Opinions on Oscilloscopes

It's my opinion that if you need two channels, you're not gonna be happy with a single channel scope.

The key to buying a scope is patience. You can't hardly go wrong with a TEK 400 or 7000 series scope. Prices run from "way too much" to "dirt cheap" depending on where/when you buy one. Patience... You can find good deals on EBAY, but you have to know the right questions to ask and hope the answers are honest. Anything that says, "can't test it" is suspect.

Search out a ham radio operator. Go visit a ham club meeting. Those guys have spare scopes for their spare scopes in the attic. Ham radio swap meets are a great place to buy scopes.

Take someone with you who knows how to test a scope. It's not unusual to find a scope can put up a green worm, but has significant problems when it comes to actual use. Scopes are relatively easy to fix, until you get to a bad part that you can't get any more. Problem is that you usually need a scope to fix a scope.

There's no reason to EVER put 240VAC into a scope. You'll always be using at least a 10X probe. Worry about the probe specs, especially on cheap probes.

I'm gonna repeat myself. Scope probes are designed to be insulating and safe to use. Hanging random wires on 240VAC and stuffing that into a scope is a recipe for DEATH. Maybe not today, but eventually. Just don't do it.

mike

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mike wrote in news: snipped-for-privacy@netscape.net:

Scope input is rated *DC plus peak AC*. At least that is how Tektronix specs their scopes. Why are you going to put 250VAC into the TRIGGER input? The scope usually has an internal trigger pickoff AFTER the attenuator,to keep trigger signals small but useable. External trigger is used when you have a clean reference signal to use,like H or V drive from a video generator when looking at composite video,eliminates the need for a sync spearator..

7313,7500 series scopes SUCK big time.and TEK-made ICs are only available thru cannabalizing other scopes,they are out-of-production and/or stock.

The worst part about inputting line voltages is the grounding;in switchers,what you think is ground may only be "common",and potentially hot referenced to earth ground.

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Jim Yanik
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Down the track (lots of learning to do), I'm interested in creating and/or working with alternative power sources (solar to battery then inverted to mains power).

I know I'll probably buy a lot of the equipment for doing this, but it always helps if you understand the circuit, especially when it stops working.

So, if the input is 12VDC and the output is 240VAC, I suspect I'm going to need to put a scope on both sides at some stage.

I'm just thinking ahead, and maybe I'll never need to look at mains through the scope.

And even if I do, I may have bought a much better scope by then. Besides, my research indicates that differential probes are specifically designed for looking at high voltages (hence putting that question in my OP).

thx, RR

If you think about the 12V DC/240V AC setup, then go thais way for a solution: 2 ch scope with HV isolator (e.g. Tektronix A69xx or something like this, Jim please help my memory and give the right description).

You will not go wrong with a Tektronix, but besides all "patience" that was recommended in buying: A good working well kept Tektronix is all time more expensive than a worn out one that (might) need maintenance.

Keep this in mind when you compare a price offer, and don't simply go for a low price -- your mileage may vary.

hth, Andreas

Don't bother with a single channel scope (why would they even make them any more!?). You'll find that it's very useful to compare waveforms, and for that you want a two-channel scope at least. But 10MHz, or more commonly

20MHz, is enough bandwidth for what you'll initially be doing.

I used a Hitachi V212 for many years, before I got my Tek 2465A. I love the Tek and use it as my primary scope; it is much more capable. But I still use the Hitachi for audio work, and it works great. I've seen those on eBay for under $100.

There are plenty of inexpensive Chinese-made 20MHz dual-channel scopes out there, at least in the states - Leader, for instance. Probably fine, for a first scope. The difference between that and something like a Tek will be in things like the accuracy of the input stages and the sensitivity of triggering on short spikes, and maybe features depending on what models you're looking at.

If it were me, I'd recommend getting a new Chinese scope as a first scope, unless you can land a used Tek or HP that is in guaranteed known good working order. There's no sense struggling to learn how to use a scope that doesn't work; even working ones can be tricky to fully understand. Later on when you understand how to work with it and what it *should* do, you can get a scope of high quality but unknown condition.

A differential probe lets you look at small signals riding on large signals, e.g., the voltage drop across a resistor on the high side of a transformer. Useful for working on power circuitry, certainly.

But for most mains-power work, you would just use a x10 probe, which (confusingly) divides the voltage by 10. This is the commonest of probes - practically any off-the-shelf inexpensive scope probe is either fixed x10, or switchable x1/x10. You can also get x100 probes, for high-voltage work.

240V * 1.4 = 336V, by the way.

In any event, as someone else pointed out, at the trigger input you would rarely want to apply high voltage.

There's an A6902A for sale here:

A lot depends on where you live. I'm surely not typical, but I've had to repair almost every scope I've purchased. And the ones advertised as "works perfectly" often took more time and $$ to repair as the ones advertised as "dead".

