Hi all, You may remember me asking a while ago about info on a 1970 vintage Sperry Mark-8 PPI radar indicator that I wanted to turn into a clock.
Well here's a progress report. I'm no newbie when it comes to CRT clocks, but this one is a real challenge. The indicator is of Japanese origin with all discrete semiconductor technology on 7 PCBs; 4 plug-in and 3 mounted, including the receiver IF strip. The whole thing is about the bulk of 3 or 4 large shoe boxes with a 7 inch diameter long persistence CRT. There were two heat-sinks on the back, each with a single TO-3 power transistor and there were also lots of unrecognisable multi-pin connectors, fuses and some power relays. It was grubby but intact when I picked it up, with no corrosion. That was it, the rest of the radar - power unit, modulator, transmitter, antenna and cables were all gone. I had already started reverse engineering one of the PCBs when amazingly a manual appeared on ebay! It cost more than the indicator but I bought it anyway and few weeks later I discovered what a complicated job I had let myself in for - just what I love! The radar was originally supplied with a rotary inverter to suit whatever the shipboard power supply was, this powered the rest of the set with a few hundred watts of 100 volt power at 1000 Hz. Not only were all three power transformers in the indicator designed for that frequency (they were actually labelled 800 Hz) but the 1000 Hz was also used as the reference for the transmitter pulse repetition rate so the indicator scanning circuits also ran at 1000 Hz. One power transformer supplied the CRT heater and panel lights, another the high voltages for the CRT including a feed for the 10 kv tripler and the largest supplied everything else through a number of secondaries at different voltages and currents. The CRT is electrostatically focussed and magnetically deflected with a 3 phase yoke not unlike the stator of an induction motor. There's also a smaller square yoke fed with DC through a couple of pots for X-Y centering. The scanning circuits in the unit were triggered each millisecond with every transmitted pulse. They generated a complex "area balanced" waveform consisting of a positive going sawtooth followed by a negative going pulse of the same area so there is no resultant DC component. This is necessary because the waveform passed through 2 transformers - first an output transformer and then it was fed up to the rotor of a resolver synchro at the masthead, rotating with the antenna. Its 3 phase wound stator developed rotary modulated sawtooth currents that were fed back down to the yoke. To add to the complication, the yoke is delta wound with no common star point. This was to be quite an issue later on! All this sounds complex but it was apparently common radar practice at that time The indicator has a "rings" feature that applies an intensifying pulse at several times to each radial scan to act as range markers - these appear as rings as the trace rotates and are very useful to check linearity of the scan; they will look cool on the final clock display! There was a set of "hole" controls to adjust the area balance system so that the scans started at the centre of the screen with no gap that would look like a hole in the middle of the display. There were also other circuits unique to radar such as video time constant controls and quite bit of switching to vary the sweep time, hole and ring spacing and intensity for different ranges. The construction was surprisingly low quality for such a piece of branded professional equipment - phenolic boards and sloppy messy soldering, but any capacitors that defined timing were of high quality. There was oily residue all over the boards, probably some sort of silicone waterproofing. Well that should be enough to illustrate what a challenge it would be to resurrect this unit as a clock. The next installment will describe how I went about it. Cheers, 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/c0b4f6d4-ccd3-496f-acb3-8964cc320a54%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
