Joel, I too have a K616. When I get a chance, I will pull it down and
look at the bias current. I also have a K610CR, and a K602, which I
looked at last week. These all use the same front-end module based on
P-channel MOSFETs with unprotected gates, differentially clamped (gate
to gate) with external diodes or transistor junctions. And, they all
have the same 5 fA bias spec.
Of all the MOSFETs from the old days, I know of only one type so far -
the 3N163, I think - that has the unprotected gate. The early MOSFETs
(called IGFETs back then) needed special anti-static storage and
handling, then later models added built-in Zener gate protection for
more durability. This is what spoiled them for ultra-low current
applications.
So, I'm pretty sure Keithley used selected and matched 3N163s or an
equivalent, for the inputs, and selected small-signal bipolar
transistors for the gate clamping B-C junctions.
My 610CR fired up OK after many idle years, and settles down to right
around 5 fA, so OK there, but is very jumpy and sensitive to every
mechanical effect like tapping the controls and cabinet. I need to do a
thorough cleaning and Deoxit treatment on all the controls, and tighten
things up mechanically. I also found either a packaging design flaw, or
that there may be a missing shield part under the input section at the
front panel. There is a huge open area around the input connector at the
bottom, where it's exposed to the bottom cabinet cover. The cover edge
is just placed in a slot in the front panel, and has a couple clips
retaining the back. The slightest vibration or shifting position causes
huge jumps that would be gone if proper shielding was inside. I'm
assuming there's a missing part on mine, more likely than Keithley
missing this vital issue - either way I need to make an inner shield
part for it eventually.
The 602 also fired up OK, after changing a couple of its six 9-volt
batteries. It settles to around 1 fA, and is not very jumpy, but the
zero drifts quite a bit. I think this is mostly due to the battery
voltage dropping during use (there is no regulation), and maybe would be
much better with all fresh batteries. BTW I modified this unit years ago
to replace the big old-style C-Zn block batteries with modern 9V ones,
and eliminated the Hg cell, by using a 1.22 V IC regulator and different
scaling resistors to get the 1 V reference for ohms mode.
These models both are subject to rather large voltage offset drift, so
need quite a bit of tweaking to keep everything good, even though the
bias currents are OK. It's interesting that there are three levels of
zero adjust - H, M, L - to accommodate some amount of MOSFET variation
and operating temperature range. But, that's what it took back then, and
it's still pretty respectable, especially considering these are about
fifty years old.
Anyway, regarding getting the bias current back to spec on a 616, a
thorough cleaning of all high-Z insulating structures should do it, but
don't forget to do the input MOSFETs. Their weak point is the package,
and possible gate leakage on the surface, from the leads to each other,
and to the can, which I think is the substrate and source, tied together
in this application. If you've tried every other possible thing but not
the MOSFETs, then I think it will work wonders. Gain effects tend to be
from leakage on feedback elements, while bias current and offsets tend
to be adjacent-node problems with DC present - even seemingly tiny
amounts. Also, since there's a digital voltmeter measurement behind all
this, part of getting things zeroed may include what's going on in there
too. I don't know offhand.
Speaking of device packages and leakage and such, if there's enough
interest, I can tell my "AD542 story" from decades ago, which explains a
lot about this subject, and why I always pay lots of attention to it,
and how to avoid or improve it. And it would include a free rant about
how crappy device pinouts are for some purposes, and how the standard
advice in every high-Z data sheet and app note ignores the biggest problem.
I'll report some more about the 417 project later, and maybe see what
the 616 looks like by then too.
Ed
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