[sdiy] Bypass caps in retrospect

[harrybissell]

Paul Schreiber wrote:

> 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.

My P5 was into the AVX era. No excuses there.

To be fair, I work in a industry (well sh!t I work FOR industry) where 11 cents is
real
cheap insurance. When our products misbehave we have to chase them into the
field and fix them online. THAT costs way more than any perceived cost.

Another point... it is possible to design for both lower cost AND higher
performance. You just have to make the commitment to do so.

> 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).

Muntz actually. The verb form is Muntzing.   This is from a TV manufacturer of
legend (named after its founder) Muntz.  The majority of his components were
'gimmick' capacitors... using stray capacitance and stray inductance as circuit
elements.  By orienting one component relative to another, you can couple, adjust,
tune etc...  Just bend it a little closer and voila.  Any thoughts on how hard those

were to fix (component replacement, new and different tube etc :^) ???

These TVs were fairly popular in Metro NYC, where powerful stations were nearby.
They did not work, at all... in the suburbs.  You get what you pay for.

BOM sniping

> 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 :)

I have NEVER accused MOTM of being 'cheap' have I ???    :^P

(even in a bargain basement kit... you could at least leave holes for the
decoupling caps)

I am tempted to post a photo of my breadboard right now. The first thing that
went in it was the power supply connector, IC sockets, and decoupling caps. That
is before the first chip goes in.  Plan ahead.

H^) harry