Oscope advice please

[Chuck H]

I want to get a used scope but I've never owned one and don't know what to get. Ebay has alot of BK and Tektronix units for what seem to be reasonable prices.

I have an older BK DMM thats been an absolutely reliable and accurate tool for almost 20 years. But I know nothing of their other products.

I know I don't need a high MHz rating, but is higher more useful sometimes for tracking down parasitics and such?

Since I've never used a scope, I'll be either trying to find one with a manual or at least one that you can still find the manual for. Probes with the purchace would be nice too.

Any advice?

Thanks

Chuck

[cbarrow7625]

Chuck,

For the most part, with audio, just about any scope will do. More MHz won't help since we are always only looking in the kHz range. The scope just has to cover the Nyquist Frequency (that is measure at least 2x as high in frequency as the frequency you want to look at). A 1MHz scope will be good to look at any frequencies up to 500kHz. A 2MHz scope will work up to 1Mhz frequencies, etc., etc.

There are some other things to consider if you are buying a digital scope but for an analog scope it's hard to go wrong when you are just looking at audio. Anything you get will be able to measure far more than you will ever see in an amplifer.

Make sure you get some good 10x - 100x probes though. You want to be able to step down the voltages going into the scope, especially when probing hi voltage DC. You don't want to blow up the inputs.

[lmolter]

I bought a Leader dual-trace, 10MHz scope for $75 on eBay. It needed a good dusting but it works fine. So... stay with brands you know. Tek scopes will be more pricey, but the quality is there. BK, HP, Leader are all good. I'd stay away from round tube scopes. Dual-trace is nice, but not necessary. I use it for some digital projects I've been working on. Dual-channel is probably more useful. You can trigger the 'A' waveform trace from the 'B' signal. Actually, I don't know if there's such a thing as single-trace, dual-channel scope. Anyway, you get the idea.

As far as probes go, you may or may not get them included in the deal. Mine came with a beat-up probe, although working, it looked awful. What it had been used for I have no idea. So I went on eBay and found the best deal for a package of 1X / 10X probes from China. Got a bag of 5 for about $30.

I can't wait till I get my amp wired so I can start tracing the audio paths and seeing the transformations from preamp through phase inverter to output stage. Should be fun (zap!).

Good luck,

Larry

[Enzo]

I have been using the cheap $30 probes now for decades and they work fine for me. Don't let lack of probes be a deal breaker.

My old B&K 35MHz scope has been on the bench now close to 30 years.

I agree that bandwidth is not an issue for audio. The extra Megahertz response won't add anyting to your success here.

I look for scopes that have vertical sensitivity that goes up to at least 10v/division. Many modern scopes only to up to 5v/div. If you use a 1x probe, then 5v is 5v on the screen. But flip the probe to 10x, and now 5v on the screen is 50v in real life.

My screen is 8 divisions tall. Division simply means the graphic grid drawn on the screen. If I move the trace to the bottom, then it can move 8 divisions up before moving off the screen. At 5v/div and 10x probe that means I can see 400v from a zero start at the bottom and stay on the screen. If the scope has 10v/div, then I have 800v of room the play in.

Obviously a 100x probe eliminates this problem, but I don't like switching my probe, I like leaving it on 10x all the time so I never have to think what it is set to. SO I look for 10v/div.

Do a search here for oscilloscope selection, we have discussed it before in some detail.

[CarlZ]

Here's a nice little primer on using an o-scope that I found on youtube. It's a really nice place to start with good clear explanations of how most all the common controls work.

http://www.youtube.com/watch?v=rC8HBcsNm2g

-Carl Z

[Chuck H]

Thanks for the tips everyone. Good tutorial Enzo. But everything I find either has a 5V grid or doesn't say. And if you've ever tried to find old oscilloscope specs...Well good luck with that.

Thanks for the link too Carl.

Now theres the signal generator...Shopping for scope is looking alot easier. all the signal generators are either old boat anchors that cost $50 us to ship, are being sold "as is" and "untested" (read-doesn't work) or cost an internal organ to buy. Any tips on signal generators? Do I need both sine and square? It seems like a good idea. Any tricky alternatives? Why do these things need to be so big and expensive? It's a test tone (wave actually above AF).

Chuck

[jrfrond]

We have seven scopes here in the shop, ranging from 20Mhz on up, mostly Tektronix, Leader and Hitachi. The two low-end 20mHz scopes are Tenma, which is MCM Electronics' house brand, and I do have to say that they've been holding up well for the couple of years we've had them, and they were very affordable.

We've purchased a couple of other Tenma items, and all have remained solid, and we are a busy commercial shop, so for semi-pro or hobbyist use, they should last a LONG time.

I've purchased both cheap and expensive scope probe, and have found that they last the same amount of time. So, I keep purchasing the Tenma 200mHz probes from MCM for $24.95. We get about a year of hard use out of them, so here again, yours should last.

I wouldn't recommend anything I wouldn't use myself, so I wanted to let you know that the Tenma stuff is solid, and affordable NEW. One thing about scopes is that you can get some used ones that seem to work but have funky switches and pots, and if they need replacing, they can be real pricey! Scopes are like cars to me: new is new.

