[sdiy] Bypass caps in retrospect

[Paul Schreiber]

People in general have short memories when it comes to 'mapping' an old synth 
today versus when it was made.

I can get a 5,000pc reel of Kemet 0.1uf 50V 20% bypass caps in through-hole for 
$58 or 1.2cents each.

Back in the early eighties, there were no monolythic ceramic bypass caps, they 
were the disc kind. And the chemistry of the dielectrics were still new, because 
0.1uf was a pretty "big" value then most radio work was in the 220pf range :)

The disc caps suffered from mechanical issues: their high center-of-gravity 
meant that they had a tendency to vibrate and either crack the solder joint or 
crack themselves. And, when they cracked the material, they failed as a *short*. 
Sequential nearly went out of business due to the "green caps of death", which 
were a 'bargin' green disc bypass cap that had like a 5% failure rate.

And, these caps were not cheap, they ran about 11 cents (in 'old money', around 
20 cents today) each. You couldn't throw 100 caps in a design, because the 
managers would look at the BOM and the *first thing* they point out is "can we 
get rid of say 40% of these caps? Will it still work?" and then if you tried to 
define 'work' they get all glassey-eyed.

The introduction of the AVX monolythic ceramic cap was a HUGE thing in the 
mid-80s. In high volume (1 million+) quantities they were like 8 cents each and 
could be *auto-inserted*. The failure rate was like 100,000X better than disc. 
Too bad the Dx-7 had wiped out everyone by that time :(

But still, that 8 cents times say 30-50 bypass caps was an easy 'target' for 
"BOM Sniping" as we called it at Tandy. Managers would argue for HOURS with the 
engineeers about bypass caps. One manager demanded that we go into the lab, get 
a motherboard running, and start cutting out the caps until it stopped working 
(this is called a "Mad Man Munson" approach, another obscure reference).

Since I'm not picky about being "facture pure" on my older keyboards, they first 
thing I do is slap bypass caps all over it, on the bottom of the pc boards.

My MOTM-300 VCO has 17 bypass caps on it :)

Paul S.