Hi all,
Part 1 described the innards of the Sperry radar indicator that I wanted to use for a clock and it's difficult internal structure. Now I'll get started on the design. The first step was to work out what frequencies and voltages I would need. Experience has shown that CRT clocks need to run at a scan frequency that is related to the mains frequency in order to prevent stray electric and magnetic fields in the environment from wobbling the display. This was not likely to be a big issue here as the neck of the CRT was shielded and by the time the beam got out of the shield it would be accelerating fast enough to not be seriously affected. Nevertheless I decided on 900 Hz as it is a multiple of both 60 and 50 and was in the design range of the power transformers. Looking at the ratings of those transformers I worked out that I needed about 50 watts, possibly less if I could derive the +/- 12 volt and the shift coil supplies from the mains. I also thought it would be pretty nifty if I could drive the scan coils from a rotary resolver driven by a stepper which in turn would be controlled by an AVR and GPS receiver. Cool huh? The only snag was that I didn't have, and would almost certainly never have, the proper resolver! I did have a couple of surplus 50 and 400 Hz synchros though, so why not try? The 100 volt 900 Hz power supply had to be a sine wave as that was expected by the area balance circuit. This meant that the power supply would be a linear amplifier driven by a sine wave source. Inefficient I know, but making a switch mode sine wave inverter was just too hard given the difficulty in finding suitable cores to work at 900 Hz. I designed up a 900 Hz exciter using a 4046 PLL phase locked to the mains followed by an active low pass filter to knock off enough harmonics to make it look like a sine wave. I included a voltage controlled attenuator to allow feedback control of the output and built it up on a little PCB - it worked very well and supplied several other outputs that I thought I would need. For the power amplifier I chose a LM3886 which was cheap and capable of generating lots more power than I needed providing it was heat sunk properly and supplied with enough volts. It would need a transformer to shift the output voltage to 100 and I raided both my and several friends junk boxes to find what I though might work. I had a suitable 80 VA mains power transformer already - it would develop +/- 25 volts and had taps to also get +/- 17 volts for a pair of 12 volt 3 terminal regulators, and 6 volts for the CRT heater thus allowing me to remove one of the 900 Hz transformers. The junk box yielded a suitable very large heatsink so away I went. I got to work removing all the unnecessary stuff from the indicator, Dismantling is definitely a lot of fun :-) Out went the heater transformer, IF strip, the power relays, fuses and the multi-pin connectors. All the associated wiring came out of the looms too. A few charred power resistors and all the electrolytics were replaced as were some messy replacement rectifiers that sat on blackened areas on the rear panel(!). The variable video time constant parts and switching went, as did the heading indicator and signal strength meter. After doing that there was enough room to mount the power transformer and the big heatsink for the LM 3886. I connected it all up, attached my 400 Hz synchro and reached for the power switch - a scary proposition for something that hadn't seen moving electrons for along time. Well, the good news was that the CRT lit up OK (whew!) and the waveform generators all worked but that's where it ended. It was pretty obvious that my little synchros would never develop enough secondary current to adequately deflect the beam. In addition the LM3886 was getting very hot and working far too hard to force enough power through my junk box transformers into the unit, even with a power factor correction capacitor - now that's a phrase I never thought I'd ever use again! On top of all that, the 900 Hz transformers howled like banshees. It was obvious a radical rethink was going to be needed! That's where I'll leave it for now, so stay tuned for the next exciting installment folks! Morris -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/c5f1b0c1-1242-49de-8153-f7908a986a8b%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
