In the early 1990s I ran my own business leasing terminal systems to travel agents in Zimbabwe. I got the terminals second-hand from British Airways (whose mainframe Air Zimbabwe used for reservations). BA renewed their terminals every 5 years, so I was able to get their old stock pretty cheaply. But there was a reason BA replaced their terminals every 5 years: They were beginning to require regular maintenance.
My second-hand terminals were dumb beasts, big and ugly and heavy, but they did the job. They were based on the old Z-80 microprocessor and had CRT monitors and big iron transformers and linear power supplies. But that made them easy to maintain, and I had full service data for them.
The power supplies probably gave me the greatest number of problems. They were linear with 3-terminal regulator ICs – the good old LM323K for the +5V for the logic, an LM317K for the +24V monitor supply, both on a big finned heatsink, and 7812/7912s for the +/- 12V. A big heavy toroidal transformer supplied them. (I later made a great lab power supply out of bits from an old terminal!)
But although they were simple, they were only just sufficient for the job. They got very hot and failed occasionally. But the weakest point was the connectors. Both the +5V and the +24V supplies reached their respective boards through only one pin on a totally inadequate connector. A bad connector would go high-resistance and get hot and go even higher resistance, sometimes eventually melting the connector body. Fortunately I had access to new connectors, and once installed they usually gave me no more trouble for a year or two.
The most interesting problem I had, though not the most difficult to diagnose, had to do with the vertical output circuit. These were magnetic deflection CRT displays with a yoke around the neck of the tube, and there was a large non-polarized 6.8 µF electrolytic capacitor in the vertical output circuit. They were about 2 cm in diameter and 5 cm long (for our American friends that’s about 3/4 x 2 inches).
When the first one went wrong on me (it failed open circuit giving a horizontal line across the middle of the screen), I scoured the local electronics suppliers and eventually found some 10 µF non-polarised capacitors with a suitable voltage rating. They were, however, only about 1 cm in diameter and 2 cm long (3/4 x 3/8 inches). I marveled at how component design had progressed since my terminals were built. I installed one of the capacitors in the faulty terminal, ran it up and it was back to normal. I left it for a few minutes and all seemed well. I put the terminal back into my spares pool. Some time later it was used to replace a faulty terminal at one of my favourite travel agencies.
A few days later I got a call from the agent.
“Dave, you know that terminal you replaced last week??”
I got an unaccountable sinking feeling in my stomach.
“Well, it’s exploded!”
I got out of the agent that it had made a loud pop and sent up copious smoke signals, causing no small amount of alarm to the poor girl who was operating it. The “pop” noise gave me a clue as to what had happened. My new, marvelously small capacitor had vented itself to the outside world. And when I opened the terminal, that is exactly what had happened, and the inside of my terminal was covered in bits of aluminium foil and fluffy dielectric.
I apologized profusely to the agent, told him what had happened and gave him a month’s free rental for the pain and suffering caused thereby. I also learned a valuable lesson about the AC series current rating in capacitors, and how component design had NOT progressed as far as I thought it had. I managed to find some 3.3 µF polyester capacitors of which two in parallel worked as well as the original non-polarised electrolytic. And you can be sure that I gave that terminal a one-week soak test before I let it anywhere near one of my agents again!
David Ashton is an Australian engineer. To this day he says he has a name for taking ownership of problems, which he says he doesn't regard as particularly praiseworthy, it's just how he is.