The 585 uses 580-series plug-ins, which look like letter-series and 1-series plug-ins but are electrically different, primarily in the impedance of the signal interface between the plug-in and the scope. The pin-out is also different: it's essentially flipped 180° from its 530/540/550 series counterparts. 'Signal out' is on 14/16 as opposed to 1/3, etc...
The Type 81 adapter allows the use of letter-series and 1-series plug-ins in a 585.
The original 585 (up to serial number 1070) has an "A" trigger circuit that has a differential trigger amplifier made of a pair of 6EW6 miniature pentodes, followed by a Schmitt trigger made of a pair of miniature dual triode tubes.
Starting at serial number 1071, the miniature dual triode Schmitt trigger was replaced by a 10 mA tunnel diode and a pulse amplifier made with an OC171 germanium PNP transistor. The tunnel diode is AC-coupled to the base-emitter junction of the transistor.
Vertical Signal Path
Like the 545, 551, and 555, the 585 uses a distributed amplifier for vertical deflection. Unlike the other 500-series scopes, the 585 uses a CRT with distributed vertical deflection plates, the T581, enabling it to have both high sensitivity and fast risetime. Total CRT acceleration voltage is 10 kV.
Several improvements were published for the 585 circuitry, including the enhanced trigger circuit. If all of those changes are applied to a 585, it is essentially a 585A.
Rackmount versions were also made, the RM585 and RM585A.