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Modding a CRT
MUFF WIGGLER Forum Index -> Video Synthesis  
Author Modding a CRT
Hi all

I've not really delved into video synthesis but I think this falls roughly into that category - there are a lot of tutorials out there for hacking old TVs into oscilloscopes etc. I scored a cheap but solid miniature B&W tv with the intention of hacking it so that I can use it not really as an oscilloscope, but just as an audio visualiser.

After powering it up, I actually quite like the analogue aesthetic and I'd like the option to keep it running to also display composite video (which will be another hack in itself as it only has an antenna input).

My question is - the info I've found seems to involve a lot of wire chopping and such. Is there a way I'd be able to make the modification reversible, so I can return it to it's standard operating settings, and put it into 'visualiser' mode at the flick of a switch?
I did some preliminary doodling on the train. Any thoughts? I might have the H/V coils reversed here but the principle is the same...
Pic uploads have been a nightmare...

For posterity; works perfectly. Did away with the vertical coil switch as it seemed unnecessary and left that side as it is.

Given that the horizontal coil measured about 1.43R, I made a 2R dummy coil that is only in the circuit when nothing is plugged into the input, via a switched stereo jack.

Impedance is important here I believe since line level signals, although affecting the wave visually, had a very limited horizontal amplitude resulting in a very narrow scope of movement even with ridiculously loud signals. I also tried a mic level output which barely registered (as expected) despite the lower impedance, and a headphone output with similar results. I had just been given a Technics hifi amp so I tried the speaker outputs and that was just the ticket - outputs were probably in the 2-8R range and gave a wonderfully wide waveform with lots of movement. The internal trimmers probably need a bit of tweaking to get the best out of it.

Next I'm hoping to add the composite input so I can send analogue video signals in, see how that goes smile
Nice and simple drawing! I guess you have been looking at the Cracked Ray Tube tutorial?

I've been having some troubles with getting the dummy load right in the CCTV security monitor I've been using. Tried a lot of stuff from open circuit to sandbar resistors to coils from a different CRT. Every time the unit starts to heat up after a while and shuts down.

Dave Jones provided me with a lot of info about how to calculate the proper impedance for a dummy resistor or coil, however it also requires an expensive "current probe" for the oscilloscope. He also mentioned that not all CRT drivers work the same, and some need the coil in place for the flyback voltage (in which case a dummy resistor won't work).

So my two options are: crack open a matching set and get the coils from there, or trial-and-error with the resistors. I'd be curious if you have any other suggestions.
Here's some examples of what I do get, using H coil only. With a more powerful amp (I only have 10W now), I should be able to get up into frequencies which allow me to manipulate the original image from the security camera (see last image).

Hiya! I'd forgotten about this... looks like you've got some cool imagery there smile

I had the same issue to be honest - that drawing can be disregarded as it didn't work like that in the end. I didn't touch the V coil and stuck exclusively to the H, which gave me a result much like your first two pics. I came up with a switching method using a 3PDT toggle that *should* have worked in theory, whereby one way it had the TV operating under regular circumstances, and the other way it shorted the open end of the circuit with a dummy sandbar resistor and connected the coil to the jack. I think my coil measured about <2 ohms (just checked the last post!) but whenever I switched the sandbar in between the two points that would have been originally connected to the coil, it just completely killed the signal to the screen. Maybe it was the wrong resistor, but I simply grounded one side of the resistor instead which worked. I moved onto another project before I went back to test and tidy it up. The TV I used is an old one from the 1970s so unlikely to have any kind of auto-shutoff circuitry in there - not sure if this is good or bad!

What kind of audio amp did you use? I could only test mine using a speaker output from a dedicated hifi amp - I guess the coil wants to see a few watts into a very low resistance to display the line correctly. I intended to build a small amplifier but again moved onto other things!

The highest frequency you can reproduce depends on the power of the amp. Below is an image of the Adafruit one I used, with 10W per channel (I used the second channel to driver the speaker of the TV with the same audio signal). If you look at the Cracked Ray Tube docs, you'll see they use a 50W one.

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