So, CuriousMarc continues his efforts at getting his extremely-vintage HP desktop computer back in operation, even resorting to wearing his quasi-replica magic pants.
(The pants don't have the true 1970s magic, as they're not proper bell-bottoms.)
Anyway. The most recent installment features a pair of vintage HP diagnostic probes: an HP 546A logic pulser and an HP 547A current tracer. They didn't work for tracking down the problem at hand, but the principle is intriguing: use the pulser to apply a signal to a net, then use the current tracer to see which trace is actually carrying current; the trace with the current leads to whatever is shorting out the net.
Hm. I wonder if anybody's refined this, maybe into something safe for boards with the latest generation of low-voltage logic...?
Apparently no. Seems like the only current tracer probes around nowadays are meant for use by electricians, and are much too bulky for poking around on circuit boards (even low-density 1970s boards). I did find a reference to someone (since deceased) who'd had a go at making some sort of 547A knockoff, years ago, for his own use.
Well! Seems like this is yet another thing to put on the Maybe-A-Product-Someday list. Gotta make the probes (including the magnetic pickup for the current probe) pointier nowadays. And... methinks selectivity could be improved by synchronizing the current tracer to the pulser, so it'd have a sample window just when the pulse ought to be happening. Maybe the pulser could have selectable output levels, perhaps 1V, 2V, and 4V (in case anyone's still using 5V-tolerant logic) [see addendum]. And, selectable peak current, or pulse energy, or something?
Limitations, though... this'll play nice with 2-layer boards, or 4-layer with the inner layers used as ground planes, but utility falls off as the layer count increases.
Anyway, not something I'll be playing with anytime soon. Maybe when I have a bunch of focused, brain-functioning free time, long about the 37th of never.
(Alternative approach to finding the short: apply a low-voltage, high-current DC source to the offending net, and use a thermal camera to see what traces get warm.)
Addendum: Seems the pulser probe accommodated a range of system voltages by the simple expedient of running off a user-provided power source and being able to handle 3~18V for TTL or [CD4000-series] CMOS.
Comments