[Premium-Rx] Watkins Johnson 8711A Repairs after 10 Years

[Gary Geissinger]

Gentlemen,

I own an 8615D, HF-1000 and other WJ gear.  After this discussion I have my fingers, toes, and eyes are crossed hoping none need serious repair.  I love my WJ gear.  It has incredible capability.

And, since I work in aerospace and design electronics for spacecraft, I certainly understand design life, component derating, MTBF, and MTTR.  

But all this starts to unravel for me when I think about this.  I have a collection of electronic countermeasures gear from WW2, Korean, and the early cold war.  It all functions.   And keeps functioning.  I can buy replacement electrical parts.  I have full schematics, wiring diagrams, and alignment procedures.  All this gear was designed and built to support a lifetime of about 5 years.  Yet it works and keeps on working.  Even my old beat up R-390A receivers, ART-13, and T-368 transmitters still function as designed in spite of multiple owners and rather rough treatment.

So while I understand the issues with the operation of units well past their design life, I am not convinced that it had to be like this.  I just got off the phone with an applications engineer for a major capacitor firm.  His "hi-rel" aluminum can electrolytic smoothing capacitors, when derated according to their specifications, have an 8,000 hour operational life.  Well, in the piece of GSE I am designing, I am applying MIL-HDBK-217 style derating on top of their specifications ... and then some.  Published data indicate this will give at least a factor of two increase in life.  Then of course inrush current limiting and fault protection will help stretch the capacitor life as well.  All this is adding about $10 to the design.  Using stacked mono-ceramic capacitors would have added $100 or so but would have almost eliminated smoothing capacitor life issues.  Adding conformal coat to the boards will improve the life as well for almost no cost.

Although it may have added a bucks to the cost, and made the units weight 1/2 pound more, I wish WJ had been a little more conservative in production and mechanical design.  But you certainly can't fault the electrical performance for their day.

Gary WA0SPM, also member US Army MARS

Gary A. Geissinger
Chief Electrical Engineer, Sr. Director, Technical Fellow
DigitalGlobe Incorporated