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
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!
InternalsIf 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:
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.
Rest of analog board photos:
Digital side board:
Service manual again:
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.
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 supplyNot 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 panelMembrane 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...
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