This DMM was acquired by our visiting member LC for his own homelab capability improvements. It was sold on secondary market as broken , but demonstrated to power up. Early serial number of this instrument suggested manufacturing date around 1990-1994. Very common issue with these older instruments is related to leaking electrolytic capacitors near hot power regulators and subsequent destruction of fragile 4 layer analog PCB. xDevs has lot of experience repairing more than 10 different Model 2001 and some Model 2002 with this issue. So this was expected to happen here too.

After all electrolytic capacitor replacement analog PCB was carefully cleaned and inspected. No significant damage around caps was observed so next step was to power up, measure power rails for health and see what self-test process would return.

Meter threw nearly all possible errors, starting with codes for ADC.

Instrument error report after recapping:

Because xDevs had multiple K2002 during meetup to be used as comparison further testing revealed that digital board is OK, but ADC module and analog boards have faults.

Internal tracker data

Replacement of the original axial fan with an Delta AFB0412SHB-SP04 (12 DCV, 40 mm², 0.414 m³/min) was done. Front/Rear Switch replaced with a brand new equivalent.

ADC appears to be noisy which is causing most of the errors. It was confirmed bad in another good K2002 unit as well. Reflowing R827 reduced amount of errors from the ADC module to 200.1, 200.2 and 202.4 in good working K2002 unit. This setup reduced the noise level by at least 100 times. Later suspect resistor network was replaced with a NOMCT16035001AT1 (isolated, 5k x 8, 0.1 %, 25 ppm/K). Tantalum capacitors (C834, C835) were upgraded to higher voltage ratings (> 25 V). Other components replaced include: U812 (SD5400C), U806 (74HC02), U807 (74HC02) and U820 (74HCT175) but it didn’t improve faults.

However the replacement of Q801 to Q804 (both VN0605T MOSFETs) cleared most errors attributed to ADC noise.

The faulty Analog board has the following issues when a working ADC is used now: 500.1, 500.2, 500.3, 500.4, 500.6, 500.7, 501.1, 501.2, 502.1. This is because Keithley 2002 also had faults and damage on analog board. U234 DG411 switch near the current sense resistor network R295 went into meltdown mode and burned the hole in PCB. Errors are most likely attributed to the damaged region near the U234 package (DG411DY).

Only way to repair such problems is to remove ALL carbonized PCB fiber/epoxy material with scalpel/dremel, restore all electrical connections on multilayer PCB.

Pads under blown PCB are removed as well to get access into board. Keithley analog board is 4 layer and does not have ground planes inside, which makes this job easy one. But steady hands and microscope are still a must.

All damaged dielectric is removed all the way thru the board, with remnant of the via hanging for it’s life on copper trace from layer 3.

Broken trace on layer 2 also need connectivity repair. For that I use single copper strand out of multistrand AWG28 cable.

And jump-wire in the cavity installed.

Common components affiliated with the 500 series errors: U233 (OP97FSZ), U220 & U234 (DG411DY), U222 (DG408DY+), U223 & 232 (MAX326CSE+), U225 (LT1097S8), U226 (LT1007ACN8), Q211 (3N163), Q212 (2N3906-G), Q213 (2SK1413), MISC: CR217, K201, R322, R334, and R342. The set of 500 series errors is related to the U229 comparator (LM339D) on the Analog board and after replacement all errors have been cleared after the U229 (LM339D) was replaced.

Keithley 2002 is repaired, fresh adjusted and calibrated. Calibration report attached. Hulk2 used as reference and verification source, except ACV function. ACV function tested with boosted Hulk1 setup (5720A + 5725A) due to volt/frequency limits.

There is minor deviation on negative polarity of 2V range, but it is likely to be systematic error due to issues with Hulks. This can be corrected in next CF2024. 2A current range points are valid only after 30 minute warm-up of shunts in K2002.