Keithley K2000 Multimeter Repair Case

Recently, Mr. Li, a Keithley maintenance expert at Antai Maintenance Center, repaired a K2000 faulty multimeter; fault description: it can be turned on normally, but the measurements are all OV. The customer described that a wire of the three-terminal voltage regulator was burned out.

After the machine arrived yesterday, I opened it and found that it was not burned, but the board was corroded after being exposed to moisture. Only the PCB solder mask was damaged.

The wires are still there, and there is some corrosion on the pins of the device. Fortunately, the corrosion area is not large, only in the power supply area.

Because the power can be turned on and measured normally, we can first conclude that the digital part has no major problems. Start troubleshooting from the analog part, first measuring the power supply part. When checking +5V, we find that the voltage is incorrect, and we don't hear the relay operate when switching gears, so we can conclude that there is a problem with the +5V part. Then we directly measure the +5V voltage stabilization part, and the voltage value is correct, so the reference ground of this part is not connected to AGND. We find the schematic diagram of K2000 shared by netizens, and find that the +5V ground is AGND, so we can be sure that the ground wire is broken.

So I measured along the ground wire section by section and found that a via was corroded. After connecting it, I could measure normally.

When measuring, I found that the resistance was OV except for the gears above 10M. In addition, I found that the voltage measurement value was too small. When running the self-test, errors 301.2 and 302.2 appeared. According to the posts on the forum, TF-245 has many faults. According to the maintenance manual, these two tests are measured by cutting into 7V and 13V benchmarks. First, I measured TF-245, 7V was 6.9V, and 13V was 13.1V, which were within the error range. At this point, I felt relieved. If TF-245 was damaged, it would be very ugly after repair.

The detection further down became difficult because of the existence of the switch, but it was certain that there was a problem with the AD signal circuit. According to the schematic diagram, switch to the resistance position so that the signal can enter the input buffer circuit from the HI input terminal.

When HI and LO are open, the voltage of the two terminals is 7V reference voltage, and it is 0.00xV after short circuit. Therefore, the resistance measurement is correct up to this point. Following the signal input circuit, the voltage measurement is correct when it reaches R134, but the buffer circuit output is wrong, so the fault point is locked at this point. During the detection, a phenomenon was found. When HI and LO are short-circuited, when the test pen is poked on the circuit board, the resistance range measurement becomes normal. After releasing, the resistance measurement shows over-range again. Continue to detect, the output voltage of U114 is wrong. This OP follows the input and then controls the voltage of U113. After pressing down the circuit board, it is found that the voltage can follow. After releasing it a little, the voltage rises slowly. Here is a detour. At first, I thought that U114 was poorly soldered. After adding soldering, it was found that it was not working, so I changed it and found that it was still wrong. I started to check the circuit at the front end of U114. When checking the resistor string R113-R107-R103-R108, I found that the signal to R108 was not normal, but the phenomenon was not obvious. I decided to weld R108, but when I welded it, I found that the coating on one end of R108 had fallen off. So I welded it on top.

Then I installed the machine, turned it on and did a self-test, and found that the 30x error was gone. When I tested the voltage and resistance again, the results were normal.

However, the self-test still has several AC-related faults, 401.2, 402.2, and 403.2. According to the AC signal circuit, the input signals of U118 are normal, but the output signal of U118 is wrong. The analog switch U111 is measured and it is found that U111-B is closed, but the output signal of U118 is not following. Therefore, it is preliminarily concluded that the internal resistance of U111 is increased. Since there is no DG211, it will be replaced later.

Finally, I attached a picture of the repaired one. The calibration voltage is 10.00170, but remember it is for AD588, so it can only be compared. It is almost stable after 5 minutes of power-on, and the reference in the meter does not seem to have aged seriously.