On 04/01/2019 06:24 AM, Gene Heskett wrote:
It once took me around 3 years to find an engineering
goofup by Chyron.
Well, here's one crazy fault that I finally debugged. It
was a rack-mount mag tape controller for a PDP-11, made by
Datum. Basically one huge 15" square board in an aluminum 3U
or so cabinet.
It was stuck in reset. The reset/ line ran all over the
board, but only to inputs, the driver was on the system
bus. And, reset/ was stuck low, and even when powered off,
it was just a couple Ohms to ground. I started cutting the
reset/ trace in spots to isolate it, and pulled the last 2
chips, but it was still grounded. Finally, I had it down to
a 2" strip of copper trace, and this was a 2-layer board,
and there was nothing on the other side, and no ground trace
nearby. I finally peeled the trace off the board, and you
could not see ANYTHING odd there! Replaced the chips,
soldered a piece of wire-wrap wire (with the insulation LEFT
ON in the middle) and it worked fine for years.
That was certainly one of the odder faults I ever had to
debug! Not an engineering goof, just a weird defect.
A piece of gear I worked on had a photomultiplier that
sensed a green light from a flame to indicate the sulfur
content of the air. This also provided a flame on/off
signal. The range of the signal from flame on to flame off
was about .6 V. They set up two transistors, one lit a
flame-out light, the other held the gas solenoid open. So,
either one transistor should be on, or the other one should
be on. But, the .6 V difference was almost the entire
signal swing, so the circuit was really flaky. I eventually
figured out how to do it with ONE transistor, as the lamp
drew so little current it could not possibly hold the
solenoid open. I put the lamp across the transistor that
switched the solenoid valve.
Since this was in a safety system, I was a bit hesitant to
be so bold with this change, but, my circuit WORKED, the
original required frequent tweaking, maybe as room
temperature changed or like voltage changed.
Jon
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