Products > Test Equipment
Keithley 2400 SourceMeter review and teardown
TiN:
Followed by my Kei2001 endevaours, i quickly found another level of instruments from this vendor.
SourceMeter series is not just a 6.5 digit DMM, but actually combines three
instruments in one. Keithley have variety of SMUs for different use requirements.
First is fast and precise 6.5 digit DMM, with ability to measure voltages, currents and resistance.
Second - precision power source, with adjustable compliance levels (limits) for both voltage and current.
Third - given it's four-quadrant power unit, it can not only source but also sink current, acting as active load.
All these functions are synced together and operate using same controls, which making use of it as easy as just normal power supply, but with much higher functional
and precision/accuracy level. These kind units are widely used in semiconductor and production testing and covered from "simple" 20W units like this 2400, to advanced
systems with multiple channels and wide ranges from picoamps to tens of amps. And even base 2400 cost significant money, I got this SMU with broken VFD,
and even then had to pay over 1000$. After salvaging display from trashed Model 2001 and replacing it - unit worked like a charm :)
Every photo below is clickable to open high-resolution, so enjoy exclusive photo coverage of Keithley Model 2400.
I was trying to find any internal photos on web before, but failed.
Exterior
It's done in common half-width 19" unit format, just like popular 2000-series DMMs, featuring same bright contrast dot-matrix VFD as Model 2001,2002,7001,7002 has.
Front panel is similar to DMMs, membrane keypad and front inputs. Due to ability of source current/voltage it have button to enable output.
Rear side have inputs as well, adding guard output and guard sense input. This comes handy to perform unique 6-wire measurements, which is useful when one trying
to source tiny voltages/currents into test device or measuring gigaohms level resistance.
Rest items on this side quite usual, GPIB port, RS232 port, mains IEC power input (have versatile 85-250VAC 50/60Hz input) and digital I/O connector.
Also have Keithley's Trigger link to team with other SourceMeter or measurement instruments for syncronised operation.
Model 2400 is fanless, so it's producing zero noise, which is nice for home use :D
Left side have massive heatsink to dissipate heat from output stage.
My unit was abused by previous owner with some sticky ugly blue liquid which is hard to clean and front panel had few cracks here and there.
Will replace front panel plastic later probably, if it's not too expensive.
So, following Dave's guidance, don't turn it on, tear it apart!
Internals
If you have 2000/2001 DMM and disassembled it, 2400 is no different. Two screws from back, two screws on bottom, and we are in.
Top board is analog voodoo section, with all sourcing/measuring magic and dozens of expensive Linear Tech, Analog devices opamps, comparators.
ADC is once again using integrating discrete solution, with digital section implemented in ALTERA MAX7160 CPLD.
Under analog board we can find switching power supply and digital brains board. Thanks to SMPS there is no bloody mains switching curcuitry like in Model 2001/2002. :)
Analog board shielded from digital and SMPS section by another PCB without any components, but only with copper polygons.
That board covers most of analog bottom side and fixed on standoffs:
Analog voodoo
Of course, there is no schematics available, just like with every recent Keithley instrument. Pity, it could be pretty educating to learn thru different solutions
used on this level of instruments.
Output stage
All bottom side of PCB on photo is output stage, which can source power with high precision. All power devices installed on aluminum heatsink
for cooling. During operation that heatsink getting warm, but not hot (+50-55C max), which is just normal for 20W capable unit.
Left corner have optical isolators for communication between digital PCB from below and analog world.
I could not spot voltage reference on this board, it's not using LM399 like Keithley DMMs, but probably use zener or integral reference. There is TP232 test point
just near serial number sticker marked as VREF tho. Its connected from LT1097 opamp output (U226)
Output stage power source, ZMA Magnetics transformer to provide -225 and 225 voltage source, driven by two transistors on right left.
All output stage section have separation and HV PCB cutouts from rest of analog stuff, nice attention to details.
ADC
Big beefy EPM7160 in PLCC84 package, just like Model 2002. It's configuration have label 2400-801A01. Clock is delivered from FOX 12.000000MHz CMOS generator nearby.
There are two 16-bit DACs, Analog Devices AD7849BR near it, and bunch of opamps.
R609 is TF245 resistor network, which was spotted before in Model 2000 DMM :)
I'll quote service manual here:
--- Quote ---The SourceMeter unit uses a multi-slope charge balance A/D converter with a single-slope rundown. The converter is controlled by gate array U610. Commands are issued by the MPU on
the digital board through communications opto-isolators to U610, and U610 sends A/D reading data back through opto-isolators to the digital board for calibration and processing.
--- End quote ---
Rest of analog board photos:
Digital side board:
Service manual again:
--- Quote ---The core digital circuitry uses a Motorola 68332 microcontroller running at 16.78MHz.
The memory configuration includes:
2 x two 256Kx8-bit EEPROMS
2 x 128Kx8-bit RAMs
They are used in parallel to utilize the 16-bit data bus of the MPU. The RAM is battery backed-up, providing
continued storage of data buffer information during power-down cycles.
All calibration constants and system setups are stored in a separate serial EEPROM.
--- End quote ---
So we are safe here and 2400 will not brick itself when battery go bad after dozen of years.
No much component magic here, it's similar to other Keithley products of that era.
Some LDO power for digital domain.
Another SMPS with ZMA Magnetics transformer.
