Purple plague? Was:Re: [Coco] Any problem with connecting 2 floppies to one power supply?

[Gene Heskett]

On Monday 02 February 2004 00:10, jimcox at miba51.com wrote:
>On Sun, 1 Feb 2004 20:00:33 -0500
>
>  Gene Heskett <gene.heskett at verizon.net> wrote:
>>That said, I did it for several years in my grass valley
>>e-disk
>>simulation running on os9, but its duty cycle was quite
>>low.  Never
>>had a problem with it, ever, other than dying wd-1773's
>>in the disk
>>controllers, it used up 3 of them in 13 years.  I think
>>the purple
>>plague ate them, a well known PMOS chip failure
>>mechanism.
>>
>>--
>>Cheers, Gene
>
>Gene:
>
>Can you elaborate on the purple plague, and does anyone
>know if the wd-1773's are still available?
>
Not a lot Jim.  Its a chemical thing thats exacerbated by heat 
somewhat, but mostly time & moisture infiltration thru the epoxy b 
related.  Even the ceramic packs had problems because their really 
not any more hermetic than the epoxy b is.  When the tops were 
removed from failed chips and a microscope applied to see what may 
have failed, it appeared there was some sort of a possibly corrosion 
related deposit that was attacking both the bondout pads and the 
actual base junctions of the transistors, and it was purple colored, 
hence the moniker.  Eventually the processes got changed over to cmos 
or fully bipolar and that particular failure mechanism went away.  As 
to the availability of wd-1773's, I've no idea.  I think if I needed 
a few, the first place I'd call would be graveyard electronics, who 
gambles and buys up the remaining stocks of chips from the makers 
after the end of life period has passed.  Even they may take a few 
tubes to the landfill eventually if theres not enough motion to pay 
to keep the shelves dusted.  I have very serious doubts that 
allied/hamilton/marshal/newark or any of the other mainline 
distributors would have any.  They were EOL'd by WD about 18-20 years 
ago now.

When atmel EOL'd the 27c4000, the worlds remaining stocks were bought 
up quite rapidly, like in just a couple of weeks, which is one of the 
reasons amigados 3.9 comes on a cd, and you have to overlay the old 
chips with dram using the memory manager if the machine had one and 
load the new image.  Having to do that was both a help and a 
hindrance, the hindrance because it ate precious dram, and help 
because the dram was 4x faster than the original eproms which had the 
same, equally oddball footprint on the motherboards.  Damned Commode 
Door anyway!  I begged atmel to make another 10,000 at least, but 
they simply weren't interested.  It wasn't that great a chip anyway, 
tending to be forgetfull because the recommended programmer sequence 
left it underburnt, but it was the only game in town for that job.
>Jim

-- 
Cheers, Gene
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