Repairing DT71 Measurement Tweezers

This instructable describes how I repaired my DT71 measurement tweezers.

These tweezers provide a wide range of measurements; voltage, resistance, capacitance, inductance, frequency and can generate signals.

I use them quite a lot but there was a period when I didn't use them for a couple of months. When I came to use them after this they wouldn't turn on. No problem I thought just needs recharging. After 24 hours they still refused to turn on. Some googling revealed that there is a known problem with the charging. If the lithium batteries in the legs drop below about 3.5V then they will refuse to charge no matter how long you try. There are two batteries one in each leg and they are not easily replaceable.

There is a destructive tear down which shows the construction.

https://youtu.be/DTdycgOalG4

Fine scalpel or craft knife.

Fine point soldering iron

33 Ohm resistor

5V power source (e.g. USB)

The first image shows the battery exposed during the destructive teardown.

I guessed one could get access to the battery terminals without doing too much damage.

I used a scalpel / fine craft knife to cut out a section of the inner plastic cover on the bottom of a leg.

I measured from the video and reckoned that a section 7.5mm up from the bottom of the inner plastic should be sufficient. I removed the metal prongs by removing the screws. Then I sliced down each side between each side of the plastic cover and the main side. That was relatively straightforward.

I then sliced across 7.5mm up. This needs to be done carefully. It is best done by making repeated strokes across until there is a pretty deep groove across almost right through. A spudger or screwdriver can then be inserted between the metal plate and the bottom of the leg. to lever up the bottom section of plastic. Some thin nose pliers can be used to wiggle the section and finally break the plastic free. It is good if one can get it out as a fairly clean rectangular block as this helps complete the repair later. The removed block has a couple of plugs which are a push fit into slots of the leg.

With the block removed one should see the end of the battery and a red and black wire going to 2 soldered connections. I soldered two wires onto these with a fine point soldering iron. I actually did the same operation on both legs to check how the batteries are wired and found they are just wired in parallel so opening up 1 leg is sufficient. The two wires showed 0V but I guessed this was because the battery protection circuit on the end of the cell kicked in.

I then attached a simple bootstrap charging circuit consisting of a 5V supply and series resistor of 33 Ohms making sure the polarity was correct. I put a voltmeter across wires going to the battery terminals. The voltage rapidly ramped up to 2.5V and then increased more slowly as the batteries started to charge. It is very important that this is closely monitored to avoid overcharging the cells which is bad news and potentially dangerous for lithium cells. A safer alternative would be to use a proper 4.2V lithium cell charger together with the 33 Ohm resistor to ensure the charge rate is kept low enough for these small cells.

I stopped the charging at 4.0V and then reconnected the normal dt71 charger via the plug at the top of the legs. This continued to charge the cells up to 4.2V.

I did a quick test to see if the meter head would now work and it did.

The repair was completed by unsoldering the attached battery wires. The main horizontal cut edge of the removed plastic block was cleaned up a bit with the scalpel and I then found it could be inserted back into the two securing lug / slots and pushed down with pliers. This gave a secure fit (second picture). One could apply a bit of filler and sand down if required but I have left mine just pushed in in case I ever need to remove then again.

One major lesson for me is to leave the tweezers on charge when not in use!