soundcard as oscilloscope

oh, I didn't even see that line in his original message ;/

Anyways, Its Jon.. but I prefer Mr. Dissonance ;) J/K

I guess. But I figured it must be a sin wave since, say, if they used a square wave then it would screw up everything? (i.e. things like transformers would behave differently and stuff?).

I guess I just though it must be a sine wave for some reason... I suppose cause all the examples I've seen show a pure sine wave and never the real thing or mention that its distorted.

I think the nuetral is about 1 V above ground or so. I'm not to worried about scewing up my sound card all that much to tell you the truth... its a POS anyways... I'm more worried about screwing up my computer but I feel save that the sound card would prevent that(atleast I hope).

Well, I don't have an isolation transformer so I don't know how to do this... I brought a lot of stuff but not nearly enough it seems. Just trying to get into some projects(such as building a PS with some cheap transformers I brought) and would like to see the output(so atleast I have some idea whats going on).... I don't like poking around with a DMM to much.

yeah, thats why I wanted some oscilloscope. I suppose I don't need anything fancy to do that(hence using the sound card and/or TV). I just started yesterday on this stuff so I still need more time to play ;)

Thanks again, Jon

Well, I wired the wall outlet myself so I know its hooked up right ;)

Are you saying that the Variac's chassis is wired to neutral? (shouldn't it be wired to ground? and ground is almost always hooked up right?)

ok... I guess I thought I was living in a perfect world or something.... I didn't realize it was such a mess.

I tried that.. I didn't change anything(that I could tell).

it could be that.

Well, I was just doing it for fun... to see if it worked. I suppose it did but I thought there was something wrong with my circuit... aslong as its not me then I'm fine ;) Now I can move on to other things.

Thanks, Jon

I don't understand this. Why do I have to run the deflection coils from the circuitry of the TV? Couldn't I just tap into the plug and use my own circuits to generate the current needed for deflection?

On the TV I have there are 4 Hot wires going to the CRT. One is to the top of the CRT and is a really thick wire that comes from this strange looking capacitor like box. Another hot and orange, that is smaller, runs to the TV tube from the same box. I Think they control the focus and strength because they are near the controls "Focus" and "Screen" and I played around with them and they change the focus and something else(its not really strength in the same sense but it makes the beam brighter(along with other things)..

Then there are some "bus" like wires that go into the TV tube each having a hot and 2 and 3 other wires(there are 2 of these busses). I suppose they control the modulation of the signal and stuff to the TV tube to generate the picture. I took both these out and there was no picture(no blue dot)... I'm not sure why though.

The coils themself are attached to some jack by themselfs. I removed them cut the wires and hooked them directly to my variac to generate some pictures. I should be able to control the coils in any way I want that is independent of the TV circuitry?

How bout rewiring? its not linear simply because of the size of the deflection? If the deflection is small then it should be ok?

rewire?

Thanks, Jon

What about a 5 yr old TV? have they changed in 20-30 years?

Well, where and when are these hamfests? If they got scopes this cheap then I'm there... I read on another site that they are a great place to find drill bits for PCB boards too.

Thanks, Jon

WHERE WHERE WHERE?

When I did a small search on oscilloscopes I couldn't find anything under

and what does "mostly working" mean? ;)

Thanks, Jon

much.

You would probably see about the same waveform if you used a $5000 oscilloscope single-ended and tied the ground to neutral. The 1 volt or so from neutral to ground has a lot of power behind it and is mostly noise. You can see the noise by connecting your sound card input from neutral to ground. This signal will be added to the true waveform. A simple cheap isolation transformer would be a Wall-Wort with AC output, or break open a DC type and remove the rectifiers. However, power transformers usually run near saturation, so you will see the clipping at close to rated voltage, but if you run the variac down the waveform should clear up.

If you have stereo input on the sound card you could try making a differential input oscilloscope, and display the difference in the two signals. You would need to make sure the external voltage dividers have exactly the same ratio, but that should work fine.

I have a specialized product that is essentially a high-accuracy AC current meter. You are welcome to download the demo and other information on my web page

I had considered using a sound card for my application, but I'm not sure if the accuracy and sensitivity is good enough. I have a special PC board with an instrumentation amp, PGA from 1x to 200x, and a 10 bit A/D on a PIC. It communicates with the PC on a serial port at 57.6 kBaud for 2400 samples per second. Lowest range is about 5 mV FS, resolution about 5 uV. The parts cost is probably only about $20.

Good luck, and play it safe!

be wired to ground? and ground is almost always hooked up right?)

I meant that most variacs pass through one of the incoming wires straight through-- if that happens to be the "hot" side of the line, and you try hooking that to the ground side of your computer, major kabang will follow. Trust me on this. I still have the charred PC board to prove it.

cut the wires and hooked them directly to my variac to generate some pictures. I should be able to control the coils in any way I want that is independent of the TV circuitry?

Okay, your're lucky. The older TV's I tried this on did not work with the yoke disconnected. They depended on the inductance of the yoke to help boost the B+ voltage.

You can buy a good scope for about $100 used........i have done quite a few from ebay for tinkering

I guess as long as they're snapping whippers.... ;-)

Cheers! Rich

Alas, Rich, i must disabuse you. They used induction motors with well understood slip and appropriate gear chains. I know i took enough of them apart as a kid. Motor shaft was typically 1750 RPM.

Alas, Joseph, i must disabuse you. They used synchronous motors with out slip but with appropriate gear chains. At the power station they just made sure that at the end of the day all clocks again showed the correct time. The momentary frequency was not very stable.

Just look around E-bay. Like I said, going price for a Tek 465 (a really nice scope) is $100-$200. And the lesser Eicos and Heathkits go for $20 (if they sell at all). These prices are more than what you would typically pay at a hamfest or flea market.

CRT may be dim, controls noisy, uncalibrated, not all switch positions may be working. But "mostly working" in that you can still hook up a signal and see it.

Tim.

I got this one and it's fantastic.

It comes with complete schematics and manuals.

Al