Tek is usually the favorite of more applied people while Agilant seems to (atleast from what I've seen) dominate the higher end engineering segment. Perhaps dominate isn't the best word, because like I said, the battle is still raging.

I personally don't like Tek scopes as much as their digital counterparts. Aside from personal beliefs, Agilant scopes usually have higher performance compared to a Tek scope of similar price. Unfortunatly, Tek scopes are also a whole lot easier to get ahold of at the price range we can usually afford. That means I have to set up a nice storage system so I can move my stuff all over the place when I need to use a scope. Oh well.

 

[stadams]

Upper end in engineering--start looking at a LeCroy.

Later,

 

[Syzygies]

I'm very tempted by the Indigo io Type II Cardbus: stereo 1/8 inch analog input and output, with a 24-bit/96kHz converter (Stereophile Review). I'd combine this on my Macintosh laptop with SignalScope and SignalSuite.

I'm sure this isn't as good as a real scope, but it probably can do things my multimeters can't do. Would this be of any help for audio DIY? Sure is portable...

I'd view the 24/96 recording and playback as a bonus, a way to justify part of the expenditure. I do sometimes record live and digitize old recordings, and I'm interested in the math of interpolating 24/96 output from 16/44.1 recordings. It stuns me that there's audible tape hiss on recent-issue CDs remastered from old recordings, our ability to manipulate sound lags behind our ability to manipulate images...

 

[bg4533]

Quote:

Originally Posted by Syzygies I'm very tempted by the Indigo io Type II Cardbus: stereo 1/8 inch analog input and output, with a 24-bit/96kHz converter (Stereophile Review). I'd combine this on my Macintosh laptop with SignalScope and SignalSuite. I'm sure this isn't as good as a real scope, but it probably can do things my multimeters can't do. Would this be of any help for audio DIY? Sure is portable... I'd view the 24/96 recording and playback as a bonus, a way to justify part of the expenditure. I do sometimes record live and digitize old recordings, and I'm interested in the math of interpolating 24/96 output from 16/44.1 recordings. It stuns me that there's audible tape hiss on recent-issue CDs remastered from old recordings, our ability to manipulate sound lags behind our ability to manipulate images...

I am all over the place here. I decided now not to buy the scope I mentioned above. This is due to many reasons, some are the scope, but most are personal.

I am going to start a new thread about software based scopes here. Aimed at the PC mainly. One of the biggest disadvantages to these scopes is the relatively low bandwidth.

There have been a few comments on the tape hiss thing. People have said it is removable, but there is always a slight loss when doing so. Most audiophiles would rather listen to the hiss than have the loss it seems.

 

[tangent]

Quote:

Originally Posted by Syzygies I'm very tempted by the Indigo io

Another one you might look at is the M-Audio Transit. Well-regarded in the audio test world, probably because it's external and doesn't suffer from frills that most other sound cards push on you.

Quote:

SignalScope and SignalSuite.

Hmmm, looks to be on par with TrueRTA level 3. The Faber stuff looks like it might have more resolution (depends on how they distribute their 4096 FFT points -- linear or log?) but TrueRTA has more technical stuff, like various calibrations.

Quote:

Would this be of any help for audio DIY?

It's okay if you just want to watch audio signals go through an amp, but it's completely useless beyond the audio band, or for signals over a few volts, peak. And, there's the danger of frying your sound card. If you're going to try and rig up a scope probe or similar so you can connect the sound card to something in the middle of the circuit (as opposed to just using regular sound cables to the input or output of an amplifier), there's the worry of slipping while hooking it up.

Quote:

It stuns me that there's audible tape hiss on recent-issue CDs remastered from old recordings

If you have some decent audio software, try playing around with its noise removal features. You'll find that the result isn't flawless.

Quote:

our ability to manipulate sound lags behind our ability to manipulate images...

Not really. What we demand from images simply differs from what we demand from sound. I do a fair bit of Photoshopping, and I can tell you that when I remove a telephone pole from a JPEG by covering it with sky cloned from nearby, you don't miss it because you don't expect to see a bird or a cloud in that place in the sky. But when you remove noise, you expect to hear new things that you couldn't hear before. Your ears go searching into the mix, and find the artifacts left behind where the noise was; granted, the artifacts are at a lower level than the noise level, but your ears find them. In the photo, your eyes ignore any minor flaws in my coverup job, because it's "just sky" where the flaw was.

 

[Syzygies]

Quote:

Originally Posted by tangent If you have some decent audio software, try playing around with its noise removal features. You'll find that the result isn't flawless.

Indeed, I'm sure that's why commercial CDs leave the hiss in. I recently mastered a CD from 15 year old language cassettes my Thai cooking teacher had recorded. She was thrilled, but I was horrified, at the tradeoffs I had to make to get to an acceptable noise floor.

Quote:

Originally Posted by tangent But when you remove noise, you expect to hear new things that you couldn't hear before.

I agree with everything you say. To cover up footprints in the sand, you have to replace the ripples as they would have been, or at least convincingly could have been. In this respect images and audio are very similar, it's a hard problem either way to model what's missing.

If you scan 10,000 19th century journal pages in the same font at 300 dpi and invest enough computer time matching like characters, you can reconstruct 4800 dpi models for each character, smooth this to a Postscript character in an on-the-fly font, and use this font to reprint the journal at any desired resolution. The key here is to use probabilistic models to match information across time.

Get far enough into telescopes as a hobby, they wax philosophic about "seeing", how watching an object for 30 seconds causes our brain to register an impression that isn't there at any one moment. Our brain is matching information across time.

This is what I'd love to do with audio. When I listen to music I want to relax, I'd rather the computer did the matching across time for me.