What do you guys think of these oscilloscopes?

http://cgi.ebay.com/DSO-2090-100Msa-s-USB-PC-Digital-Storage-Oscilloscope-/130301821160?pt=BI_Oscilloscopes&hash=item1e569800e8

http://cgi.ebay.com/Uni-T-UT81B-Oscilloscope-8MHz-w-USB-and-vivid-LCD-UT81-/370473601888?pt=BI_Oscilloscopes&hash=item5641f28b60

http://cgi.ebay.com/Handheld-Digital-Multimeter-Oscilloscope-UT81B-gift-/260724693021?pt=BI_Oscilloscopes&hash=item3cb467181d

I can't speak for the PC based scopes but I would stay away from hand-held scopes. I used a hand-held Fluke Scopemeter (http://www.fluke.com/fluke/caen/portable-oscilloscopes/fluke-190-series-ii-scopemeter.htm?PID=70366) for audio and considering its price tag I was very disappointed. Not to say that the ones you are looking at would be the same but personally I would stay away.

As far as the PC based scope goes, I'm very interested to hear what people have to say. The only other scope I have access to is a tank and along with the ancient signal generator I borrow from work I'm in danger of putting my back out every time I want to set up some gear.

canidoit said:

http://cgi.ebay.com/DSO-2090-100Msa-s-USB-PC-Digital-Storage-Oscilloscope-/130301821160?pt=BI_Oscilloscopes&hash=item1e569800e8

http://cgi.ebay.com/Uni-T-UT81B-Oscilloscope-8MHz-w-USB-and-vivid-LCD-UT81-/370473601888?pt=BI_Oscilloscopes&hash=item5641f28b60

http://cgi.ebay.com/Handheld-Digital-Multimeter-Oscilloscope-UT81B-gift-/260724693021?pt=BI_Oscilloscopes&hash=item3cb467181d

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Depends very much on what you're looking to do with it.

Will this be for modern electronics where you need to capture and store fast action, or for classic electronics?

Do you need to measure digital/logic timing? Do you need to decode I2C control logic?
Do you really need high frequency (>100MHz) as this bumps up the price?
Do you need high voltage support?
Do you really need it to be portable or bench based?

How many simultaneous channels do you need?

Is a FFT spectrum going to be useful, or can you just use a standard sound card and some open source software on your PC (e.g. spectrum lab)?

If you're looking to calibrate or professionally service stuff against a standard then you probably need something with a calibration sticker on it.

I initially bought a cheap and cheerful modern (Owon) 2 channel DSO and found the input noise and vertical resolution to be truly awful, and sent it back for a refund.

I then bought a ~30 year old second hand Tektronix 2445B 4 channel 150MHz scope on evilbay for far less including probes. It's really fantastic for what I use it for. The 10megohm 400V peak input and 50V/division (with 10:1 probe) is absolutely ideal for hooking up directly to tube circuits. On the finest trace setting I find you can actually see better resolution and with less artifacts than on a digital scope. You can really see if something has small HF oscillations or low frequency motor boating. No chance of calibration or spares, but you can probably measure levels better with a modern true rms voltmeter than with a scope. So my scope is more important to me for fault tracing and quickly looking at wave shapes along the signal path. I also got some Wandel and Goltermann audio test sets really cheap (25 euros!) from an army surplus depot. I combine these with PC software via a standard sound card if I ever want to look at audio spectra and harmonics.

MeToo2 said:

I then bought a ~30 year old second hand Tektronix 2445B 4 channel 150MHz scope on evilbay for far less including probes. It's really fantastic for what I use it for. The 10megohm 400V peak input and 50V/division (with 10:1 probe) is absolutely ideal for hooking up directly to tube circuits. On the finest trace setting I find you can actually see better resolution and with less artifacts than on a digital scope. You can really see if something has small HF oscillations or low frequency motor boating. No chance of calibration or spares, but you can probably measure levels better with a modern true rms voltmeter than with a scope. So my scope is more important to me for fault tracing and quickly looking at wave shapes along the signal path. I also got some Wandel and Goltermann audio test sets really cheap (25 euros!) from an army surplus depot. I combine these with PC software via a standard sound card if I ever want to look at audio spectra and harmonics.

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When you do find oscillation with your oscilloscope, even minute, what is the usual fix for it? Is it to replace components or is it a matter of trimmer adjustments?

Thanks for the info, I was about to consider an OWON.

Can Oscillsoscopes get damaged from the units they test, I am concerned about buying those USB/PC oscilloscope units and damaging my laptop with my inexperience.

