Keithley 2110 - Documenting / Fixes

[beenai2018]

Hello
I have recently acquired a Keithley 2110. This is the same meter as the picotest M3510A. It is 5.5 digit  basic bench meter

The unit I had was listed for parts  ,but had a  working display(cant bet too bad to fix,  I thought,  ). Made an offer and got the meter.

The good news is ,it partially works .Im not looking for anything cal lab level ,but something that works and help out  with my hobbyist needs.

Review by Gerry Sweeney here - 


I have opened up and  looked . My suspicion is the Amps protection /range switch ., I also found the a nice copper power ground pin (near the 400V AC Capacitor),and all DC voltages/ Ohms relative to this are accurate.

System has a 602 Ram Self Test error ( No option to skip or pick tests in this meter  ). Skipping this issue  for now,till  the Front Ends are ok.


Current State

DCV - Has a 0.170v offset from power ground  as measure on Input Low.  Nulling this out ,meter measures ok. When measuring with Input Hi and Power ground ,voltages look good . Checked the AD584 and Max 6341 references.

ACV - seems ok , but has small mV level offset .

Currents -Havent tested ,but seems to have some offset.

Ohms - 2W . With the Input Low/ Hi jacks. each range seems to have a different ohmic offset .   When measured between Input Hi/power ground copper pin , it works to good accuracy  .I have a few .01% resistors and they all look good

Freq - Works
Cap - Does not work ,always "OVld".

RTD /Thermocouple - Works (Note this is an independent measurement circuit on front panel) . I used a k type probe on components and they were all 30-40 C


I read this post  https://www.eevblog.com/forum/testgear/0-2v-offset-on-dc-measurements-with-picotest-m3511a-dmm/ ,seems to be similar but not same as that meter has more items that work.

I need suggestions to check and maybe help with  some components . What is Q208/Q209 A22    ( maybe a mosfet / jfet  , the picotest has it as a 703w/F2GZA , Keithley has it as A22 for both ). Heating the part changes the offset .

Pictures !!

[Kleinstein]

One possiblitly could be a battery buffered RAM holding the cal constants that went bad. I have not checked if this meter uses this and not flash.

It looks like this meter uses a SD ADC chip (ADS1271). 170 mV is a lot of offset and would be pretty obvious if a good ground / ref. point is found.
U501 pin 6  (output of the OP-amp) should have more or less a buffered signal that goes to the ADC that may be worth testing. At least in the 1/2 V range it should follow the input.

One may have to draw a crude reverse engeniered schematics ot the front end, to at least get a general idea and see how the ground / low sides are connected.
Like in the other thread on the M3510 the problem seems to be on the ground side somewhere.

For the SOT23 parts there are tables with SMD codes.

[beenai2018]

Update-Fixed issue
 
All functions  work and are in spec.Meter possibly needs a cal,but is so close in my basic standards that I'm not too bothered .

Fix Notes
The 4 transistor near the current range are a pair of 4393 fet and (mostly,possibly) a pair of 2n7002 mosfets.

The mosfets gates are connected to the lm393 comparator inputs ,and also the mc14094 (u1003) .switching ranges sets 3.2v on the gates.
The comparator connected to the 4393 fets and drives them near the input hi.the mosfet conn3ct and disconnect ground and current shunt resistors.

The lower mosfets (q208,q209) names  A22 were bad . A22 coded devices are transistors,mosfets,diodes. Intuitively this could be a mosfet switch,but not clear.

Looking at the available service manual of the big brother picotest m3500, the most common 3 pin devices were 4393 fet and 2n7002 mosfet.

Since these were ground switching, and driven by mc14094 I guessed these must be n channel mosfets.


These were replaced with 7002 mosfets salvaged from elsewhere ,marked SS.


This cleans and fixes all ranges. 

[digitaloz]

I, too, recently purchased a Keithley 2110 that was sold as-is and the self test is showing the same fault code. The pico test service manual has u1701 listed as a 1mbit sram in a 44tsop package. When I looked for u1701 on the pcb, I noticed that it is not populated and there is nothing there along with some other parts around that same area. I noticed your pcb is the same way. The footprint for u1701 is different on the keithley however.

These are the parts that are missing on my board:
u1701
r1704
q1701
r1703
r1702
r1701
r1705
c17015

These were missing but in a different part of the board:
u1203
d1506

Does anyone know what these parts (most importantly, u1701) are for the keithley 2110?

[beenai2018]

I noticed the missing RAM IC as well .

However  I believe the available Picotest Service Manual  is for the higher grade  instrument (the 2100).

https://www.tek.com/en/products/keithley/digital-multimeter/2100-series

2100 design based on Semi Discrete  ADC vs the integrated chip  ADS 1271 in the Keithley 2110 . The 2100 also seems to use  an  FPGA   for Instrument Front End /ADC State control .

For  your 2110

The fault code may be  incorrect . A simple test is remove the Q208 Q209 transistors  .This will remove the RAM fault and display the actual fault codes.  When I changed the Q208 / Q209 all started working .

[wigahluk]

Apologies for resuscitating an old thread. I recently got a Keithley 2110 with the same problem (voltage readings had an offset of about 150mV, other readings were also off). I replaced the mentioned Q208/Q209 A22 with 2N7002 mosfets (https://www.onsemi.com/download/data-sheet/pdf/nds7002a-d.pdf) and the meter seems to work correctly now, except for the 2w ohm readings that have an offset of 3ohms consistent across different ranges. 4w readings seem correct, as well as any other mode I tried. The self test failed once after the replacement but has been passing ever since. Considering that the offset seems to be exactly 3ohm, I wonder if there is any other issue. Any ideas on what should I look for? Thanks in advance!