Yesterday I postulated that an engineering training is valuable in architecture because it teaches cross domain skills, and in some cases reinforces these with physical harm or danger; a very effective teaching aid!
A good example of this was an experience I had soon after I graduated, well before the PC era. I was working on the EHT power supply design for the first ever colour display (an IBM 3279 for the consegnenti) and we were testing the first few units. The design of EHT power supplies providing highly stabilised 25Kv with huge load changes is well understood now but in those days was very challenging. In particular one of the strange things that occurs with CRT’s is that every now and again, for no apparent reason, there is a “flashover” of the 25Kv to the anode pins and hence to the low voltage and digital circuitry. Most chips don’t like 25Kv on their input pins so protection circuitry had to be included and then tested.
This testing of the protection circuits was a problem because we had to have a flashover for the test. The cause of the flashovers was unknown and only happened once a year; clearly waiting for natural ones wasn’t going to work. We built a device called the mousetrap which connected the 25Kv supply to the anode pins but the spike characteristic wasn’t the same as a natural flashover in the CRT itself. We played with lasers (fun!) trying to ionize a path in the CRT but there was not enough gas in the evacuated tube to ionize with the low powered lasers we then had. We were keen to get a mega powered laser, mainly to see what we could burn holes through, but that was blocked by management (whining about budgets!). Clearly we had to come up with another mechanism.
So I had the bright idea of adding some gas to the CRT so that the laser could ionise it. General agreement ensued and so we broke the sealing nipple where the tube had been evacuated with a pair of pliers and immediately slapped some epoxy putty on the hole before too much air rushed in. A highly scientific implementation; hey we were engineers! We then ionised the gas with the laser and applied the 25Kv supply to the cathode.
Ok, so who can see the problem here?
Well what happened next was a small bang followed by a very large implosion of the tube which lifted all the ceiling tiles in the lab and brought people popping out of offices all down the hall like gophers from holes. It would have been funny if we hadn’t frightened ourselves witless; have you ever heard a TV tube implode? Very noisy and a lot of bits of glass too!
So what happened? Have you worked it out? The answer is in the feedback.
Anyway this was a case where some basic thought in the chemistry domain would have avoided an explosion and change of pants in the physical domain. Cross domain analysis driven home with a Bang!
The problem was that the putty was epoxy based and so had hydrocarbons in it. These were sucked into the tube along with the air so we had a oxygen / hydrocarbon mix in the tube which was ionised with the laser and then had a massive spark put through it. This caused a weak explosion in the tube which was designed to resist external pressure rather than internal. The overpressure of the tube broke the glass at the same time as the mixture inside decayed back to near vacuum, so creating an implosion.
Did you spot it?
I love this story Mike! Thanks for sharing!
Thanks Jimmy, it's nice to get feeback like this, motivates me to keep going