Vintage Audio: Tektronix Model 105 Square Wave Generator

This trip to the attic brings us a 105 Square Wave Generator.

Mention Tektronix and most people think of world class oscilloscopes. What is less well known is that the Beaverton, Oregon company also made other types of test equipment. As they were constantly pushing the envelope of oscilloscope performance, it soon became apparent that there was a lack of devices in the field capable of keeping scopes in calibration. So Tek got into the test equipment business as well.

It may be hard to believe in the 21st century, but its frequency range of 25 Hz to 1 Mhz, and 0.02 microsecond rise time were pretty impressive specs back in the early 1960s. It also had sync input and output capability, as well as a frequency meter with 3% accuracy. With the right combination of terminating resistors, attenuator pads and coaxial cable, the 105 offered a wide output range. At the high end, with no attenuators or termination, it could put out square waves from 7 to 100 volts at 600 ohms. On the low end, with a 50-ohm termination and three 10X  attenuators, output ranged from .26 to 4.0 MV. Circuitry of the 105 included a conventional multivibrator with two 6CB6s. This was followed by two limiter-amplifier stages driving an output stage. The clever bits that gave it great specs include direct coupling between stages, strategically located HF tweakers and an inverted output amplifier with the plate connected to ground through the load resistor.

In oscilloscope calibration, the 105 was used to adjust the vertical amplifier and delay line. (Delay line adjustments were among the most dreaded portions of a scope recalibration, owing to the number of interactions between several controls.) Other applications included testing the transient response of amplifiers, checking console sources and testing line amps for ailing vacuum tubes and coupling caps. At the transmitter site, it was great for tuning up high-level plate modulated transmitters for a more robust low-end sound. Of course, the most memorable thing about Tektronix gear is the craftsmanship. Flawless sheet metal work, solder connections and wiring harnesses, and premium-quality components end to end.  Not to be overlooked are the tech manuals that cam with this equipment: hand-drafted exploded diagrams show the location of every part, and are works of art in themselves.

These days, the 105 sees occasional duty calibrating vintage Tek oscilloscopes. It's a bit of a beast, weighing 35 pounds and containing 26 vacuum tubes. Portable? Sort of. It does have a handle on the top. Left on standby mode with its internal fan running, this square wave generator can do double duty warming up the shop on cold winter mornings, although you might not want to look at your electric bill afterward!

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