First impressions - General Radio 1632-A Inductance Bridge - Page 1
Forum section: electronics / general

author: JuiceKing
author_rank: Regular Contributor
timestamp: November 15, 2012, 12:42:03 pm
content: Bigger images here:http://www.flickr.com/photos/the-maestros/8188158254/#This very tall and handsome bridge had been languishing in eBay at a prohibitive price for a long time. I couldn't stand it any more and so I made a reasonable offer; 5 minutes later, it was MY white (dark grey) elephant!It was pretty filthy, but inside was OK. The bridge must have been left idle for many years because the inductance decade switches had very noisy contacts. I popped it open, removed the interior shield, and cleaned and relubricated all the contacts with isopropyl alcohol and Caig Deoxit 100. That was all that it needed to be usable again.Removing the front panel dress and knobs was very straightforward, and it was pretty easy to clean off all the dirt and label residue without damaging the paint. Conrad Hoffman is right when he says not to use any solvents stronger than water or WD40. Pretty much anything else will show grey paint on the towel. You've been warned. This is why I gave up on removing the white paint you can see on the low-right. No big deal. Removing the dress panel lets you see the mechanical guts of chains, wheels and gears that automate the placement of the decimal points for the G and L readings. I should have photographed it while it was open; it was very elegantly engineered and made.This bridge has no oscillator or detector, so I used my function generator, a milli-voltmeter and scope. It works, but it's not nearly as easy to use as the GR 1608-A and has the same 0.1% absolute accuracy. (For finding a bridge balance, turning dials is much easier than adjusting digits.) Happily, the 1632-A, as an Owen Bridge, has minimal interaction between the two adjustable bridge arms, so it's pretty quick once you get the hang of it. It reads in conductance/inductance, which means that you have to do a little math to convert to Q/inductance figures for the 1608-A. Anyway, some simple tests on some inductor values from 5 - 500mH showed that the two bridges matched within 0.1%. Good!The real charm of this bridge is its ability to measure extremely low inductances, with resolution to the thousanth of a micro-henry. Unfortunately, to do that, you need a 1632-P1 impedance matching transformer. Which I don't have. It probably came with the original unit, and was lost in the mists of time, along with the generator/detector shielded patch cords and aluminum end frames that must have been removed for this to be rack mounted.The 1632-P1 transformer seems to be nothing more complicated than a 20/5-1 transformer to make it possible for a signal generator expecting 600-ohm loads to deal with the low bridge-input impedance that you must drive for low-range settings. I will look into getting an equivalent. If you know of one (or, better yet, where to find the original 1632-P1), please let me know.Also, because iron-core inductors introduce non-linear harmonics, you do need a tuned detector to sniff out the traces of the test frequency. My simple millivolt meter/pre-amp + scope is pretty clumsy for this, but at least it works for now.Even though this bridge has 0.1% absolute accuracy, it can resolve values far beyond this, so it's possible to compare inductors down to a few PPM. I like it!

author: saturation
author_rank: Super Contributor
timestamp: November 15, 2012, 04:35:30 pm
content: Nice, thanks for the review. I wish you had taken photos of the interiors when you were working it, its worth 1000 words, particularly the before and after. Once all those years of grime are done, its gone forever!PS The rest of your collection shown on Flikr is A1+ worth a look by all. Your restorations on many GenRads are fantastic.

author: JuiceKing
author_rank: Regular Contributor
timestamp: November 15, 2012, 06:19:24 pm
content: Quote from: saturation on November 15, 2012, 04:35:30 pmToo late for me to restore the original grime, but here are some interior shots. I'll save the gearing guts for another time. I think they need a video anyway.- KenInterior with shieldingInductance switchesInductance switches - shield removedRange switch - shield removedRange switch - shield removed (overhead)All interior - shields removed (Conductance switches on left - I'm not tampering with them for now)

author: saturation
author_rank: Super Contributor
timestamp: November 16, 2012, 09:38:31 pm
content: Many thanks, and great photos! Not just clarity but the composition. They remind me of many scenes in old 1950s sci-fi movies, got your catalog from Unit 16 to build an interocitor?