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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..

Key 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.


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.


Some Parts used in the 503

Page Class Description Used in
6DJ8 Vacuum Tube (Double Triode) dual triode 132 502 502A 503 504 515 516 519 533 535 543 544 545 545A 545B 546 547 549 555 556 565 581 581A 585 585A 661 2A60 2A63 3A8 1121 1A4 67 81 82 86 3B1 3B4 B O W Z Telequipment S32A Telequipment D52 Telequipment S52 Telequipment S51 Telequipment Type A
6DK6 Vacuum Tube (Pentode) high-frequency miniature pentode 180 503 513 514 541 543 535 545 545A 551 555
6DQ6 Vacuum Tube (Pentode) power pentode 503
8393 Vacuum Tube (Triode) Nuvistor triode 453 454 1A6 1A7 503
T5030 CRT CRT 503 504 560 561 661