[GreenKeys] Teletype REC 10 power supply available

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I'll add that the degradation process doesn't stop after three months.  The 
change in characteristics from the ideal is primarily a gradual decrease in 
front to back ratio and secondarily an increase in forward drop 
(resistance).  My personal experience is that the shelf life varies from 20 to 50 
years.  Shelf life defined as the point at which it no longer works well enough 
to continue to be usable.  And time includes those rectifiers installed in 
equipment that hasn't been used in decades.  The end point is essentially a 
low value resistor.  DOD agencies began issuing silicon retrofit kits at least 
as early as 1962.  My practice for about the past decade with military 
equipment containing seleniums is usually to replace them before firing up the 
equipment the first time except in some cases where the function is as a 
shunt regulator rather than a rectifier.

In a message dated 09/14/2011 11:27:05 AM Central Daylight Time, 
bill at blcain.com writes: 
> Thanks Kevin. Nice info to have!
> Bill
> 
> On 9/14/2011 7:58 AM, airrelic at juno.com wrote:
> >Bill,   A year or two ago I had the same question about Selenium 
> rectifiers bouncing around in my head and I started looking for why they go bad. 
> Here is an excerpt from Army TM 11-473 dated 1944 on Central Office 
> Maintenance.“Rectifying Elements, sub-paragraph c. Aging. A rapid decrease in 
> the amount of current which the disks will pass occurs during the first 3 
> months of service. This aging also takes place, but not as rapidly, if the 
> rectifier is placed in storage and not used. The decrease in initial output 
> does not generally amount to more than 25 percent of the rated output, and 
> is provided for in the design of the rectifier by the addition of taps in 
> the secondary winding of the transformer, to increase the current output as 
> the deterioration increases. These transformer taps vary the ratio of turns 
> in the secondary winding from those in the primary winding. Aging of the 
> rectifier elements is brought about by internal changes and temperature. The 
> speed with wh
> >   ich aging occurs depends also upon the operating temperature; thus, 
> the aging is more rapid when the rectifier operates under heavier current 
> loads.”   In short, if it works at all, it won’t for long. The 
> designers knew it wouldn’t last for long but Selenium was cheap and 
> plentiful, and so was the labor to replace it regularly.   Kevin KD8QGX
> >
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480