The demo of an early teletype machine produces some rather spectacular, if unintended, results
Years ago I worked for a large manufacturer that was a leader in designing and building teletypewriters. For the younger set, a teletypewriter had a keyboard and printer with no computer between them. It had serial input/output capabilities allowing it to communicate with remote similar machines. One large user of such machines was Western Union for their telegraph service.
At the time of this story, these units were mostly mechanical plus a few switches, solenoids and relays; no electronics. They were mostly assembled from parts punched out of steel. Data moved into and out of them at an astounding 10 characters per second.
Many of this company’s products were used by the government and military, so we had some impressive test equipment to verify that the products would work over the needed environmental range. For instance, there was a walk-in chamber capable of greater than 120F at greater than 95% humidity, another capable of simulating an elevation of 30,000 ft. (Note: All numbers are subject to poor memory.)
Management was so proud of the facilities that they held an annual open house for employees and family.
One year, they added a shake table to the test equipment facility and wanted to showcase it at the open house. Its purpose was to vibration-test products over a frequency range from 0 to 5000 Hz. For example, you might want to test your product to make sure that it didn’t rattle apart when you installed it in an airplane and the engines created a beat note at say 10 hz.
If I remember correctly, the shake table was built by a company then called Ling-Temco-Vought. It consisted of an electronic amplifier and a loudspeaker-like voice coil, and it was big. The final amplifier, which was visible through a window in the cabinet, consisted of two vacuum tubes each about the size of a 5-gallon bottled water tank. These were air cooled, and glowed dull red-orange in operation. The voice coil looked like a large kettle drum. It was five or six feet in diameter, water cooled and capable of shaking a load on the order of 100 pounds that sat on the “drum” surface.
For the open house, someone decided to have the shake table introduce itself to the visitors as part of the demonstration. It could do this since the speaker assembly made audible sounds as it shook the payload. They also decided that the unit under test for the demonstration should be a brand new teletypewriter that was under development.
This "new" product was an ASR-33, pictured here:
The shake table had two sources of test frequencies. An internal oscillator could be used to provide the needed 0-5000 Hz at controllable amplitude. There was also an input jack allowing an external source of oscillation.
For the demo, a tape recorder was hooked to this external input with the recording, “I am the Ling-Temco-Vought model xxxx and I test …………” After the tape recording introduced the shake table, the operator was to switch the input to the internal oscillator, which would run through a pre-programmed frequency profile. Of course, this all worked well in rehearsals. On the evening of the open house, with about 50 people gathered around, the operator started up the shake table and the tape recorder. The Ling-Temco-Vought successfully introduced itself. Then the operator threw the switch to activate the internal oscillator.
However, he forgot to turn down the gain control before he did that. You know that little static pop that happens when you switch from one audio source to another? Magnify that by, oh quite a bit, and you can probably visualize what happened: The shake table voice coil gave a mighty pop, and the teletypewriter which was operating during the demo was destroyed. Steel parts were actually launched through the plastic cover, and the cover was broken free from the base of the unit. It was very impressive, and needless to say, the demo was finished for the evening.
The next day a warning sign appeared on the front panel: “Turn gain down before switching input.”
Charles Glorioso has a BSEE from Purdue and an MSEE from the Illinois Institute of Technology. He has over 40 years experience in electronics design and management for industrial and consumer products.