Keithley 610C repair

Well, when you get round to selling your building, you can probably afford to store a couple more warts, even. ;)

Cheers

Phil

On Sun, 10 Jan 2016 08:51:12 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 01/09/2016 08:45 PM, John Larkin wrote:
><snip>
>> I don't think the mosfets are the problem on the lowest ranges. The
>> problem seems to be random charge transfers and maybe leakages.
>>
>> At heart, the mosfets are the front-end of a voltmeter. On
>> voltage-input ranges, even with the multiplier cranked all the way up,
>> it's stable. So the fets aren't drifting. They might be leaking, but I
>> doubt that. A little pressure on the side of the box, bending the
>> metal a tiny bit, makes the 10fA range go wild. Mosfets aren't doing
>> that.
>
>I think it's mostly mechanical instability combined with fairly large 
>internal E fields.  The power supplies are +- 55V or something, and all 
>the resistors are hanging in midair.  If you've got 100V gradients 
>running around, with a node capacitance of 5 pF and a typical separation 
>of an inch, you can get 10-fA currents from a velocity of
>
>  ~ 25 mm * 10 fA / (100V*5pF) = 0.5 microns per second.
>
>That's an average, of course, so in real life the motions would have to 
>be much bigger.  Still, it makes sense that flexing the front panel 
>(where the switch is mounted) by a millimetre or so might make it go nuts.
>
>A bit of strategically placed foil inside would probably help a lot.
>
>
>> My home-made pA meter was more stable, maybe because there wasn't so
>> much stuff inside to make e-fields.
>
>The combination of a 100-fA FS homebrew box and a 610C is a pretty good 
>solution.  I like your pluggable thing, which is nice and flexible.  For 
>current measurements, making the loops and nodes smaller would make it 
>faster and more stable, so I'd lean towards a fixed 1T resistor and a 
>metal box meself.
>
>Cheers
>
>Phil Hobbs

I built that before I got the Keithley, and still use it now and then.
I have an aluminum chassis box that drops over the top to shield Z1
and Z2 when needed. 

I was looking for a low-leakage diode. The PAD1 is sold as such, but
it's actually a jfet with s and d shorted. It has lots of series r,
lots of shunt c, guarantees 1 pA, and it's expensive.

Turns out that small transistors can be excellent low-leakage diodes.
The c-b junction of BFT25 is amazing: sub-pF capacitance, good forward
conduction, leakage hard to measure, 10 fA maybe.

One could charge a film capacitor from diode leakage, for hours or
days, and probably resolve attoamps.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Sun, 10 Jan 2016 04:30:08 -0800 (PST), Phil Hobbs
<pcdhobbs@gmail.com> wrote:

>>> Well, the full scale range is switchable from 10 femtoamps to 300 
>>> milliamps, so the correct answer is "yes". 
>
>>The discussion was to find ways to improve the drift on the 10 fA range. 
>
>>Would your customers let you get away with such an answer? 
>
>I don't recall ever having been asked such a stupid and hostile question by a customer. 
>
>Cheers
>
>Phil Hobbs

Especially one that expects you to work for free.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Sunday, January 10, 2016 at 5:51:19 AM UTC-8, Phil Hobbs wrote:
> On 01/09/2016 08:45 PM, John Larkin wrote:
> <snip>
> > I don't think the mosfets are the problem on the lowest ranges. 

> I think it's mostly mechanical instability combined with fairly large 
> internal E fields....
> A bit of strategically placed foil inside would probably help a lot.

If the box in question has been around the block a few times, it might
have a shield missing; I've seen factory-fresh meters that
were too sensitive to nearby items, and on calling the factory,
was advised to reseat the shield.   

"What, there's supposed to be a shield there?"  I replied.

>> I think it's mostly mechanical instability combined with fairly large 
>> internal E fields.... 
>> A bit of strategically placed foil inside would probably help a lot. 

>If the box in question has been around the block a few times, it might 
>have a shield missing; I've seen factory-fresh meters that 
>were too sensitive to nearby items, and on calling the factory, 
was advised to reseat the shield. � 

>"What, there's supposed to be a shield there?" �I replied. 

;)

Cheers

Phil Hobbs