I just want to callibrate my EQN, NV73, 1176 and La2a and maybe some form of SSL type compressor and Tube preamp in the future.

Thanks for the response.

Hi, I have the Hantek DSO-2150 pc based, and is pretty awful :S in weak signals you can't see what is happen... the signal goes crazy and the waveform is always deformed, is never clean and round (in sine wav case)... has always some noise attached! of course, if you want to check out the phase, and some voltages is ok... but to get the whoke pic, is preety bad... try any good old cinescope's oscilo! analog one, 20Khz 2 channels.... best way for audio...
Cheers,

Eddie

I use one of these http://www.vellemanusa.com/us/enu/product/view/?id=351263when I'm working away from here. It's a hell of a lot less hassle to take on site, especially if I'm working abroad, & lets face it if you're fault finding & testing most of the time you just need to see a sine wave & whether it's distorting.  The velleman also has some usful metering functions for audio testing, like dBv, dBu, etc.

I have a regular dual beam 20MHz scope that I use at home.

>I am concerned about buying those USB/PC oscilloscope units and damaging my laptop with my inexperience.

A VERY valid concern.  The short answer is yes, you can smoke your your PC.  I only use my software based telemetry on know working subjects.  Measuring response and distortion.  I never use software based telemetry during surgery.  Far too risky especially with tube circuits. 

canidoit said:

MeToo2 said:

I then bought a ~30 year old second hand Tektronix 2445B 4 channel 150MHz scope on evilbay for far less including probes. It's really fantastic for what I use it for. The 10megohm 400V peak input and 50V/division (with 10:1 probe) is absolutely ideal for hooking up directly to tube circuits. On the finest trace setting I find you can actually see better resolution and with less artifacts than on a digital scope. You can really see if something has small HF oscillations or low frequency motor boating. No chance of calibration or spares, but you can probably measure levels better with a modern true rms voltmeter than with a scope. So my scope is more important to me for fault tracing and quickly looking at wave shapes along the signal path. I also got some Wandel and Goltermann audio test sets really cheap (25 euros!) from an army surplus depot. I combine these with PC software via a standard sound card if I ever want to look at audio spectra and harmonics.

Click to expand...

When you do find oscillation with your oscilloscope, even minute, what is the usual fix for it? Is it to replace components or is it a matter of trimmer adjustments?

Thanks for the info, I was about to consider an OWON.

Can Oscillsoscopes get damaged from the units they test, I am concerned about buying those USB/PC oscilloscope units and damaging my laptop with my inexperience.

I just want to callibrate my EQN, NV73, 1176 and La2a and maybe some form of SSL type compressor and Tube preamp in the future.

Thanks for the response.

Click to expand...

As others have said, I like using an analogue scope on analogue kit for fault finding and signal tracing. I don't want 300V going through my apple imac or expensive monitors. That is certainly possible with (faulty) tube gear. For tube kit you probably therefore want a cheap/classic 2nd hand dual beam 20MHz+ scope with high voltage input/ protection. If it ever does blow up (unlikely), buy another one.

I actually use this series of tips for testing/turning on tube gear for the first time, especially the series 40W light bulb test: http://hammondb3organ.net/restore.php

Once the circuit is up and running and you are confident that the input and output levels are not going to fry your other test kit/ computer you can hook it up to other stuff.

Especially for calibrating compression levels and EQ curves of audio in and out, you can get away with a normal sound card/ your standard balanced audio input and output on your PC together with free software. In this case a spectrum analyzer (FFT function) is usually a lot more useful than a scope. You can then directly read off the compression levels whilst at the same time watching for harmonics/loss of signal etc. For EQ you can inject pink noise at the input at a specific dB/sqrt(Hz) level using a software noise generator and simply look at the EQ curve and -3dB points directly on the spectrum analyzer output. Check out http://www.qsl.net/dl4yhf/spectra1.html There are plenty of others (and possibly better and more user friendly) pieces of software out there, but this at least gives you some idea of what you can get for free before you decide to shell out for that Agilent baseband analyzer on eBay.

Re: your other question. Basically an oscillation is due to a phase shift of 180 degrees at a point where the open loop gain is greater than 1. As for getting rid of oscillations, well you have to find the source of the feedback loop. That's often difficult, but much easier if you can immediately see the results of your change on a scope. Fixes can range from re-twisting and rerouting wires, changing a tube, adding/changing capacitors, adding grid or screen grid stopper resistors, to changing transformers or op amps.