Keithley 610C Cable

[CalibrationGuy]

Just found a Keithley 610C without an input cable. Lots of contradictory information on modern replacement equivalents. Does anyone know where to get this cable, or how to replace it?

TomG.

[EC8010]

The 610C is from before triax inputs, so replacement is easy. Any RF coaxial cable having a decent (no holes) braid screen will do. RG174 would be fine, but RG59 (75 ohm) or RG58 (50 ohm) are not so good because their braid has holes. I'd replace the input connector as well. Not because it was a poor original choice but because it has probably got dirty (leaky) over the decades. That's why electrometers should have protective caps over their input connectors (and also over their associated cables). Be aware that the longer the input cable, the higher the shunt capacitance and noise gain of the meter. Keep it short.

All electrometers are horribly sensitive to capacitively coupled hum (hence that need for a good screen) and you should always have an oscilloscope on the pre-amp (chart recorder) output to check that you have minimised hum before believing any DC measurement. The best electrometer measurements are made inside earthed biscuit tins connected by a short cable to the electrometer.

[CalibrationGuy]

Thank you for your response. I bought it to check our fluke 720A divider. Nulling it with our 3458A was an exercise in frustration and futility.

TomG.

[CalibrationGuy]

And yes, I read their white paper on how to do it.

TomG.

[EC8010]

You've lost me. How would an electrometer allow you to check a Kelvin-Varley divider?

[CalibrationGuy]

The 610C has a center zero setting with 1mV full scale range.

TomG.

[guenthert]

The 610C has a center zero setting with 1mV full scale range.

TomG.

   That might be so, but iirc the 720A wants a null to be less than 0.5µV to meet specifications.  Fluke recommends (of course) their 845 Nullmeter, which has a minimum full scale range of 1µV.

    Don't know about the 610C, but the little more recent ("brown era") 617 has an output taken directly from the pre-amp.  There a more sensitive instrument could be attached to.  The combination is still noise limited though.

[CalibrationGuy]

You are correct, this is not nearly sensitive enough.

TomG.