Oscilloscope... this good?

[bluefoxicy]

I need an oscilloscope to do some tests on modified amps and on ones I build, anyone think this will do it?

http://cgi.ebay.com/TEKTRONIX-455-A2...QQcmdZViewItem

I'm going to be looking for harmonics to test for distortion, for a start. Gotta learn to use one but gotta get one I can use for the purpose first.

http://cgi.ebay.com/ws/eBayISAPI.dll...id=p2759.l1259

This too

[BackwardsBoB]

I like the Velleman HPS10SE. It's around $160 new, doesn't take up any bench space, runs on batteries so you can float it, works on signals up to 600V out of the box, and has digital storage, so you can capture intermittent events like the attack on a played note or what happens when you come out of standby. You'd want the SE - special edition with the backlight. The even cheaper one without the backlight is very hard to read.

A function generator to make sine waves of different frequencies as a signal source will be handy too.

For another $350, you can get Velleman's two channel USB scope and their USB function generator, and make frequency response measurements on your computer.

[Enzo]

That is a very nice scope as long as it is indeed in the shape described. Notice he says the controls need cleaning. This would be more than enough scope for about anything you'd ever do in guitar amps.

Note also it does not mention probes, so I would assume there are none. You'll need to buy a probe or two. The good news is I have always gotten reasonable service from the $29 probes on the market. You can find them that cost hundreds.

[BackwardsBoB]

You have to worry about the time-base knob on old Tek scopes. It's a complex assembly with the pull-time base switching and stuff.

[Steve Conner]

The Velleman thing is digital. I much prefer analog scopes for working on audio circuitry. I don't know why, I think that noise is easier to assess on an analog scope.

You probably won't go far wrong with a 455, I don't think Tektronix ever made a truly bad scope. However in the case of that particular Ebay auction you linked, the abnormally short traces worry me. The trace should cover the whole width of the screen, and the trace length is a preset adjustment that shouldn't have drifted so far out. It could need some TLC. Then again, maybe someone was just messing with the trace length trimmer. Your call.

I have a Tek R7603, Tek 222A, and Pico ADC212/100 PC-based scope. The timebase knob on the 7603 never gave me any trouble. I bought the 222A on Ebay because it looked so cute, but it actually turned out to be very usable. I only use the Picoscope when I need to capture one-shot events and analyze them in gruesome detail.

A good signal generator is very useful too. For audio work you only really need the simple ones, that are basically modern versions of the HP 200C. I have a digital one, but mainly because a friend was selling it cheap It has one cool feature that I've been using quite a lot lately: a gate mode that pulses the output on and off. It's almost like a standardized, repeatable pick attack.

[mooreamps]

That's a good looking scope. Better grab it, before I do.....



-g

[bluefoxicy]

I think the 455 goes from 100MHz to 350MHz, and I need like 20Hz to 20KHz (or more) to work on this stuff... I'm not sure. I know zero about this.

[Enzo]

No, the scope responds from DC (zero Hz) up to 350MHz or whatever. There are some REALLY old scopes that did not respond down to DC, but I haven't seen one in decades. The MHz rating of a scope is the highest frequency signal it can display reliably.

But for the record, my old beater 35MHz scope can still just barely detect the 178MHz output from a wireless mic transmitter. SO the upper limit is where it starts to roll off, not a brick wall upper limit.

In fact I don't think there is a scope made that doesn't have more than enough freq response for your needs.

[Bruce / Mission Amps]

Among others, I have two Tektronix scopes, a 455 and a 465 and they both are superb... and major overkill for about 99% of all the audio work I do.
Although my former employer supplied killer test equipment for my commercial bench, for the first few years of my tech life in my home shop, I use and old and very seriously gronky, microwave oven sized 500Khz Hewlett scope that worked flawlessly for a whopping $10 investment and that included an extra set of tubes.
Moral.... just get any analog scope and learn to use it as they are a wonderful investment for a home shop even in it's most basic use.

[Jag]

So did you get it? I see the auction closed with one bid.

I bought a cheap Hitachi ($35.00) on eBay a couple of years ago. Works just fine for me.

[bluefoxicy]

Originally Posted by Jag

So did you get it? I see the auction closed with one bid.

No, I missed it, and also still thinking. I'm currently very poor anyway so....

[Alex/Tubewonder]

Originally Posted by bluefoxicy

I need an oscilloscope to do some tests on modified amps and on ones I build, anyone think this will do it?

I'm going to be looking for harmonics to test for distortion, for a start. Gotta learn to use one but gotta get one I can use for the purpose first.

An oscilloscope will not show you harmonics as such, it shows signal in time domain not in frequency domain. A spectrum analyzer does a much better job at this.
Of course, you can make a pretty good judgment of harmonic content based on the shape of distorted sinewave once you learn the basics of it. The effect of 2nd and 3r on "clean" sine is easy to identify, higher are more difficult to untrained eye, however, high levels of 5th & 7th you should hear easily as not so much a distortion but rather something "disturbing" in the total complexity of the tone.

[martin manning]

If you have a windows computer and you want to play with a PC 'scope and spectrum analyzer, here is a freebie you can get:

http://www.trueaudio.com/rta_down.htm

There is a built-in signal generator too. I played with the scope some to observe output waveforms and it worked fine for that. You need an attenuator between the amp and the computer since it uses the microphone input to the computer's sound card as its input- a headphone or line-level source is what you want to feed it.

MPM

[Paul P]

Originally Posted by martin manning

You need an attenuator between the amp and the computer since it uses the microphone input to the computer's sound card as its input- a headphone or line-level source is what you want to feed it.

I bet you do ! I wonder what 400v would do going into your microphone input...

There are definitely some accessories that you'd need before using your
computer for a scope, like the attenuator (I don't know what that would
look like) and a 1kv probe.

I bought a dual-trace 40mhz Japanese scope from a local classifieds for 100$,
including 4 probes, and it works great for amp building and testing.

Paul P

[lmolter]

I picked up a Leader 100MHz, 2-channel scope for $75 on eBay. It needed cleaning and the timebase switch was dirty. But it cleaned up nice and the switch works fine after a shot of tuner cleaner. Also got an Eico signal generator for $25. Wasn't sure it worked until I got the scope. So... my advice is to keep looking on eBay until the right one comes along for the lowest price. The last time I looked, there were 900+ oscilloscope-related ads. Personally, I'd stay away from anything old with a round tube. You don't want to have to fix the scope before you can use it. Stay with Tek, HP, Leader. Not sure about Eico. And you can get a set of 2 or 4 probes for less than $40 (for 4).

Now, if I only had something to fix. Anyone want to send me their broken amps? Kidding. Or not.

Larry