June WMATA crash - AC track circuits

-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that\'s close........[v].(301) 56-LINUX Unless the host (that isn\'t close).........................pob 1433 is busy, hung or dead....................................20915-1433

Or, at least, would seem to mandate the use of careful physical and electrical separation between the "exciter" and "detector" portions of such systems. The fact that these two system components shared a common element (heatsink and power supply) was a big part of the failure, since it allowed the existence of the sneak path.

I'll be interested to hear more about just why the exciter circuit developed a parasitic oscillation. I ran into something of this sort a couple of years ago, when diagnosing a two-tone audio oscillator circuit that a local ham had built (from an article in QST) and had never been able to get to work stably. The 2N2222 "twin T" oscillator circuit turned out to be prone to parasitic VHF oscillation at the peak of the audio-waveform cycle (low-voltage or high-voltage point, I forget which)... it really should have been designed with a few ohms of anti-snivet resistance in the base lead. A ferrite bead around the base lead of each transistor fixed it up quite nicely.

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