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Roy Lewallen · August 27th 06, 09:56 PM posted to rec.radio.amateur.homebrew

wrote:
. . .
It's relevence is I've seen this before and understood it's origin
and also elsehere. The other aspect is that if a commonly
accepted part is not fully understood and can lead to undesired
effects then, why not others. . .

Very nearly 30 years ago, I was looking into "feed forward" circuits, a
technique developed by someone at Tektronix for ultra-low distortion
amplification. It turns out that the topology of the MC1350 is similar
to what's needed, and a feed forward amplifier can be made from one plus
just a few external components. But even by then, I'd learned that it's
risky to use components for other than their intended purpose. So I
collected 8 or 10 samples from various vendors (the part was widely sold
then), and opened them up. Those in cans were easy, using a little can
opener that worked like a tubing cutter. Some of the plastic DIP ones
were more difficult, but one of the labs at Tek was able to dissolve the
plastic while leaving the chip intact. Then I examined them carefully
with an inspection microscope. Here's what I found:

1. There were at least three very different designs. The chip size of
the largest was several times that of the smallest.
2. Some designs were inherently better balanced than others. Some had
resistive "cross unders" where traces cross, which weren't the same on
both sides of the circuit.

Based on this, I decided it was too risky to make a design based on that
part number, since a vendor could change chip suppliers or designs
without notice.

Interestingly, about six months later, I got a call from the component
engineering group asking if I still had the chips. It seems that one or
more of the vendors supplying that part (which was used for other
applications at Tek) had changed their design, causing failure of some
products and the shutting down of their production lines. Tek was big
enough that vendors were often required to give advance notice before
such changes, but they hadn't given any notice in this case.

I'm bringing this up because I'm hearing the MC1350 being spoken of as
though all are the same. It wouldn't surprise me if, after all these
years, they're now all being made with one design from one foundry. But
those ones in your junk box might be way more different than you think.

This is almost certainly true of just about any IC.

Roy Lewallen, W7EL