Now you people gonna see what happens when I catch just the right buzz. lol
There are a jillion old scopes out there. We can still somewhat use the 20M hz jobs for something, but anything lower than that is worth shit. Then the re are scopes that don't quite work right but have good deflection outputs at least. They might not trigger or some shit. Maybe the vertyical calibrat ion is so frigged up there is no hope, shit like this.
But in cases where the finals are at least good and we don't have to deal d irectly with the CRT, life might just be good. First of all this applicatio n doesn't need DC coupling, in fact might do better without it. For the mom ent, I shall treat the subject as if we have a normally operating scope. Tr iggered, maybe, but we will be using the external input. Not trigger, the i nput.
To preface this - I have been known to use a 1Khz square wave fro certain t hings, one of which is to check electrolytic caps. Tis is better and faster than Dick Smith, The Wonderbox or anything. The Wonderbox is pretty damn f ast being analog, but the scope method with the 1Khz square wave beats it. I will supply details one of these days in a separate article.
In using a 1Khz square wave for signal tracing in audio circuits, you will notice a change in the waveshape when equalization of any type is applied. Try it, roch the bass and treble controls, or better yet get a five or ten band equalizer and play with that.
So you get used to that and then, like me, you might think that that wavefo rm is pretty predictable. When you adjust the bass or treble or anyting lik e that, going through the phono eq it should have a certain look. And so...
To make a dynamic response indicator out of a scope :
I estimate that a sqaure wave of about 300 Hz should be used. This is off t he top of my head but is based on the fact that most bass controls, even wi th the (more desirable) bass control tunrovers, seem to tilt the entire fla t part of the square wave. I know this is not what happens to the frequency response. Ther is also phase involved and all that shit, but the idea is t o get it to more closely approximate a real time frequency response curve.
I know this can be done with a chip and an FFT but this is quaint becasue i t is analog.
So, this square wave goes through the DUT and when it comes out we know wha t the curve is because it is displayed on the scope screen. To do this we r everse the horizonrtal sweep of the scope.
Now the horizontal waveform must not just be of the polarity to make it swe ep right to left, it must also be non-linear. Some sort of logarythmic or s omething. With a bunch of math, it could maybe be made to read octaves on t he scope's graticule. But if it isn't that accurat that's OK too. I even do ubt we can do the whole audio spectrum without some sort of ranging. Like h igh and low, with different frequency square waves.
Eventually, after all this is developed, a calibrated microphone can be add ed and room acoustics etc. could be measured.
I guess this would qualify as a "poorman's" version of what is actually out there. But instead of buying something this could be a way of using someth ing that could be thrown out, but why ?
Whaddya think, is this kinda doable I think it is. Hell, I got the Wavetek so I got the square and a triangle, and a half sawtooth. Between all this I won't need a hell of alot of components to at least get a working model t o tweak it.
Really, to be any kind of useful, this thing should have three ranges I thi nk. Low of course to screw around with the woofers. One full range range, 3
00-400 Hz I think, then maybe half that or a bit less for the low end. Mayb e a little higher for the high setting, 2 Khz ?I'll draw up a little something. ... ... ... ... (I shit you not, I just di d...)
Fig 1 is the scope display. The green line represents the backward waveform of the top of a square wave after going through whatever device under test .
Fig 2 depicts in black the normal, linear horizontal scan of a typical scop e, the brown shows whe needs to fed in. It makes the sweep slow at the begi nning and fast at the end. Or do I need it the other way around ? Whatever. that kind of thinking doesn't have to be done tonight.
What else was I going to think about on a Saturday night after everyone lef t and I ain't ready to hit it yet ? Plus I got this friend who keps running across scopes.
If you thinkk it is a waste of time, good. Let's do it anyway.