Keithley 2420 Sourcemeter repair help

[PTR_1275]

Hi everyone

I recently picked up a Keithley 2420 source meter really cheap because it was dead. No fault description, but it looked in OK condition.

When I got it onto the bench, power it up and it turns on OK, will source the correct voltages / currents, but then after a few minutes I can hear a relay click and the output glitches. These clicks and glitches get more and more frequent as the SMU warms up. It is almost like it is trying to auto range.

After a while, it stops, but I now get no output. Sourcing 20V with 105uA compliance and I am reading 3.8mV on my bench meter. No more relay clicking.

The service manual troubleshooting section isn't that great and the voltages aren't matching what I should be seeing for the analog section, but I have no idea where to go from there. The service manual is a table of voltages. Maybe I am just used to the HP troubleshooting flowcharts which give useful information.

I am hoping someone with more knowledge on these can give me some pointers on where to look next

TP200 is measuring -2.9977 while sourcing 63v
TP201 is measuring 35mV while sourcing 10v
TP202 is measuring -10.146v while sourcing 10v and measuring uA
TP203 is -2.7mV while sourcing 10v
TP213 is 0.3mv while sourcing 10v

TP232 (Vref) is 6.24365v

These are with respect to TP500


With respect to TP501
TP218 is 7.799
TP219 is -7.810 (Manual says this should be 7v, but the silk screen says-5, so this might be a typo in the manual)
TP214 is 8.05 while sourcing 20v and measuring uA



Power supply checks look to be OK
TP507 14.983v
TP508 -15.184v
TP510 4.963v
TP502 83.155v
TP503 (Couldn't find it on the PCB, but measuring the negative leg of C538 I get-83.161v)
TP504 39.343v
TP505 -39.344v


Any advice would be greatly appreciated.

[MadTux]

Look into the Keithley 236/237/238 schematics and the output stage there, it's more or less the same.
Most probably a toasted transistor/MOSFET on the output stage, at least that was the only problems I had with K236/237 so far.
If it is something digital, you're more or less screwed with all these unobtainable parts in there.

Edit: Just had another look inside the 2400, digital board isn't that bad as I remembered it, apart from the Altera CPLD

[Ice-Tea]

Heh, well if it's the same as the 236/237/238 add a cooked -15V zener to the list 

[plurn]

If it works for a while then fails - cold solder joint perhaps?

Maybe solder melted away on an overheating transistor leaving a bad connection - hence the issue taking a while to show up?. Just basing that on this though he had different symptoms (power supply issue):

http://blog.julian1.io/2017/02/05/keithley-repair.html

Also - a failed solder joint issue   "TSP #95 - Teardown, Repair & Calibration of a Keithley 2400 Source Measure Unit (SMU)"

I did not put any real thought into diagnosis so take my comments with a grain of salt.

[PTR_1275]

I watched the TSP video, not sure how that one got through QC...

Sadly I can’t see any issues on the pcb with bad solder joins. While watching the TSP video I desoldered the TIP and IRF power packages from the board and have tested them. Nothing looks out of place, no bad joints, no shorts. They all tested ok with a meter, then used the peak atlas dca75 and they all tested OK.

I’ll have a look through the 236/237/238 schematics and see what I can find.

Before I started dismantling it tonight, the unit is doing exactly what the one in the video is doing. Turning on but no life, so there’s a chance it is on the digital side. I am thinking something is intermittent because it kinda worked, then it doesn’t.

Thanks for the suggestions.

[goaty]

I got a defective 2420 just now.
Power and digital seems ok, but Analog ist blown. Burnt components are
CR400,401
Q410,411,420,412,414,415,416
Q526,527,530,531
R517,524,564,565,566,567
R569
U405
VR400,401
Switches on, enables output, relay click, but reads "Overflow" in amps and unplausible value in volts.
I´ll try and get the parts, but I think from the blast it got there´s more components damaged where the case is still intact (the others have literally exploded)
Anyone got a bit more schematic besides the partial and the 236/7/8 one ?
If I can provide any measurements or photos let me know.
Thomas

[JxR]

Anyone got a bit more schematic besides the partial and the 236/7/8 one ?

Getting schematics these days is a relic of the past for more recent commercial instruments unfortunately. If the US Gov ever sourced the units though a schematic outside of Tektronix control should exist.  Although the chances of it surfacing is low.

[goaty]

I´m about to continue my repair attempt on the 2420,and wanted to post a few things here.
- List of Test points and differences in names to 2400/2410
In brackets names from other device (2400/2410)
* means comment taken from other forum post

TP500 FCOM (* This is the positive output I/O-HI)
TP501 OCOM (Is this I/O-LO ?)

Power (referenced to OCOM):
TP502 +85V
TP503 -85V
TP504 +42V
TP505 -42V

Power (referenced to FCOM):
TP510 +5VF
TP507 +15VF
TP508 -15VF

Power (referenced to ?):
TP218 +5VL (* 5V local supply for low side buffer drived from +30V referenced to OCOM)
TP219 -5VL

Signals (referenced to FCOM):
TP200 NAVMDAC (* Negative Approximate Voltage Feedback Mode DAC)
TP201 NVDAC (* Voltage DAV output 0..-10V)
TP202 BIAS
TP203 NIDAC (* Current DAC output 0..-10V)

TP204 NADAC

TP205 VDRIVE (* voltage POS / Output buffer or input buffer compliance)
TP206 NVDRIVE (* voltage NEG / Output buffer or input buffer compliance)
TP207 IDRIVE (* current POS / Output buffer or input buffer compliance)
TP208 NIDRIVE (*current NEG  / Output buffer or input buffer compliance)

TP209 +VC (* Buffered TP205 /  Positive voltage compliance, enabled when positive)
TP210 -VC (* Buffered TP206 / Negative voltage compliance, enabled when negative)
TP211 +IC (* Buffered TP207 / Positive current compliance, enabled when positive)
TP212 -IC (* Buffered TP208 / Negative current compliance, enabled when negative)

TP213 ERRDRIVE (* driven by TP205 or TP207)
TP214 MAINDRIVE (EAMP in 2400 / * Error amplifier output, goes directly to driver amp)

TP215 AFB (* Aproximage feedback output, provides feedback during switching ranges)

TP220 ISENSELO
TP221 ISENSEHI

TP222 -SI
TP223 AIFBLO

TP224 IGUARD

TP225 AVMFB
TP227 AVFBHI (+SV in 2400)
TP228 -SV

TP230 VFB (* Voltage reading from outputs, feeds ADC and compliance)
TP231 IFB (* Voltage proportional to output current, feeds ADC and compliance)
TP232 VREF (* Diode reef voltage, 6.2..6.6V, feeds to amp to raise to 10V)
TP233 A/D-IN (* ADC input after multiplexer)


* R238 is a 10k resistor network for current DAC.
Pin1: DAC
Pin2: DAC + feedback
Pin3: feedback