Does this Keithley 616 electrometer look operational?

[ikrase]

I'm in the market for a cheap electrometer/picoammeter because I'm trying to build a mass spectrometer on a shoestring budget and it seems like some of the cheaper options are less expensive in sweat and treasure than mucking around with op-amps, especially given my limited electronic design skills.

I found a few Keithley 616's which I've seen show up in some posts here before. One of them has the meter reading "+1___" (3 LSD's blank) which according to the manual indicates an overload. The meter is set to 10^-5 A range, sensitivity 3 clicks to the right, Fast, Zero Check off, and of course it isn't possible to tell the setting of the zeroing knob.

Is this probably just dirty / have the zero adjust rotated to the extreme end of the range, or is this indicative of a serious problem?

[Kleinstein]

Having a overload in the 10 µA range at least looks like a fault. So with only limited electronics skills, this might be serious. i might as well be just an oxidized switch, but could also be a blown / corroded unobtanium chip. Anyway a repair might be more demanding than building a small pA range current to voltage converter.

When building a cheap mass-spec, building the pA meter your self is one of the few places where one could get away cheap. This is about the easy part. An electrometer might be useful as a tool for development and testing  though.

[Chris56000]

Hi!

I've downloaded a copy of the Keithley 616 Manual and I'll post back a bit later with Fault–Finding and Repair Hints once I've had a chance to study the circuits!

Chris Williams

[cncjerry]

I suggest you read through some of our old posts on the 616.  There are a few of us that posted a couple of threads in the past year.  The 616 is a very sensitive meter and has some difficult to source components and a couple of areas you don't want to touch if you want to keep the high ohm readings functional. 

The next biggest challenge when you get one that is operational are the triax connectors and cables.  I got lucky finding a guy that was a ton of help.  All he sold were cables for all types of applications.  He sent me samples of a couple and multiple triax connectors.  I finally got to the point where I had two sets of cables with quality clips on the ends.  Many of the meters have one or two of the pins ground off the BNC connector to accommodate a triple pin vs dual pin, IIRC.

A properly functioning and setup electrometer will measure people walking into a room.  I'm sure there are a number of religions that use them to measure peoples' auras.  They are a ton of fun to explore ridiculous properties and charges.  They also have shocked me a few times due to the high voltage on the open ohm ranges.

I remember a video where someone charged a cap and measured its voltage on a Keithley Electrometer and a high impedance voltmeter.  The high input impedance of the electrometer is readily apparent as it took forever for the cap to discharge through the electrometer.

For grins, I got it out this morning and measured two points on our Christmas tree.  Depending on the branches I selected it read has high as 10E7 ohms and as low as 100k.  I wonder if you could use one to test the freshness of a Christmas tree?

If you get hung-up on cables or connectors send me a PM as I have the Ebay seller's name someplace.

~Jerry



 

[Chris56000]

Hi!

I've had a chance to study the circuit-diagrams now!

As Member CNCJerry said, there are a number of areas where you must be specially careful, both for your own personal safety and to prevent damage to sensitive semiconductor devices in the pre-amplifiers!

In particular this instrument requires a 550V H.T. supply in it's preamp circuit in order to provide a sufficently high source voltage for the Giga-ohm ranges, obtained from two voltage-doubler circuits in the power-supply fed from a 110-0-110V secondary on T101, providing +275V and -275V with respect to chassis-earth 0V.

First of all, you MUST be happy measuring high voltages and have a good-quality multi-meter with sound test-leads to make the tests suggested below!

1) Measure the voltage across C202/R211 with the multi-meter black lead on chassis/earth or the metalwork. The reading should be +275V DC  +/-20V. If very low or missing, check D201/D202 for short or open-circuits and C201/C203 for open circuits. If the reading is particularly high you may have R211 open-circuit or a fault in the positive-side of the current-source in the preamplifiers, which I'll come onto later.

