A lightbulb moment comes too late to save a mainframe engineer's blushes

Throwing light onto a frozen bit of big iron

On Call The weekend is almost upon us – a time for adult beverages and ill-judged foodstuffs. Unless, that is, you're one of the unfortunates on the other end of the phone. Welcome to On Call.

Our story today comes from "Will", who told of his time as a City of London mainframe engineer in the swinging sixties and slightly more sombre seventies.

His tale opens, as do many On Call episodes, with a telephone call. This time from a colleague who was working at a major London stockbroker.

"They had two systems in their very modern prestige computer room," he recalled, noting "it even had a visitors' gallery."

However, one of the systems had "frozen" and would not do anything. There were no warning lights, nothing to indicate things were broken, but the mainframe was locked solid.

"I was nearby," Will told us, "and went round to the customer and sure enough nothing would work.

"We even tried a POR (power on reset), which was a big no-no in those days, until it became the popular fix for Microsoft and most modern devices – which still left the system in the same frozen state."

A small crowd of techies had gathered by this stage, and all were scratching their heads as they gazed upon the controls of IBM's machine. In this case, it was a System/360 Model 40. This particular bit of big iron first made its debut in 1964 and was eventually withdrawn in 1977. A common configuration [PDF] would feature a mighty 128KB of storage, card readers, tapes and printers. Yep - all the toys a business could want.

In this case, however, the toys were broken and the machine was (relatively) silent. Ideas were tried and discarded. Right up until an engineer, running low on suggestions, jabbed a button on the front of the console. The lamp test button.

There was no helpful screen in those days, lights were the order of the day and every bulb lit up. Except one.

"This," sighed Will, "was the one that indicated that the console typewriter was out of paper."

The System/360 Model 40 was paired with the IBM 1052 typewriter keyboard, which consumed the same fanfold continuous stationery used by the bigger printers.

"The sensor was on the back of the console typewriter, so the machine had paper visible but was about to run out," recalled Will.

Nobody had thought to check the console – after all, you could see the paper and the warning light wasn't on. It must be OK, right?

Hotblack Desiato Will wasn't, and the bulb wasn't a black light that lights up black to let you know that something was wrong.

Changing it and the paper resolved the problem and the computer sprang into life.

Trickier, however, was explaining to the customer why a printer running out of stationery had taken a football-squad of techies and a large chunk of the day to resolve.

Every struggled with a problem only to find it was the fault light that was, er, faulty? Or had to make that explanation of shame to a customer? We've done at least one of those things. Have you? Confess all with an email to On Call. ®