Re: How inaccurate is a 555 or 7555 REALLY?


John Fields wrote:
On 8 Dec 2006 14:06:22 -0800, bill.sloman@xxxxxxxx wrote:


John Fields wrote:
On 7 Dec 2006 17:30:41 -0800, bill.sloman@xxxxxxxx wrote:


John Fields wrote:
On 6 Dec 2006 16:18:51 -0800, bill.sloman@xxxxxxxx wrote:


John Fields wrote:
On 5 Dec 2006 21:21:13 -0800, bill.sloman@xxxxxxxx wrote:


Phil Allison wrote:
<bill.sloman@xxxxxxxx>

John Fields:

<snip>

Yes. I've seen them. All very nostalgic. Your recent exercise with a
4024 (the last post in this thread) reminds me of stuff I was doing in
1974 with a 4040, though I didn't decode with three diodes and a
resistor, even back then.

---
Didn't know how? It's just an RDL AND...$

Of course I knew how. But the resistors I would have had to use were
0.6" long and 0.2" wide on the board, so it would have wasted space,

---
Really?

ISTR that 1/8 watt resistors and 1N914s were available back then, so
the entire decoder would have used up the same real estate as an 8
pin DIP, as well as not using nearly the amount of power the TTL
glue you'd have to otherwise make the decoder from would.
---

1N914s weren't a problem, but company policy had fixed on one style of
600mW Philips metal film resistor, and 1/8 watt resistors weren't an
option.

---
Still, you could have mounted it Jap style.

Not according to the QA department. This stuff went into power stations
and oli refineries and everybdy involved was rather picky.

Oh, well...
---


not to mention confusing the final test technicians and the service
engineers.

---
LOL, you design to keep from confusing folks?
---

Of course. Unconfusing them takes time ande costs money. All other
things being more or less equal, I'll go for the transparent design
every time.

---
So you're a proponent of recurrent production costs in lieu of
proper training? How much money do you think you've "saved" over
the years by doing it that way?
---

Quite a lot. In particular, service engineers who are prepared to
travel all over the globe are a rare breed, and both George Kent and
Cambridge Instruments hired as many as they could find. They could
afford to be too picky about the their trainability. The design
engineers understood that keeping the service engineers happy was part
of their job - if the service engineer got fractious, we were next in
line to spend six weeks in Seoul.

The worst I copped was a week in Nice in the middle of winter, and that
was sorting ot a mess that the marketing department had got us into,
but we were all encouraged to be careful.

Final test was a different kind of problem. They worked down the
corridor, and if they got stuck the good ones had a tendency to stick
their head around the door and ask or advice, which used up design
time. The bad ones invented their own solutions to what they perceived
as the problem, which you'd find out about when it appeared in the
"proposed modifications" file - usually some two hundred items long,
which appeared on the desk of any design engineer when he (or she) was
between projects.

If the design wasn't transparent to the final test technicians, you
always ran the risk of the machine going out the door distinctly
maladjusted, and the quality of the final test technicians was always a
bit erratic - if we we doing well, we'd just hired a bunch of new
technicians of variable quality who were still being trained,and if we
were doing badly, the good technicians would get themselves a more
secure job somwhere else.

One time I designed a circuit that used an un-trimmed - thus cheap -
Analog Devices multiplier to do a job, and wrote a procedure that let
me set up the two trimpots required in about ten minutes. I got called
into final test when the first machine went through to find that the
boss of final test and his star technician had been fiddling all day
trying to do the adjustments that I then did in the usual ten minutes.

Total cost (in techniican time) was about $300. I promptly modified the
circuit to use the more expensive laser trimmed multiplier, and did the
offsets on the rest of that batch (about 5 electron microscopes)
myself. We weren't expecting to sell more than about fifty of that
model before we obsoleted it with the first fully computer controlled
electron microscope, so my blunder with the cheap part cost us more
than I could have saved over the whole production run by using the
cheaper part.

--
Bill Sloman, Nijmegen (but in Sydney at the moment).

.

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