Downgrading My Best DMM

... or should the title rather be 'upgrading'? Feel free to decide for yourself, but please read before you do :-)

When tinkering with electronics for many years (i.e., decades), you for sure collect more than one multimeter, more or less automatically. For instance, for measuring current and voltage simultaneously you need two of them.

My newest and best one is the one pictured above, the InLine VA40R. It is moderately priced, and I purchased it about four years ago, mainly for two of its best features - it has RMS measurement (i.e., it also measures non-sinusoidal voltages and currents accurately), and because it connects to a PC via a dedicated optical link for automated measurements.

It comes with some drawbacks, too: The German-language user's guide is very difficult to read because of its size and layout - in fact, it is copied to two DIN A4 sized scraps of paper. Reading them is impossible even with my reading glasses, I need a magnifier! In addition, the two main features are wrongly described. What's more, the meter loudly (and very annoyingly) beeps whenever operating the range rotary switch (but I'll come back to that in the end).

What I consider most disturbing is the fact that the test probe sockets are a tiny bit too small in diameter for accepting my preferred banana plugs, so reliably connecting my time-honoured test cables is impossible. Most of these cables are equipped with the lovely 4 mm 'MultiContact' (MC) banana plugs that I have been using since shortly after the dinosaurs became extinct. They are stackable, gold-plated, and Swiss made. Even the modern, fully insulated MC plugs don't fit because the sockets are somewhat short.

The 'MultiContact' company, btw, was founded close to Lake Zürich in 1892. Today, they call themselves 'Stäubli', after one of the two founders. Their modern, fully insulated connectors - that look a bit different from the ones I prefer (and carefully use) - are labeled 'MC' nevertheless.

The first picture shows two of my preferred banana plugs mentioned above. They feature several spring-loaded, gold-plated fins that provide multiple contact points (thus the company's earlier name). The next two pictures show the more modern, fully insulated, foolproof MC banana plugs - but even these cannot be inserted well into the DMM's sockets.

In the fourth picture, you see the front part of the PCB (printed circuit board) of this DMM, and you can also see three of the four old sockets that are simply soldered into the PCB. One of them has already been unsoldered, and you see a hole with its surrounding, rather narrow 'donut' to which it was soldered. The diameter of the holes in the PCB is 3 mm.

Now, some time out was required for thinking - what should I do to increase the internal diameter of the sockets so that my plugs may be used? Simply drilling them a bit larger seems like a not-so-bright idea, because the coating (nickel, I assume) will be drilled away, and the sockets will corrode with time. But what materials do I have available to replace them? As you perhaps know from my earlier Instructables, I prefer using materials from my junkbox...

In there I found some press-in, 4 mm sockets from the same manufacturer. They are machined from brass and gold-plated, and I thought about using them as replacements without their standard insulating sleeve. Brass can easily be tooled, so I thought about shortening them to the length of the original socket and drilling a thread for M3 (3 mm dia.) machine screws that also fit into the PCB's holes.

To check whether this might work, I stole a drawing from the current Stäubli PDF brochure, modified it in my vector graphics application ('Original' in the fifth picture above) and changed it, then, according to my idea ('Modified'). The length of the socket's body is reduced to 12 mm to match the original's size (which is, I found out after having finished, is a bit short, unfortunately).

What I can tell you here is that shortening, drilling and cutting a thread is much easier on paper than in reality :-)

The picture above shows the original socket at the far left, and then the new one (Stäubli order no: B-EB4-AU) in its original condition - it's too long, and its connecting tab doesn't match, obviously.

In order to match the new sockets to the DMM according to the drawing, I proceeded as follows:

1. I shortened the sockets; this went nicely using a stationary belt sander with a perpendicular work table. I clamped the socket's connecting tab with a solid pair of pliers and ground it down to the required length (take care when touching the socket afterwards - friction generates heat, and a lot of it!).
2. I used two aluminium brackets from the junk box as a jaw protection in a machine vice. I placed the sockets' connecting tabs into it in such a way that they were in a vertical position. Using a drill press I drilled a 2.5 mm dia. hole in each socket's base, deep enough to also perforate a part of the connecting tabs. The 3rd socket from the left in the picture above is shortened and drilled (it is, in fact, a discard since the hole was not exactly vertical). Thanks to the aluminium brackets I avoided damaging the vice's jaws. The drill bit will automatically find the center of the hole being at the lowest point within in the socket, refer to picture no. 5 in the previous step.
3. I used a 3 mm ('M3') machine screw tap to cut a thread into the 2.5 mm pre-drilled hole. After that I removed the connecting tab by simply breaking it off, and cleaned the end of the socket with a file; the 4th socket from the left in the picture above shows us its rear side, and the thread is clearly visible. This is, btw, also a reject from the first try that became much too short.
4. I had some brass M3 screws (3 mm dia.) in my junk box. They were my preferred choice since brass can easily be soldered. After shortening (beheading :-)) them I inserted them into the sockets no more as necessary and soldered them in place, for improved mechanical stability and reliable electrical contact - see picture above, at the far right.

In case the holes in your PCB should be smaller, there is also a press-in socket version available with a cylindrical, 1.9 mm dia. connecting pin, Stäubli order no. B-EB4-IR_AU. This makes the installation much easier if the holes match.

After soldering the new sockets into the PCB (see picture above) while taking care that they are placed vertically, the DMM could be reassembled.

Before that, however, I stuck a small piece of adhesive tape over the sound hole of the piezo buzzer to soften its annoying volume.

... and after reassembly, when plugging in my banana plugs, I realized that the sockets are a bit short for a good footing of the plugs, as indicated in the drawing in step 1... but alas, there is no such thing as foolproof :-)

I thought about an improvement. There are two possibilities: Either remake somewhat longer sockets, or better drill the openings in the DMM's plastic front a little larger. Both methods allow the plug to dive-in a lettle deeper. I decided to use the 2nd solution because it's easier and cheaper. The last picture above shows the new recess at one of the sockets. A small part of the socket's label was lost with this method, unfortunately, but I'll be able to live with that.