The Tektronix 535 is a 15MHz oscilloscope that accepts letter-series and 1-series plug-ins. There is also a 535A. There are two timebases and triggering is done with a Schmitt trigger made of a 6DJ8 tube. The vertical and horizontal output amplifiers are all tube and non-distributed. The 535 is functionally the same as a 545, but with half the bandwidth. Early 535s were brown. Later ones were Tek blue. All 535As are blue. Early 535s had selenium rectifiers.