555

The 555 is a dual-beam CRT oscilloscope made by Tektronix from the late 1950's until the late 1960's. The system is split it two pieces: the main indicator unit and the external power supply unit. The power supply unit contains the transformer, rectifiers and regulators for all of the voltages except the filament and CRT HV supplies. The filament and CRT HV voltages are generated in the 555.

The 555 takes letter-series and 1-series vertical plug-ins.

The 555 uses special horizontal plug-ins that contain the trigger and sweep. Early instances of the 555 came with the 21 and 22 horizontal plug-ins. Later instances came with the 21A and 22A horizontal plug-ins, which use tunnel diodes for triggering.

Regulation of the filament supply happens as follows: There is a feedback loop that is formed between the indicator unit and the power supply unit. The power supply sends AC through the interconnection cable to the indicator unit, where it is connected to the primary of the filament transformer. A sample of the output of the filament transformer is sent back through the interconnection cable from the indicator unit to the power supply unit. This performs two functions. First, the filament voltage that is sent back to the power supply unit is used to heat the cathodes of tubes in the power supply unit. Second, this filament voltage is the feedback signal to a regulator in the power supply unit that controls how much voltage is sent to the filament transformer. One of the consequences of this scheme is that it is impossible to energize the power supply unit without the indicator unit connected. The regulator tubes in the power supply would remain cold without filament power, and would not pass any current.

• 555 manual on BAMA

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  front view: two timing plug-ins on top and two vertical plug-ins on bottom

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  external power supply

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  left side view

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  right side view

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  top view