Battery for RAM and two configuration jumpers. Based on silkscreen labeling, same digital board is used for both Model 2400 and Model 2410 SMUs (2410 can source up to 1100V)
Interesting find - PCB capacitors (?). Maybe it's used to read mains frequency due to AC coupling, but that's really just my guess :) Would be interesting to know
what are those square pads connected to mains input actually do :)
Mains power supply
Not much interesting here, it's KTH-7063 model power supply, probably made by some OEM for Keithley.
Voltage output is 5V 1A and +12V 2.9A.
And, front panel
Membrane keypad with rubber keys, just like good old 200x DMM series.
This is how i got it, VFD glass broken, magic vacuum is gone.
Back side.
Desoldered VFD, using solder wack to suck all solder. Thanks to little bigger pads, i could manage to free display from PCB relatively easy.
I had one of 2001's with lots of components missing, so used VFD from it. New VFD ready prior to soldering:
There is minor difference - 2001 VFD had straight row of pins, while 2400 PCB had checker formed pins. Had to do some creative forming to solder new one on 2400 PCB :)
All done, exploded view of all parts before assembly:
Assembled, first power on:
It works! I already updated firmware to C21, downloaded from Keithley website.
Last cal was 13 september 2007, which is recent, compared my 2001's 1993 and 1995 years, haha.
Some simple checks, 87V and my first 2001 measuring 1.23455V output from 2400.
2V output:
210V output:
Hope you enjoyed reading :)
If anybody have idea what to try or show more on this SourceMeter, feel free to shoot :)
I think volt-nut horde got one more follower, just ordered today bunch of Linear tech opamps, couple DAC and ADCs and first LTZ1000A...
chickenHeadKnob:
@TiN: Thank you for so thoughtfully documenting your efforts.
I am going to read it multiple times. In many respects a teardown of an old Keithley is more relevant to us than robrenz new SMU as most of us won't buy one of those and the used market is nill for the same. One thing that disturbs me about Keithleys, they do seem to get sick and die a lot, at least much more than the equivalent HP/Agilent. That might just be a false impression as the repaired units get talked about on the EEvblog and working units don't. Looking at all the "my 2000x multimeter ist borken" threads I imagine the ratio of living to dead Keithleys is inverse to that of HP3440x which appear to be tanks.
TiN:
I'd like to see teardown of 2450 either, but we got what we got here :)
As for repairs, i think it's wrong impression, maybe because of age of those units.
So far i did not found on web anybody saying about faulty keithley's earlier than 2005 year or so.
I bought myself 7 2001's (5 dated 1992, 2 dated 1994) and this 2400 and two 7001's.
And all this was possible only because i bought them all broken and did not mind to take risk
of fixing them :)
2001 did have issue with voltage drive circuitry in old units, which was fixed on later ones.
None of instuments of 20 year old age safe from caps bulging and leakage either. And broken VFD in this case?
Can happen to any meter, and I still prefer VFD over anything else for usability reasons.
Hell, even space satellites fail :)
But i think positive side - due to age and abuses from owners, normal hobbyists (almost like me, lol) can buy
broken precision instruments, spend some time fixing them up, and get a handy and highly useful tool for
almost any kind of use. As everyone starting in EE, i had choice to buy some cheap bench meters and sources,
from rigol or other brands (which still do job in most of cases) or get some not so fancy but reliable and trusty
tools which i can just use, not worry about knobs and fancy menus? Well, i voted my bucks on Keithley, Tek and HP years ago already.
Did not regret a second still by today.
chickenHeadKnob:
--- Quote from: TiN on January 07, 2014, 05:40:43 am ---
As for repairs, i think it's wrong impression, maybe because of age of those units.
So far i did not found on web anybody saying about faulty keithley's earlier than 2005 year or so.
I bought myself 7 2001's (5 dated 1992, 2 dated 1994) and this 2400 and two 7001's.
And all this was possible only because i bought them all broken and did not mind to take risk
of fixing them :)
...rest elided
--- End quote ---
I know, the image of your stacked "keithley reserve cache" is what gives me the wrong idea, I am a visual animal. Even if Keithleys are less reliable, like a high maintenance girlfriend that can still be fun.
As far as the picture of those square capacitor pads purpose - I am thinking isolated zero cross detect.
TiN:
What I meant, is fact of having bunch of dead keithleys in one hands and couple repair threads on popular forum does not mean that they are more or less reliable.
Maybe that person just a zombie addict and was looking for dead meters on purpose, for collection. I got couple dead HP gear too, 33120 and now 4263B (just arrive today, still in post office),
but it says nothing about reliability. That's it, back to topic :)
Yes, that's what i thought about zero-cross or frequency readout as well, as that's all my knowledge in analog stuff can cover for, as for today :-//
Actually this 2400 already on my desk for a while, almost a year, use it on almost daily basis. Very handy for curcuit probing, as current/voltage levels can be set flexible.
Assembled small buzzer with NFET for one of digital outputs, and setup it to buzz on compliance FAIL condition, so turned 2400 into simple diode-check buzzer :-DD
Only difference that current and voltage levels can be set accurately, so i can easy distinguish dead short from 1 ohm resistor, for example.
Also useful to test current sources and zeners. Just connect zener, source voltage mode, set compliance current to 10mA and you see zener voltage on screen right away.
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