i wanted to know if you could build an o-scope from a television.

is this idea rediculous.  it okay if you think so.

if you can, do you know of any link with some schematics.  

thanks.

One way it has been done, is to compare the vertical and horizontal sweep voltages of the TV, with the audio signal and an external oscillator respectively. When the two are both 'close', you gate the video on.
It isn't high resolution, but it does work, there was a project in Electronics Australia many years ago (think around 1960s).
Much easier is to use a computer with a soundcard, and run one of the 'software' scope programs people have mentioned here. (though, they don't go down to DC, so it might be misleading unless you limit yourself to examining audio waveforms).

Perhaps it could be done, but would it be cheaper and/or work better than the Heathkits and Eicos that go for less than $25 on eBay?

I just saw a Tektronix T922 go for $25.

I got a fully working Tektronix 2225 with four probes for AUD$60, which at the time was close to US$30!

There are kits that interface probes with PC sound cards, then all you need is the software.

One thing first: possible, but dangerous,
because most older TVs are connected to the mains directly, with no isolating transformer.
With the old tube-b&w-TVs, it was quite easy:
Disconnect the 2 deflection coils from the TV-circuit, and feed them
from the Speaker-outputs of 2 audio-amps (~10 Watt) with proper isolated
power supplies.
The amp that feeds the horizontal coil gets its input from a simple sawtooth sweep-generator ("timebase");
the other amplifies the signal to be scoped.
This is basically an AC-scope for up to 20kHz bandwidth.
Now you only have to find the point in the TV-circuit,
where you have to (capacitively - for insulation) inject a signal to blank (mute") the ray at the instant of the sawtooth`s down-ramp.
(oh yes: and hope, that the TV`s horiz. and vert. amps withstand
being run loadless...).