The Tektronix 7503 is a 100 MHz 7000-series mainframe with 3 module bays. In contrast to most other 7000-series mainframes, which have switch mode power supplies, the 7503 uses a linear power supply.
The 7503 project was managed by Phil Crosby, and was supposed to be a 7504 but with one fewer plug-in. The 7503 was Tek's first scope to use a harmonica connector for inter-board wiring. Prior to this, inter-board wiring had either mainly been point-to-point soldered. Other approaches were taken on certain instruments, for example the 7704, in which pins sticking out of the chassis mate with holes in the boards that receive the pins and make electrical contact.