Sunday, August 28, 2016

Geeky TDR Tip

     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.
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* 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.

10 comments:

Joe in PNG said...

Smart fella with a knack for the soldering iron and a bunch of schematics could probably make a decent living fixing and McGuyvering old electronics, I reckon.

Old NFO said...

Nicely done... :-)

Keads said...

Nice!

Chuck Pergiel said...

". . . and before you can read War And Peace even halfway through . . ."
Nice turn of phrase there, kiddo.

Anonymous said...


Taking a look at that web page you linked, on the photos of the internals I see something potentially worrisome - That whiz-bang doo-dad has the infamous 'orange drop' capacitors, not known to age gracefully. (Unlike you, of course. 29 still? *wink*)

Anyways, over the past few years the 'net has turned into a veritable gold mine of repair tips for obscure (and non-obscure) electronics. I really fear losing that knowledge to the transient ways of websites, and hope the WayBack machine has rat-holed that data away somewhere...

Roberta X said...

Oh, there's plenty to worry over inside a 1502. So far, so good....

wheelgun said...

I thought you could use an antenna analyzer (at least some of the modern/computerized ones) to measure coax and find faults, calculate velocity factor, etc.

Roberta X said...

Yes, but you'd have to have one. I own one, good though the upper end of HF, but my employer does not. We have a network analyzer but that's a bit much.

Anonymous said...

"then" ... even ubiquitous Radio Shack was a supplier of not-so-bad parts ...
Q

Rich P said...

Someone at the laser farm has been known to revive elderly computers with AA cells and black tape. Because we need to elderly computers to run the antique software.