2) Measure the voltage across C204/R212 with the multi-meter black lead on chassis/earth or the metalwork. The reading should be -275V DC  +/-20V. If very low or missing, check D203/D204 for short or open-circuits and C202/C204 for open circuits. If the reading is particularly high you may have R212 open-circuit or a fault in the negative-side of the current-source in the preamplifiers, which I'll come onto later.

3) Check the low-voltage supplies:-

a) +12V @ pin 10 of QA201 (a standard 723-type regulator) - any faults should lead to a test on the '723 itself (test by substitution), bridge rectifier D205 and it's reservoir capacitors C205 & C207 - you can use a standard 220uF to replace these.

b) -9V at TP2 - if low or missing check D206 for leaks or short-circuits and R201 (801 ohms - this odd value may be difficult to find but I think it should be available from a supplier of "E96" series resistor values);

c) +5V on TP4 - this is obtained from a TO-3 can type 5V monolithic regulator - if faulty (unlikely) an LM323K/STEEL (Mouser) can be used to replace it.

d) -12V at TP5 - any faults should lead to a check of QA202 and Q201, the rectifiers D209 & D210 and the reservoir electrolytic C213.

4) If all the power supplies test OK, the next test is to see of the meter can be made to read zero. Refer to the circuit diagram of the pre-amplifier stage drawing no. 25764E and unplug the Pre-amplifier Module PC361. Place a 100 ohm resistor between Pin 1 of J104 and Pin 4 of J104.

Switch the instrument on. The display should be within a few digits of zero - not more than +/-0.005. If you are still getting a "+1." overflow indication the fault may be in the positive/negative pre-amplifier source circuits Q105/Q108 (positive) or Q106/Q109 (negative), or a fault in the associated resistors R126-R136. If the transistors are leaky or test unserviceable, you may need to get a "parts mule" (another unit the same) as these are specially selected Keithley low-leakage devices.

***CAUTION*** - make sure the 275V smoothing capacitors C202 & C204 in the PSU are completely discharged (zero volts) before making any component tests in the pre-amplifiers!

The resistor values should be stocked by Mouser, etc.

5) If you can't find a problem with Q105/Q108 & Q106/Q109, leave the pre-amplifier PC361 out and transfer the 100 ohm test resistor to J118 & J119 and again switch the instrument on and test for a very small or zero reading. If you still get the "+1." overflow indication with a 100R across J118/J119 there is a fault further forward in the A/D Converter and digit-drive logic, which I'll cover in a further post if needed!

6) If you DO get a zero reading (or very close to it) when the 100R is connected between J104-1 and J104-4 the fault is either in the Preamplifier Module PC361 or all the associated range-switching prior to that, which, again, I'll cover in a later post if needed!

Let us know how you get on so far!

Chris Williams

PS!

The manual is too large to post as an attachment at about 4.4M but it is freely downloadable from k04BB or "Dxdevs" that specialise in Keithley DMM repair, and possibly Electrotanya as well! Print it out and have a good read of it to begin with - long experience has taught me that trying to fault-find gear off a computer or lappy screen is a frustrating exercise - an open book with it all laid out in front of you is so much easier!

[ikrase]

Oh, wow, thank you! That's even more than I was asking for!

(I have access to the manual.)


Part of why I was looking at these is because some of the components in them -- especially that great big rotary select switch with the teflon terminals -- seemed like they might be useful even if the meter overall is broken. (I've done my homework concerning electrometers, but when one goes beyond "OPA128 in an altoids tin" to selectable ranges and stuff, I start getting lost.)

I also think I have a source for parts to make test leads.


As far as the whole mass spectrometer thing: yes, that's exactly what I was thinking of. Without knowing what range of ion currents I will be measuring...  a small op-amp in an altoids tin would probably make a good collector amp once I can size the resistance or if I want to have multiple collectors, but until then, protection circuitry, easy connections, and range settings are important. However, I kind of wonder what parts of the mass spectrometer you think that I *can't* save money by DIYing.