TV to vector display conversion - Page 1
Forum section: electronics / projects

author: Kevman
author_rank: Regular Contributor
timestamp: May 21, 2013, 01:27:14 am
content: Hello,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:http://spritesmods.com/?art=bwidow_fpga&page=4without 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?

author: amyk
author_rank: Super Contributor
timestamp: May 21, 2013, 08:01:45 am
content: CRT scopes use electrostatic deflection for a reason; bandwidth. The beam can change direction much faster than on a TV with magnetic deflection.

author: Kevman
author_rank: Regular Contributor
timestamp: May 21, 2013, 12:49:02 pm
content: 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?

author: amyk
author_rank: Super Contributor
timestamp: May 22, 2013, 11:30:42 am
content: 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.

author: GK
author_rank: Super Contributor
timestamp: May 22, 2013, 01:13:41 pm
content: Follow the posts from here on:https://www.eevblog.com/forum/projects/home-brew-analog-computer-system/msg222430/#msg222430In 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.

author: Kevman
author_rank: Regular Contributor
timestamp: May 23, 2013, 01:15:05 am
content: Yes, I figured I would have to compensate in software for the varying frequency response of the display coils, but was really hoping to get it closer in hardware first. Since I don't have any sort of spectrum analyzer (just my trusty Tek 2445b), I figured I'd have to do it through experimentation.Could I design the software to try and run a 'fixed' frequency and still draw what I want? I'm having trouble wrapping my head around that.The supply to the amplifier is +/- 15V, and I'm not seeing anything close to that on the scope yet, so I think I'm good there.I just tried Youscope and you can definitely see what's going on, but yeah... The ringing. I think that's killing me. I'll do some research on what to do about it, but suggestions are appreciated.

author: GK
author_rank: Super Contributor
timestamp: May 23, 2013, 01:21:25 am
content: If the coil current is ringing at the coils self resonant frequency, a resistor of sufficently low value put in parallel with the coil will kill the ringing. 1k would be a good start.What is the inductance value for your re-wound coils? It is quite possible that the coils self resonant frequency is much too high for the chip-amp to damp out on it own, due to its inability to maintain a low output impedance well above the audio band.

author: Kevman
author_rank: Regular Contributor
timestamp: May 23, 2013, 11:01:29 pm
content: Argh, I don't own a meter capable of inductance, or a function generator to do it with the scope.Man, I'm really tempted to pick up a function generator. I may have a look around eBay.There's already a 39 ohm resistor across both coils.

author: Barry Ward
author_rank: Newbie
timestamp: November 30, 2021, 03:08:54 am
content: I know, old reply but...your issue is impedance matching. A resistor across the coil is sort of ok but your amp with low output impedance and no wide band phase correction will not work well... AMHIK.You would be much better off with a series resistor that matches the driven coil impedance. Yes, you will need to burn 1/2 your power in the resistor(s) (or your image will be 1/2 size) but it will prove that the system will work.Then you can work on sensing the coil current and applying phase / frequency compensated correction to the amp input.Barry Ward