| Electronics > Projects, Designs, and Technical Stuff |
| Home Brew Analog Computer System |
| << < (16/66) > >> |
| SeanB:
With the CRT you are using did you connect up the degaussing coil ***EXACTLY*** as it was in the set originally, along with connecting it up to power to operate when on the bench in the position it is in. Certain tubes are incredibly sensitive to stray fields, and this often will affect only the gun nearest to the source, or where the change is greatest. You might want to try using a rolled Mumetal shield around the CRT base that envelopes the whole gun assembly and the magnets, just do not kink it, and solder a wire to connect it to the CRT earthing band. If the magnets are as they were in the original set then it is better not to touch them, as they are both going to be interactive with each other and probably have been glued to the CRT neck as well during tube alignment. I have had CRT monitors where the tube back was covered with plastic strips and full of small ferrite plastic magnets to do beam correction across the tube. Others just had under 5 for the same model. |
| GK:
--- Quote from: SeanB on April 10, 2013, 06:38:22 pm ---I have had CRT monitors where the tube back was covered with plastic strips and full of small ferrite plastic magnets to do beam correction across the tube. Others just had under 5 for the same model. --- End quote --- Thankfully this CRT doesn't have any of that; it just has three pairs of rotatable magnets on the CRT neck, as can be seen in the photo I posted. They are not glued to the actual CRT at all, but poorly "locked" in place with small dabs of that common white compound that sets hard and brittle (which is also visible in the posted photo). I don't think it should be too much of a hassle to tweak, but I think I shall refrain from playing with it until the whole thing is reassembled into its new chassis with my own electronics. I'm going to have a 19" rack mount front panel made by Front Panel Express, with a proper cut-out and mounting holes for the CRT. The rest of the chassis I'll fabricate myself out of galvanised sheet steel and aluminium ribs. I will shield the CRT entirely in its own metal box. This will be necessary as there will be some hefty power transformers to power the linear deflection coil driver amplifiers. The degaussing coil is currently still connected as original and operational at each power-on cycle. That part I have not reverse engineered yet but I'm pretty sure it's not much more complicated than a series R and an NTC thermistor, though there might be a TRIAC switch and timer or similar to shut off the degaussing coil current completely after it has done its job. Worse case, if the red beam convergence cannot be adequately aligned with the green and blue beam with the CRT assembled into its new and final magnetic environment, I'll just design the digital control of the guns such that the only displayable trace colours are only either red, green or blue. No convergence issues if you’re only ever running one beam at a time ;D |
| GK:
OK, I now know how to adjust the convergence. This is from the Commodore 1084S computer monitor manual, rather than a Palsonic TV, but the CRT and magnet assembly is practically identical. |
| GK:
Finally have the horizontal deflection coil driver up and running. Now I have highly linear horizontal deflection. In the vid the vertical coil(s) are being driven directly from my AWA G251 oscillator. The schematic diagram *.pdf for the horizontal deflection amplifier is attached. The vertical coil driver will be very much the same, with the exception of operating on +/-100V for the discrete power output stage, as the vertical coils have much higher inductance than the horizontal coils. This CRT originally have the two vertical coils wired in series (140mH total). To reduce (halve) the required voltage compliance for the driver amplifier I have rewired them in parallel. |
| GK:
I needed a break this evening from doing PCB layouts for this huge project and decided to fire up the soldering iron instead. I couldn't wait until all the boards are etched so I quickly knocked up a "dead bug" version of the vertical deflection coil current amplifier (the mess in the foreground of the attached picture). I decided in the end to orientate the CRT vertically. It's looks way cooler this way when simulating and plotting missile trajectories and like stuff. However there is also a sound technical reason. The vertical deflection coil(s) has a great deal more inductance and a much lower self-resonant frequency than does the horizontal deflection coil. In the end I had to frequency compensate the vertical coil driver amplifier for a small signal bandwidth one tenth that of the horizontal coil driver. In a display such as this, vertical bandwidth is much more important than horizontal bandwidth. The horizontal bandwidth just has to accommodate the horizontal sweep/repetition rate; and that only needs to be quick enough for a flicker-free display. So, by flipping the CRT on its side, the slow vertical deflection becomes the horizontal and the fast (comparatively) horizontal deflection becomes the vertical. There was of course another option - I could have just rotated the yoke 90 degrees instead, but as I mentioned already, my preference if for the CRT on its side. The attached pic shows the prototype monitor displaying my simulation of the trajectory of two bounding balls simultaneously (one ball a little heavier than the other and subjected to a larger constant for gravity). Tomorrow evening I will "dead bug" the colour gun multiplexing and get the individual ball trajectories showing in different colours (and perhaps modify the simulation to simultaneously compute another ball trajectory or two). |
| Navigation |
| Message Index |
| Next page |
| Previous page |