90903 Cathode Ray Oscilloscope

The 90903 rack panel oscilloscope was part of a series which included the 90902 and 90905 models. The last digit of the model number represents the CRT size in inches. Thus, the model pictured above has a 3" CRT. These are basic scopes comprising the power supply and general brilliancy and focus units. For all practical purposes these were used with external sweep units such as the Millen 90921. However, as an AM transmitter monitor the user could feed modulated carrier RF into the vertical input via a pickup loop and audio modulation voltage to the horizontal input. By proper adjustment of levels well known trapezoidal monitoring patterns would result.

We have knowledge of at least two 90903 versions. Early edition models used a 2X2 mercury vapor rectifier power supply tube which could be seen at the rear of the unit (schematic date 1948). This version had a front panel like the one shown above. The later version 90903 integrated the Millen 90202 module plug-in power supply (solid-state) and utilized a completely different front panel layout (schematic original date 1966). On the later version the four adjustment controls are located two each in the vertical on each side of the CRT. The older version used a wide chassis cabinet and the CRT neck would stick out the back of it. The later version chassis cabinet was narrow and protruded out the backside completely housing the CRT. It's this elongated cabinet that resulted in the front panel controls being brought in closer to the CRT display. Strange that Millen didn't change the model numbers between these two versions. Both schematics indicate 90903-1, however a mechanical drawing I have for the later version indicates it's a 90903-3 which would reflect a model version change. What adds to the confusion is the -3 would represent a version 3 scope. If anyone has a -2 version and manuals please contact the society as we would love to post information on that one too. It should be noted that all known versions use the 3KP1 CRT.

The photo above is courtesy of Joe Rose WA2PJP.

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