"DoN. Nichols" wrote in message
...
On 2017-03-16, Jim Wilkins wrote:
.....


I use a lab supply set to 1.00A constant current. Then milliVolts
equal milliOhms.

I've used the same trick, set to a limiting voltage low enough
to not fry TTL ICs to find a fault in a card for a MITS Altair 680b
(I
think that it was a memory card, but I don't remember for sure these
days.) The fault was a short between adjacent traces (5V and
ground)
*under* the solder mask. The local vendor was trying to blame it on
his
assembly practices (really well done, BTW), when it was in reality a
faulty board from the factory. (The Altair computers could be
purchased
factory assembled or in kit form.) They had others to fix the
Altair
8800 (Intel 8080 CPU), but I was the only one for the Altair 680b
(Motorola MB6800 CPU).

I had to scratch through the solder mask and the short to fix
it.

The semiconductor test equipment maker I worked for pushed the state
of the art and went through a lot of circuit board revisions as a
result. They used layout design rules too tight for the board vendor
to consistently meet so internal shorts were an issue.

20A at 1V usually identified and cleared the short, literally in a
flash. If not, the voltage drops could be followed to locate it, even
in a solid plane. I cleared one by slowly milling down until a copper
chip appeared.. They would complete and test that one board to check
for other problems before sending out another rev.

That was in the first half of the 1980's. By the early 1990's the PC
board vendors had substantially improved their process control and I
had little trouble with shorts despite substantially tighter spacing
rules. They gave me enough detail on the processes to know what I
could likely get away with at standard or higher cost, ie more labor
and lower yield. I've seen a circuit board that cost the Air Force
$30,000 and heard of yields of 1 in 20 for high layer counts.

For example the fiberglass threads deflect a fine drill bit and
determine the amount a pad has to be larger than the hole. They don't
stack the boards as high on a small prototype order so I could use
smaller via pads than would be acceptable in production. The size of
the board also affects misalignment between the drill and the pads due
to uncertain thermal expansion when they press the layers together.
Making power planes a lattice instead of solid and filling unused
spaces helped keep my multilayer boards flat, unlike some other
people's.

BTW -- the MC6800 had an undocumented instruction dubbed "HCF" (Hang
and
Catch Fire) which tri-stated the data buss, and cycled the
address bus through all 64k addresses. Made it easy to spot
problems in address decoding. :-)

I built mine around an 8080 chipset that the company gave me after I
finished the lessons on a microprocessor trainer. The were very
supportive of home study as long as we weren't caught using their
parts to build pirate HBO decoders.

They generated a lot of useful scrap like the 9" square wirewrap cards
I built it on. I had to make a manual wirewrap tool from brass tubing
for their 0.062" square high-current backplane pins.

I used counters for the front panel address registers so it would auto
increment when toggling in bootstrap code. The 8080's Wait signal
could trigger a variable one-shot controlling Ready that decreased the
bus speed to 1/2 - 200 cycles per second as well as single step. It
was very helpful to fast-forward through subroutines, using the
address jump to detect the Return.

I learned enough from it that later I was able to design a
special-purpose DRAM controller IC.

[ ... ]

Some of those Mil connectors and tooling were unavailable even to
Mitre. I had to make the special lock nut spanner wrench for a HAVE
QUICK microphone connector.

Now Mitre could have been a more convenient place for me to
work. There was at least the octagonal mushroom building belonging
to
them a lot closer to my home than Ft. Belvoir was. :-)

Enjoy,
DoN.

Mitre was a real country club, which is great for the golfers, not so
much for the caddies attending to them. You weren't squat without at
least an MSEE. I got by on the unusual breadth of my practical
engineering experience rather than its rather shallow theoretical
depth.

-jsw