Question

What should I look for when purchasing a used oscilloscope?

Answer

I would recommend purchasing an oscilloscope made by a top-tier manufacturer such as Tektronix or Agilent (formerly HP). These are quality professional instruments which you should be able to get easily serviced or purchase parts for.

Besides checking for the usual physical defects like rust, cobwebs, a thriving insect colony take a look at the following --

There is usually a calibration signal on the front of the oscilloscope. For each channel --
- connect the calibration to the channel input and adjust the voltage range selector.
- change the trigger source to the channel that the signal is connected to. Adjust the trigger level and verify that the oscilloscope triggers when the level is set to value within the trigger range. Also verify that the oscilloscope *does not* trigger when the trigger level is outside the signal range.
- Change the voltage range selector knob and verify the display is measuring the correct levels.
Verify that the timebase function works.
Oscilloscope probes are impedance matched to the input of the oscilloscope. If the oscilloscope comes with the proper probes this can save you a considerable amount of money.
Original manuals are useful but most manuals can be downloaded form the manufacturer.