The most important information about a used product is, "Have they tried to fix it?" I've had good luck with busted stuff from people who don't know anything about it. I've had BAD luck with "needs minor repair" stuff from experts, or from hamfisted repairers. This goes for test equipment, cars, washing machines... I've also had some very interesting email discussions with a well-known equipment refurbisher who advertises stuff on EBAY as "no way to test it" under an email address that looks like an individual. They routinely calibrate this stuff. Why would they advertise it as untested??? And why would they vanish when you called them on it?

So, bottom line (for me) is, "Price is THE ONLY object." Assume everything is busted and offer accordingly. YMMV. mike

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Good point. I don't think I've EVER seen a 7300 or 7500 mainframe for sale. I'll ammend my recommendation to 7700 mainframes. I've had excellent luck with $10 7704s.

One more point. If you buy used equipment, try to get manuals and probes with it. A single manual or a single probe can easily cost you more than the equipment itself.

mike

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Yes, a new Chineses scope is the way I'm leaning. My local (Australia) electronics chain has a 10MHz (1 Channel) for about US$120. The 20MHz (2 Channel) is about US$300.

The low cost of the 10MHz seems great to get started for me. Then when I get to the point I need a "real" scope, I might look at a PC-based digital like the Stingray DS1M12 USB- I've got a couple of old PCs (133 and 120MHz) sitting around doing nothing.

Though I keep reading mixed messages about PC-based scopes. OTOH, it seems to me that to look at signal all you need is a fast A-to-D converter and the right software; it's simple and obvious - unless I'm missing something.

But, I hear what you're saying about the 2 channel. So maybe I should go for the 2 Channel Chinese and then I'll have a usable analog scope that will last me for many years.

It's not like I'm a "professional repairer" and it's not going to be used 10 hours a day. In fact, it will probably spend 11 months of the year just switched off (after my initial burst of study).

Thanks, yes I reviewed the RMS definition.

Thanks again for your advice, and also to others for their comments.

I must say the second hand market is a bit too hard for me to figure out as a beginner.

Though I missed out (forgot to bid) on a Tek 2336 last week that went for about US$300. The individual seller claimed it worked perfectly except the screen was a little darkened and it was missing a case clip. It certainly seemed like a real bargain at that price.

There's now a Tek 2215 in perfect order (with probe) currently bid at about US$340. Very tempting....but like posters have alluded, who knows about its calibration, and whether it will die in short time. And what about parts, etc. etc. etc.

regards, RR

[snip...snip...]

There are fast A/D converters and there are FAST A/D converters. Pretty much every PC-based USB scope that I've seen tops out at 100 Msps (often shared by two channels). For comparison, the lower-end Tek digital TDS-1000 series sample at 1Gsps per channel. Their high end digitals sample at 20 Gsps; those are the DAMN FAST A/Ds.

Ignore the "equivalent sampling rate" numbers.

--
Rich Webb   Norfolk, VA

I would really push you to go with two channels. Being able to do things like looking at both the good channel and the bad channel of a stereo, at the same time, is very useful. And you need two channels to do Lissajous curves, and to do curve tracing of semiconductors - all useful stuff.

As I said earlier, one of the big differences between okay and excellent scopes is the quality of the triggering. That statement is probably meaningless to you now, but you'll come to appreciate it over time... and good triggering is all about ANALOG, even for a digital scope. Being able to reliably trigger on a short erratic pulse, being able to *ignore* a short erratic pulse - this is stuff that can't be done in software; you have to just plain have good analog circuitry. Maybe there are PC scopes that have good analog sections, but I'm sure there are more that don't :-)

But don't sweat it too much; you'll get there eventually. Get a cheap dual-trace one way or another and start playing with it.

I'm convinced. :-)

short

have

Yes, that makes a lot of sense.

Thanks again. I'm going to take your advice.

cheers, RR

Have you had a look at the DSE 20mhz 2 channel scope for $399? Looks like a good buy to me, and I plan to buy one next week

David - who bought > Hi,

a

Yes, that's what I'm looking at getting. I think Jaycar have the same or similar unit.

Thanks for that....I won't buy one of those then.

cheers, RR

"RR" wrote in news:V7i%e.3574$ snipped-for-privacy@news-server.bigpond.net.au:

I find 2200 scopes to be very useful,good scopes.I have a 2213 myself. Far better than some other TEK scopes such as 434,any 5000 series scope. IMO,2213/15 are one of the most repairable of the newer TEK analog scopes. Calibration is not meant to be "long-lasting",TEK recommended at least yearly calibration.

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Jim Yanik
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For a scope to "fool around" with, I would suggest the OP look at a Tek

2225. They typically go for $50 - $100. They are also almost completely discrete, so repairs are feasible in most cases. Definitely not a "high-end" unit, but certainly enough to teach you the basics.

"BFoelsch" wrote in news: snipped-for-privacy@giganews.com:

A great beginner scope would be a T922 or T932/35;they have no TEK-made ICs or special transistors in them.Very simple switches,too.

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Jim Yanik
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