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

[Karl-Arne Markström]

When reading this thread, I just wonder what mistake the Swedish ITT 
subsidiary Standard Radio & Telefon (SRT) made in the early 70's when 
they
designed the "System 300/500" receivers and exciters. 

Many of the early production CR302A (1973/75) receivers are still 
operational after nearly 40 years of 24/7 operation. 
After ironing out some initial problems, such as the ageing of the
early mechanical filters and "Purple Plague" internal corrosion 
problems in early TTL, the equipment has 
shown a remarkable track record. 

The manufacturer had dimensioned the recommended spare part 
inventories and service organisations from a calculated 4000 h MTBF 
and a service life of 10 years,
and they planned for selling another system generation in the mid 80's 
when the "System 300" was expected to be worn out.
Instead, the system worked so well, that the customer decided to 
"jump" at least one generation of equipment.
Postponing the new procurement finally ended up in the financial 
troubles that caused the first and second bankrupcies of SRT.

In reality, the field MTBF is in the order of 40000 h, and the end of 
the service life (or the "foot end of the bathtub curve") 
has not been reached yet.  This means in practice that I have to 
attend any of the 10 operational receivers at the Stockholm Radio
receiving site at Enköping statistically twice a year.

The main causes for repair are still dried out electrolytics and some 
random failures of TTL circuits.

Are there any similar stories around about other manufacturers making 
"too good" equipment?

73/
Karl-Arne
SM0AOM



----Ursprungligt meddelande----
Från: ggeissinger at digitalglobe.com
Datum: 2013-07-30 17:16
Till: "premium-rx at mailman.qth.net"<premium-rx at mailman.qth.net>
Ärende: Re: [Premium-Rx] Watkins Johnson 8711A Repairs after 10 Years

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

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