Electronics > Projects, Designs, and Technical Stuff

TV to vector display conversion

(1/2) > >>

       I've been watching the EEVBlog for a while. Sorry my first post in this forum is an ask for help...

I've taken an old TV monitor (you know, a TV from the 80s that doesn't have a tuner...) and have been converting it into a vector display.

I bought a Sparkfun O-scope clock to get a test image from, and used LM3886s to built a simple current amplifier to drive the deflection coils. I also disabled the blanking on the display and rewound the coils to drop the inductance.

The amplifiers are basically this:


without the goofy input crap.

I attached the image I get below.

Question is, does anyone have any suggestions on how to get this thing under control? You can definitely tell its a clock, but I'm not sure I understand why there's so much distortion.

I took an oscilloscope to between the output of the op-amp and ground, and it looks very clean. However, when I use the 2 add-invert across coils to get the true voltage (because the shunt could throw it off), I see what I think are large voltage spikes from the inductance of the coils. I wonder if that's the problem? I haven't been able to blank the beam on demand, so I don't know how big of an impact that will have on the image...

I've been fighting this off and on for a few months and thought I'd ask some people who know what they're doing. Any suggestions on where to go?

CRT scopes use electrostatic deflection for a reason; bandwidth. The beam can change direction much faster than on a TV with magnetic deflection.

Yeah, that's because scopes have to operate at 100Mhz+.

Vector displays don't go too far beyond audio frequencies, and have been used in large CRTs for years. But now that you mention it, I wonder if I should try Youscope on it, since its designed for audio frequencies and the Oscope clock is built solely for scopes...

Should I try to add a zobel filter to combat the inductance in the coils?

Youscope would be worth a try, but keep in mind the TV vertical frequency is only tens of Hz so even audio range might be too high.

Follow the posts from here on:


In my CRT the unmodified horizontal (now vertical) coil has a self resonant frequency of around 350kHz and the (much higher inductance) vertical (now horizontal) coil has a self resonant frequency of approximately 20kHz.

I have since designed an equalizer board for the horizontal channel now utilizing the 20kHz coil and both channels are amplitude flat and phase matched to +/-1 degree from DC to 1kHz. The vertical channels small signal bandwidth (using the low inductance coil) is 45kHz. The other channel is a bit over 5kHz.

The voltage developed across an inductance is proportional to the rate of change of the current flowing through the the inductance. To handle moderate frequencies you need considerable voltage compliance. A fast rise or fall transition will produce a potentially very large voltage spike across the inductance, causing your driving amplifier to clip against the rail(s) and effectively slew-rate limiting di/dt to the driven inductance. it looks like you also have ringing and oscillation issues. Plonking the inductance inside the feedback loop of a power op-amp isn't so simple. Damping control and frequency compensation must be specifically tailored to the characteristics of the inductance.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version