I got out the TDR Friday -- an elderly Tektronix 1502 Time Domain Reflectometer, filled with unobtanium tunnel diodes, and it was dead. D-E-A-D as in won't even turn on.
They do that. There's a big ol' NiCad battery pack in the middle of the thing and Tek was worried they would be burning up costly TDRs if they weren't very careful about charging the batteries and as a result, if the battery is way low, or getting puny, or even just having a bad day, the power supplies crowbar before they even turn all the way on. Usually you can flip the thing on and off a few times and catch it looking the other way, at which point it will start to charge up the batteries but still won't quite work and before you can read War And Peace even halfway through, the thing will be happy enough to run.
Not this time. It was not waking up. And it's not like the corner Tek store* is going to be selling replacement battery packs, custom fused, heat-sunk, waterproofed-sealing as they are. The local battery-everything places can maybe rebuild it, but not quickly.
Quick, to the Web! And there I discover this is a known issue and what you want is about 200 - 270 Ohms at ten Watts with 2200 uF at 25V in parallel across the banana plugs where the battery pack should plug in. A nice big handful of 2W carbon resistors later (seven of them, because if 10W is plenty, 14W should run cool) and an electrolytic capacitor later, I was in the TDR business -- inelegantly, with a little box marked "Fake Tek NiCad Pack" on rubber-covered test leads plugged into the thing, but it was working and it saw the coax cables.
They don't look too bad. Or anyway, not too dissimilar to one another and there aren't any glaring discontinuities. So I'm hoping that's a good sign.
* This actually used to be a thing. There were Tek service centers in all the major cities, usually with a salesperson working out of 'em, and you could drive over there, drop off equipment for calibration, buy common parts and order uncommon ones, maybe get shown the latest new toys-- That was Then. Now, not so much.
1 month ago