Upgrade the TL-21 Test Lead (DE-5000 LCR Meter)

I have 6 multimeters. A couple of them very good, others are simply good or just for simple checks like "is this battery dead?". Some of them can measure capacitors, but none of them are able to measure inductors or measure important parameters of these components, such as ESR (Equivalent Series Resistance) or Q factor, among others.

For this reason, after spending some time reading reviews and searching, I decided and purchased a DE-5000 LCR meter with some accessories. This is what I consider sufficient for my needs, and it seems to do a pretty good job for a fair amount of money.

One thing I do not like (I'm not the only one) is the length of the TL-21 test lead wires, something like 4 centimeters. I decided to improve it with a pair of long Kelvin alligator clip cables and in this post I show how to make such modification.

DE-5000 LCR Meter

In the following image you can see the LCR meter and the accessories I purchased. The TL-21 is the test lead that I will show how to modify, the TL-23 is a guard line lead (not shown in the image), and the TL-22 is a twezzer for measuring SMD components. It all cost me about U$S 126 in March 2021, not bad really for such a capable instrument and all the accessories.

(From Left to Right) TL-22 SMD Tweezers, TL-21 Test Lead and DE-5000 LCR Meter

It is not my intention to review the DE-5000 LCR meter, (at least not at this time), so let's dive right into the reason of this post.

Tools and supplies required

In the following image you can see the two elements that we are going to use to perform this modification and the DE-5000 LCR meter. These elements are: the TL-21 test lead and a pair of kelvin alligator clips with shielded cables.

(From Left to Right) Kelvin Connection Alligator Clips, TL-21 Test Lead and DE-5000 LCR Meter

I am not going to post where or from whom I bought the Kelvin cables since the quality of this type of items bought from online stores sometimes varies over time. The ones I bought are not terribly bad, but they are not excellent either, as an example, the center conductor insulation doesn't hold heat very well so you have to solder it fast so it doesn't melt.

You will also need the following:

• Soldering iron and tin solder (1mm or less).
• Desoldering braid (to completelly remove the solder).
• Wire cutter.
• Philips screwdriver.
• One small cable tie.
• Some cotton swabs.
• Alcohol to clean the solder.

Modification Procedure

The first thing to do is to open the TL-21 by removing the two philips screws on the back. In the following image you can see what we find inside, a pair of printed circuit boards soldered together and in one of these printed circuit boards are soldered the wires coming from the alligator clips.

Inside of TL-21 Test Lead - Two PCB and Kelvin Connection Cables

The Kelvin connection is a 2-wire connection technique in which two wires are used to connect each terminal of the measuring device to the component to be measured. This results in a low impedance connection at each terminal, which is especially important in certain circumstances, such as measuring very small resistances $\class{inlineFormula}{\rm{(R \leqslant 0.5\Omega)}}$.

Each alligator clip is connected by two wires to the board. There is an additional wire corresponding to the cable shield, which is connected to the guard terminal of the TL-21. We see that the black alligator clip is connected with two wires (red and white) to the PCB terminals marked "-1" and "-2" ("Negative 1" and "Negative 2").

Now we have to prepare the Kelvin aligator cables to solder them in the PCB. In the following images you can see the process of preparing the Kelvin alligator cables.

Kelvin Coaxial Cable BNC Connector Disassembly

Once the BNC connectors have been removed, strip off approximately 2 centimeters of the outer plastic shield and foil shield. Strip about 5 millimeters of the coaxial cable and join the coaxial cable shields together.

Kelvin Coaxial Cable Prepared for Tinning

At this point it is a good idea to completely remove the solder on the PCB and tin both coaxial cables as well as the joined shields. It is necessary to keep the amount of solder to a minimum on each wire so that it is easy to pass them through the holes in the PCB.

Kelvin Coaxial Cable Prepared for Soldering

Now solder the three wires and cut off the excess. The solder joints should be shiny, otherwise they could be cold solder joints that are prone to cracking, and meniscus-shaped, avoiding excess tin during soldering.

Kelvin Coaxial Cable Soldered and Cut

Repeat the same with the other alligator clip and clean all solder joints with alcohol to remove any residue. Install a cable tie in the same way as the original one, but do not tighten it yet.

Both Kelvin Coaxial Cable Soldered and Ready for Reassembly

Install the TL-21 PCB inside the plastic case and tighten the cable tie so the two pairs of coaxial cables rest on top of each other, this way it is not necessary to cut the plastic housing. Cut the cable tie flush with the head.

TL-21 Ready to Close the Case

In the following picture you can see the modified TL-21 test cable installed in the DE-5000 LCR meter and ready for testing.

Modified TL-21 Installed in the DE-5000 and Ready for Testing

Checking the Modified TL-21 Test Lead

This is by no means an exhaustive test, but rather the verification of a couple of measures to check if it is working properly. The results of these tests are not valid from a metrological point of view since no calibration was performed, and the one that was done was done a long time ago, in other conditions of temperature and humidity.

The first test is to check the resistance of the wires by connecting both alligator clips together. In the following image you can see that the measured resistance is $\class{inlineFormula}{\rm{(0.02 \: \Omega)}}$ which seems to be normal.

Modified TL-21 Installed in the DE-5000 - Checking the Kelvin Connection Resistance

Next, we compare the measurement of a capacitor $\class{inlineFormula}{\rm{(100 \: nF)}}$, using the direct connection on the front of the instrument and the modified TL-21. In the following pictures you can see the comparison. The results do not differ too much, which seems acceptable.

Modified TL-21 Installed in the DE-5000 - Comparison of capacitor measurements