[Chuck H]

Thanks jrfrond. A warranty does sound good. I'll look into the Tenma products. When you consider the cost of a used unit will include the unit (beat or otherwise), shipping, hunt down a manual, find and buy probes and possible having to reship it and pay for calibration, and thats without any warranty, well, new might be a good idea. But I've never owned a new car either. New is new, but I once figured the per mile driving cost of any used car I ever owned against several new car options. No matter how I sliced it the new car was at least twice as expensive to drive. But it is new.

Some things I do insist on getting new:

toilet seats
cooking and eating utensils
condoms
food
fishing bait

Chuck

[jrfrond]

Thanks jrfrond. A warranty does sound good. I'll look into the Tenma products. When you consider the cost of a used unit will include the unit (beat or otherwise), shipping, hunt down a manual, find and buy probes and possible having to reship it and pay for calibration, and thats without any warranty, well, new might be a good idea. But I've never owned a new car either. New is new, but I once figured the per mile driving cost of any used car I ever owned against several new car options. No matter how I sliced it the new car was at least twice as expensive to drive. But it is new.

Some things I do insist on getting new:

toilet seats
cooking and eating utensils
condoms
food
fishing bait

Chuck

I guess women didn't make the list, huh?

[Enzo]

There are plenty of signal/function/audio generators out there, even new ones for not a lot.

A quick visit to MCM:

The house brand Tenma 72-455A for $205


The B&K 4001A for $195


They also sell this little item for $60


But you could easily build one of those from a project line or a kit line.

Certainly ther are old Heathkit audio gens on ebay. And search out the local amateur radio operator, "ham fests" occur all the time and are a great place to get old gear cheap. A ham fest is a flea market for electroniocs geeks.

And on scopes, don't sweat the 5v/10v thing, it is just me personal preference. If there is a front panel photo, you can usually see the vertical scale knob to see if it goes up to 10v.

[Chuck H]

Thanks Enzo. I figured out the signal generator thing. I looked in the archives and found an old post of yours that gave me some ideas. I found that the trick on Ebay (for finding the affordable ones that weren't used by your grandpa) is to search for "audio oscillator", sine square, "audio generator" and similar type phrasing in "titles and descriptions". That weeds out alot of the RF and MHz stuff and opens every other option.

As for salvage wearhouses, hamfests and geek conventions...Well, I'm kinda positioned in Ice Station Zebra as far as that kind of thing goes. I live on an island in the PNW US. Lots of grass fed beef, micro brews, farmers markets and homemade breads and jams. It's an incredible place to live. But technology peaked here with the horseshoe. I am exaggerating of course, but not by alot.

Unspoiled women are also in short supply

Chuck

[Enzo]

That is exactly the sort of environment where amateur radio thrives. You might be surprised how many there are around you. Look up the ARRL and see if any chapters are in your area. Many ham radio guys are very helpful folks.

I make regular visits to the salvage yard at Michigan State here. It is the facility that disposes of anyting the University wants rid of. I once found an old HP 200 audio oscillator there for $5. Not working. It has a large several sections ganged variable capacitor to tune the freq. One of the plates was bent and touching another. Three seconds with my needlenose pliers and I had a working Hewlett Packard oscillator.

Technology peaked with the horse shoe? Ahem. Here we sit in the middle of a web forum dedicated to vacuum tubes. That HP 200 is made of vacuum tubes and is nonetheless a fine piece of gear. Perhaps Joe Blow engineer who retired to your island in 1989....

[Chuck H]

Enzo, of course my last post was sarcastic. In fact, there is a naval base here. So there must be SOME stuff around. I haven't yet looked into any local organizations. But I have been to several swap meet, Lions club, community yard sale and church fund raisers. As far as I can tell there's not much going on. If it does't cut wood or catch salmon it's a hard to come by item here (also sarcastic). I have seen one ham antennea in all my travels here. The closest thing we have to an electronics store is a Radio Shack. And if you look in the phone book for electronics repair you find bumpkiss. You have to take a ferry and then drive another sixty miles to find a real electronics store and one or two repair shop options. Or, we have a bridge at the north end. You could drive about eighty miles to find something like "Zeik and Bobo's Old Timey TV Repair" (sarcastic). Otherwise you have to keep going till you hit Seattle.

So yea, there must be something going on here. I haven't found it yet, but I am sure by what I've seen so far that it isn't much. But on the up side, it has proven to be a good place to salvage cool stuff like old tube console stereos and other repairable stuff both electronics and otherwise.

Chuck

[Enzo]

Oh don't worry, I see the tongue in your cheek.

[BackwardsBoB]

I still like the little Velleman hand-held scope. Here's why:

1. It doesn't take up a big chunk of your bench.
2. You can position it so that you can see it and your probing hand at the same time.
3. With digital storage, it captures transient events, like turn-on and turn-off, and you can trigger on the attack of a note.
4. You can run it on batteries, so you can float it.
5. You can run it on batteries, so you don't get any ground loops.
6. It does 600V stock.
7. It performs measurements.
8. It's cheap.

The down-side:
1. Low-res display.
2. A bit hard to see (be sure to get the "SE" special edition with the backlight).
3. Single channel.
4. The user interface isn't intuitive.

Here's one now:
http://www.apogeekits.com/oscillosco...ld_hps10se.htm

They're all over eBay for $129.

For a bit more, at the same site and elsewhere, you can find PC based scopes and signal generators that will automate frequency response measurement, display at higher resolution, provide a better user interface, and let you save or print your results on your PC, though the idea of a wire running from a potentially defective amp to my PC doesn't really excite me.