503

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Tektronix 503
450 kHz differential-input scope
Tektronix 503

Produced from 1960 to 1973

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The Tektronix Type 503 is a low-cost, low-bandwidth monolithic oscilloscope introduced in 1960 specializing in X-Y mode but also containing a sweep circuit. It features differential signal inputs. There is a similar sister model, the 504, with single-ended inputs. A rack-mount version, the RM503, was also available. The 503 was $625 in 1960 versus, for example, $1000 for the 516 in 1960.

Due to is its low bandwidth, the 503 can operate at high sensitivities while not responding to interference from broadcast transmitter RF fields, making it suitable for biomedical work.

A large market for the 503 was in electronics labs of educational institutions. The differential inputs are useful for entry level students to experiment with observing voltages not referenced to ground..

Specifications

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The 503 features two identical phase matched amplifiers with true differential inputs. The scope has a bandwidth specification of 450 kHz and sensitivity to 1 mV/cm.

The standard version 503 utilized a P2 phosphor CRT that provides a slightly longer trace decay time for ease of photographing the display.

Internals

The 503 is mostly a vacuum tube design with a differential transistor stage following the differential input amplifiers.

The CRT (T5030) does not use post-deflection acceleration, resulting in the CRT cathode operating at -3000 V.

The power supply design is unique in that unlike other scopes where the high frequency transformer only generates the CRT operating voltages, in the 503 it generates all the operating voltages other than the two filament circuits which are supplied by the AC mains power transformer. It is basically a tube-based (secondary) switch-mode power supply (SMPS). A 6DQ6 power pentode, V620, feeds switched primary current to transformer T620. The switching frequency of V620 is between 25 kHz and 30 kHz, enabling T620 to operate efficiently with much lower core mass than would be required for a transformer operating at mains frequency.

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