485

Manuals – Specifications – Links – Pictures

The Tektronix 485 is a dual-trace, portable analog oscilloscope with a maximum bandwidth of 350 MHz. The impedance of the inputs can be set individually to 50 Ω or 1 MΩ. The scope achieves its maximum bandwidth when the inputs are in 50 Ω mode.

The 485 was introduced in March of 1972. It includes a switching power supply and uses custom integrated circuits for most of the gain blocks. Triggering uses tunnel diodes.

A distinguishing feature – at least in a portable oscilloscope of its period – is the Alternate Horizontal Mode. In this mode, the B timebase is shown in a separate full-width scan, as opposed to the more common Mixed Mode, where the B timebase is shown on the same line as the A timebase, after the delay time. At the same time, The project leader for the horizontal section of the 485 was Murlan Kaufman.

The 465, which was introduced after the 485, only received this feature in its "B" incarnation (465B).

Key Specifications

Input Impedance	50 Ω / 1 MΩ // 20 pF
Rise time	1 ns (50 Ω) / 1.4 ns (1 MΩ)
Bandwidth	350 MHz (50 Ω) / 250 MHz (1 MΩ)
Deflection	5 mV/div to 5 V/div in 1−2−5 sequence, 2% accuracy
Time Base	1 ns/div to 0.5 s/div in 1−2−5 sequence
CRT	154-0652-00 and 154-0652-05, 21 kV acceleration, 8x10 divisions (0.8cm/div)

Links

• Tek 485 @ amplifier.cd
• 485 troubleshooting @ YouTube

Maintenance issues

A healthy Tek 485 CRT has been observed to have a filament current of 98 mA. A broken Tek 485 CRT that lost its vacuum was observed to have a filament current of 153 mA.

Pictures

External

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Internals

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  Top, Horizontal Amp board

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  Underside, Power supply, A-Trig board

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  LHS, Vertical Amp board

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  RHS, Sweep/Inverter board

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  A-Trigger board

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  Vertical attenuator Assy

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  Vertical attenuator Assy, 50Ω Board

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  Vertical attenuator Assy, Hi-Z Board

Custom ICs used in the 485