Tektronix engineer Frank Hood recollects:
We started to move into the new Sunset plant in June of 1951. In the same year I started the design work on our first compact scope, the Type 315. This instrument had several innovative features. One was a sweep circuit designed by Dick Ropiequet which was several times more stable and accurate then any other then in use. With this circuit, the actual time of the sweep could be calibrated with such accuracy that we decided to start calling it a “TIME BASE”. The vertical amplifier was also more stable and accurate. A man in our test department, John Kobbe, suggested a direct coupled unblanking circuit. Another employee, Ted Goodfellow, a musician with ceramics as a hobby, suggested making a ceramic strip with silvered notches to act as insulator and support for the components. This smaller scope had less space on the front panel for the necessary controls. A Machinist, Jim Morrow, suggested combining some of the controls, by making them co-axial (that is, a shaft within a shaft and separate knobs.) We tried to purchase such controls but could find no one interesting in making them, so Jim machined sample controls. He even machined the dies to produce our own plastic knobs. All these suggestions were incorporated in the 315. Several patents were granted, which later led to a patent infringement suit which we brought against the Government, (we eventually won the law suit)."
The 315 is mechanically similar to early 500-series monolithic scopes such as the 511A. The scope slides out of the case instead of having removable side covers as were used in the later 53x and 54x, and the 55x, 56x, 57x, and 58x designs.
The move to use a 3” CRT rather than a 5” was to produce a “portable” oscilloscope, which the 315-D was promoted as. With dimensions considerably smaller than the 5” models and only weighing a modest 36 pounds, it offers a lot of performance in a small package. In 1956, Tek introduced a rack mount version, the 315R. One has to wonder what the benefit of a 3" screen is when it was packaged as a rack sized instrument.
As with earlier 5” Tektronix scopes, the “D” suffix indicates the inclusion of a delay line in the vertical signal path. The delay line resides between the pre-amplifier, where the trigger signal is picked off and the final vertical amplifier. The delay line allows time for the sweep to start and the unblanking to turn the trace on slightly before the trace is deflected, so the user can see all of the rising edge of the signal which triggered the scope. The original 511, Tek’s first scope sold, did not have a delay line. It was added with a few other circuit enhancements to become the 511-AD model.
The delay line was offered as a “delete” option on early scopes, with the version without the delay line selling for $50 less. Apparently most customers ordered the delay line versions, and the 315 was the last scope introduced with the D suffix. Even so, the 1953 catalog which was the first the 315-D appeared in did not list the version without the delay line. It is quite possible that no plain 315 models were ever sold.