555

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Dual beam 30 MHz scope

Produced from 1959 to 1970

Manuals

German Introduction the 555 (PDF, OCR)

555 CRT Specs (PDF, OCR)

555 Factory Calibration Procedure (PDF, OCR)

Type 21 and 22 Factory Calibration Procedure (PDF, OCR)

(All manuals in PDF format unless noted otherwise)

Manuals – Specifications – Links – Pictures

The Tektronix Type 555, nicknamed "Triple Nickel", is a dual-beam CRT oscilloscope introduced by Tektronix in 1959 and made until the late 1960's, when it was replaced by the 556.

The system is split across two cabinets, the main indicator unit and the external power supply unit. The power supply unit contains the main 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 indicator unit.

The Tektronix 555 takes two letter-series or 1-series vertical plug-ins

Type 21 and 22 Timebases

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.

Type 21 is suggested for the upper beam and type 22 is suggested as the lower beam time base, as type 22 contains delayed trigger, and only the lower beam horizontal section is provided with delayed trigger pickup circuitry. The Delay multiplier control pot is also provided on the lower beam section.

A rigid plug-in extension for the timebases (part number 013-013) was included with every 555.

Type 21A and 22A Timebases

Later instances came with the 21A and 22A horizontal plug-ins, which use tunnel diodes for triggering. Delayed triggering is supported. Since the 555 is a dual-beam scope, it is also possible to start one of the sweeps while the other beam is sweeping. The upper-beam sweep and lower-beam sweeps are, in fact, totally independent.

Power Supply and Filament Supply Regulator

The external power supply unit generates all the B+ Voltages (-150,+100,+225,+330,+350 and +500V) required by the indicator unit. This is fed to the indicator unit through the interconnect cable.

The heater/filament supply for all the tubes inside the indicator unit are generated by a transformer inside the indicator unit. Output of this heater/filament transformer is regulated by the external power supply unit by controlling the primary voltage to it.

There are two transformers inside power supply unit. One to generate heater voltage for all the tubes which dont get a regulated heater voltage from Indicator unit. Second transformer is used to generate all the B+ plate voltages. There is also a saturable reactor inside the power supply, which is part of AC regulator for heater transformer inside indicator unit.

Regulation of the filament supply happens as follows: The power supply sends and control AC to the primary of the filament transformer in the indicator unit through the interconnection cable .

There is a feedback loop that is formed between the indicator unit and the power supply unit. This is achieved by sending one 6.3V winding output of the heater/filament transformer in the indicator unit through the interconnection cable (part number 012-032) back to the power supply unit.

This performs two functions. First, it is used to heat the cathodes of specific tubes in the power supply unit - V624,V634,V664,V684,V694.

Second, this filament voltage is the feedback signal to a filament voltage regulator in the power supply. A regulator Tube - 2AS-15, a Temperature Limited Diode - is used to sense this filament voltage and uses it to control DC current through a saturable reactor. The voltage to the primary of the heater/filament transformer in the indicator unit is fed through this saturable reactor and send to the indicator unit via interconnection cable (part number 012-032). This forms a closed loop to regulate heater voltage in the indicator unit.

One of the consequences of this scheme is that it the power supply unit will not energize the plate voltages (except +100V) without the indicator unit connected.

This is because of the following reasons:

1. Heater to V624,V634,V664,V684,V694 are taken from the 6.3V feedback line coming from filament transformer in the indicator unit. Even though the main series pass (6080s and 12B4s) are heated from a local heater transformer, the 12AX7 comparators and 6AU6 error amplifiers are heated from the indicator unit.

2. The Primary of the plate voltage transformer inside the power supply unit is looped via the thermal cutout in the indicator unit. So the plate voltage transformer in the power supply will not get power unless the indicator unit is attached.

Even if you try to power up power supply itself by bypassing the thermal cut-out and applying local heater voltages to the tubes which gets heater voltage from indicator unit, the power supply will not regulate as the design employs shunting resistors across the series pass tubes. Hence without load, it will not be able to provide regulation.

Vertical Amplifier

The Tektronix 555 has a six-section differential distributed vertical output amplifier made of 6DK6 tubes. It is the same as the vertical output amplifier inthe 551 and the 545A. An L-C delay line is between the vertical output amplifier and the CRT vertical deflection plates for each channel.

Architecturally, the 555, with its four plug-ins, is similar to the 7844.

CRT Circuit

The CRT circuit in the 555 is really two separate modules. It is split between two internal metal enclosures, each one containing a high voltage transformer and associated tubes. The lower beam HV power supply is based on transformer T901 and produces:

CRT anode voltage: +8,650 V
lower beam CRT cathode voltage: -1,350 V
lower beam CRT control grid (blanking) voltage: floating, -1,500 V

The upper beam HV power supply is based on T801 and produces:

upper beam CRT cathode voltage: -1,350 V
upper beam CRT control grid (blanking) voltage: floating, -1,500 V

Rectification and voltage multiplication is performed by a total of six 5642 rectifier tubes. Each of the two HV power supplies is independently regulated using a two-stage common-cathode amplifier based on a 12AU7 dual triode. The oscillator tube is a 6CZ5 power pentode.

The 555 uses the T5550 CRT.