Keithley 610C repair

(following on from the discussion of leaky DIP sockets)

I could really do with an instrument which measures down at the picoamp level or lower. The Keithley 610C seems to be highly regarded around here, but they very rarely come up for sale here in the UK. Keithley's equivalent current model, the 6517B, is $8K, and from what I hear Keithley's build quality isn't what it once was...

There is one used 610C on eBay, not working, in the US for about $100. Anybody have any experience of repairing these things? I found a sevice manual online, they don't look particularly difficult to work on, but I'm guessing one of the the most common faults is blown input transistors which are a matched pair on a module, only available from Keithley.

Any advice? For $100 I'm tempted to take a gamble and have it delivered to our US office, one of our guys can bring it over next time he visits. OTOH if the parts are unobtainium I'll wait for a working one to appear.

TIA

Reply to
RBlack
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The switch and meter movement are pretty nice, so even if you can't get replacement MOSFETs, you could do a brain transplant with something like an LMC662 or 6001 and have a fairly swoopy gizmo.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

If you search back postings in this NG a year or two ago John Larkin posted schematic and photos of a box he built around an LMC6001 which do pretty much the same job as the 610C for femto/pico ampere sleuthing.

piglet

Reply to
piglet

I think it was the same discussion where I was talking about using a charge dispensing loop with a 100-pF Teflon cap to do the same sort of thing. It's easier without resistors.

Voltage measurement is pretty easy--you can make a follower and measure its output with a normal DVM.

I have a couple of dozen MOSFETs without gate protection, and you can still get 2N7002Es, some of which leak only a few electrons per second.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Yeah I might think about building my own too. I've measured pA with a "leaky" Fet opamp. The LMC662 looks nice also (as Phil noted.) And it's a lot cheaper... you could blow up a few for ~$1.5 each.

George H.

Reply to
George Herold

The Keithley is a fabulous instrument, light-years better than the thing I built. I'd buy the dead one and try to fix it. It might be something easy like the power supply, and even if the input fets are blown, that can be fixed.

I see four 610s for sale on ebay at this instant. Used equipment brokers often have them, too, and those can be bought guaranteed working.

Reply to
John Larkin

At $100, there's almost certainly something wrong with it. Still, methinks it's worth trying to fix it. 610c manual at: The schematic is dated 1968 so I would suspect that the big electrolytics in the power supply have dried out. If that doesn't do the trick and it looks like the two input FETs are fried, carefully remove them (using static protection) and replace them with whatever you can find that's close, just to see if it works. If it works, but leaks badly on the higher sensitivity scales, try to find suitable substitute FETs. The manual does not list a replacement part number for the input FETs, which are designated as part of the input PCB, Keithley 23733A. However, the photo looks like they're easily replaced: Note the black Fairchild xsistors above the FETs. They're epoxy filled and might be IR light sensitive.

Internal detail and photos:

Calibration costs about $250 to $300 which might make it more attractive to buy something that works and is calibrated.

Interesting readings and photos:

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

You can get CD4007UB with several pairs of MOSFETs, for very cheap and experiment doing the replacement. Or, ALD sells well-matched pairs, PMOS or NMOS...

There's some bootstrapping in the Keithley input circuit, so the replacement's leakage will be far below the 'tested' leakage-under-bias spec.

Reply to
whit3rd

...which brings up a question: can one count the electrons (like Millikan oil drop bit)?

Reply to
Robert Baer

Hard to do at room temperature, especially in a relatively large device like that. You can do it in small devices at low temperature. (Google "coulomb blockade".)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

One way to measure the charge of the electron is by measuring the shot noise. (and the DC current)

George H.

Reply to
George Herold

I've thought that some simple semiconductor experiment could demonstrate charge quantization. It would make a nice science project.

The numbers are intimidating but maybe not impossible. You might not see single electrons, but could dig out something statistically.

An eprom is a possible detector. Or maybe a cmos imager.

Reply to
John Larkin

What are you thinking? Charge quantization.. things.

Shot noise. SPAD's sorta start out as a single electron... like pulse counting in a pmt. Quantum conductance. (You can see steps in the resistance of two thin gold wires that are intermittently touching each other.)

George H.

Reply to
George Herold

That is what is in a 6517 anyway!

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John Devereux
Reply to
John Devereux

That's not as appealing as seeing single-electron steps. Some day I may try the eprom thing.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

I don't know eproms that well... what are you going to see?

The bouncing gold wires/ quantum conductance thing is sorta single electron. The contact area is so small that there can be only one electron state in it. A box so small it can only hold one particle. (really two electrons 'cause of spin)

Here is some old data... circa 1995.. Yellowing dot matrix printer paper.

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Constant voltage source across the wires and the Y-axis is the current. In theory each step should be 12.7 k ohms... or something near to that.

George H.

Reply to
George Herold

The charge stored in an eprom cell is fC. And it's accessable to UV to discharge it.

The idea would be to scan an eprom and snoop the cell contents. Modulate Vcc and apply UV, and look for bits that are marginal, namely toggling 1/0 as Vcc changes slightly. That will plot as a step function. Now apply some UV; maybe absorbing one UV photon will cause one (or possibly more) electrons worth of discharge, which would make a horizontal jump on the plot. Multiple jumps should be quantized to the value of e.

It might be possible to detect electron charge steps in a discrete fet of some sort, but the capacitance involved will be hundreds of times the capacitance of an eprom (or ram, or CCD) cell. One electron charges 1 pF to 0.4 uV, which is intimidating. I wonder if there is some statistical way to tease 400 nV random steps out of a lot of noise.

There may be some other IC that has an internal floating node that has very low capacitance and can be accessed somehow.

I should do the math on LTC6268-10; not right now.

Reply to
John Larkin

Well some synchronous, lockin type detection. ramping a current back and forth into a cap and seeing voltage steps would be awesome. Currents from a voltage source through a resistor have no shot noise.. which I think means the electrons are coming at you in a well ordered train.... hmm I guess there is no guarantee that the first "train" of electrons and the next have any phase relationship.. steps are smeared into the expected ramp. Next idea please... (That's me talking to my brain, or God, or wherever ideas come from.)

George H.

Reply to
George Herold

I think it is less than 400nV. By V=Q/C I get Qe/1pF = 160nV

piglet

Reply to
piglet

OK, mesure the transition point, make a UV blip, measure again, repeat.

I expect that low currents have very small electron correlations, so shot noise returns.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

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