A couple of weeks ago, I was using an ANNE-50+ 50-ohm SMA termination from Mini-Circuits, but noticed that it did not seem to work right. When I measured it using an ohm-meter, the resistance was around 100 ohms instead of the expected 50 ohms. I put the termination aside and I just got around to taking a better look at it since I was curious about how it was constructed inside. Below are two photos of what the termination looks like.

ANNE-50+ termination

Before I started taking it apart, I remeasured the resistance. This time it was no longer 100 ohms, but rather an open circuit. So something had apparently happened inside it, although I am certain that it was not used while waiting for the autopsy.

Then it was time to start uncovering the secrets of the termination. I began by filing down the back side of it, which consists of a black plastic cap inside a brass tube.

Less than a mm below the surface, some more metal was revealed in the center of the device. This metal turned out to be electrically connected to the outer shell (ground).

Further filing next to the central metal piece allowed the plastic to be removed, revealing a 360 degree low-inductance metal connection between the center stud and the shell.

Going a bit further allowed the central stud to fall out. It turned out to be a (cracked) resistor. It seems to be either a carbon film or metal film resistor on a ceramic core. The fact that it has somehow become cracked is of course the reason the device failed in the first place.

A few final strokes with the file allows the remains of the pressed-in metal piece that connected the resistor to the connector shell to be easily removed. The center pin of the connector with its (teflon?) insulation and the other end of the broken resistor also easily falls out after this. The different pieces are shown in the photos below.

One can see that the diameter of the hole in the shell of the termination is different in the section with the connector pin and the section with the resistor. The first diameter is dictated by the dimensions of the SMA connector and is almost certainly designed to give a 50-ohm impedance with the pin diameter and the dielectric used. The second diameter might be chosen to give the best possible return loss given the dimensions of the film resistor, but it might also be chosen primarily for mechanical convenience.

This termination is specified to have a return loss of better than 21 dB up to 18 GHz. I have heard that some other terminations which are not specified to perform well up to such high frequencies use standard SMD resistors inside, but performance to 18 GHz probably benefits from the coaxial design of this device.

So what might have caused the failure of the termination? I do not know for sure, but since the center pin is largely held in place by the apparently brittle body of the resistor, it seems like a strong force or torque on the center pin is the probable cause. This might have occurred because the termination was mated to a damaged SMA connector or for some other reason.

I recently had a duh moment. I was trying to repair a broken fan motor and I had determined that the immediate problem was that a thermal fuse used to protect the stator windings from overheating was broken. I ordered a few new thermal fuses, soldered one in and carefully insulated it electrically using heat shrink tubing.

When I tried to power up the fan, I was surprised to find that it was as dead as before. Some quick troubleshooting revealed that the new fuse was broken. Since the motor had not even made the tiniest jerk when I plugged it in, I assumed the fuse must strangely enough have been dead on arrival and I removed it from the motor.

This is what the fuse looked like when I had removed it:

Fortunately, I had more fuses, so I proceeded to solder another one into the circuit. Wise from my previous experience, I verified that it was working before the operation and I also measured it after I had soldered and heat shrunk it into the motor circuit. To my surprise it was suddenly broken! And I had not even powered the motor up!

This is when the duh moment occurred.

Thermal fuses blow in an unresettable fashion when they reach a certain temperature, in my case around 120 °C. This is the whole point of the component. And I had failed to consider this when I happily soldered the thing in, just as if it were a resistor or some other normal component. There is obviously a big risk – not to say certainty – that both during soldering and while shrinking the tubes, the fuse gets heated to higher temperatures than it can stand.

So how do we get around this problem? I did not have any thermal fuses with long leads that would insulate them during soldering, but with some care it is in fact possible to solder a thermal fuse. Here are the tricks I used in my third attempt:

1. Do not shorten the leads of the fuse. Leave them as long as they are.
2. Put several alligator clips on the lead between the body of the fuse and the tip where you apply solder. This is to lead away heat so that it does not reach the body (see picture below).
3. Solder for as short a duration of time as possible. Preferably less than a second at a time to prevent heating of larger parts of the lead. But be careful to not get a cold solder joint despite this. It’s a delicate balance.
4. Do not shrink the tubes. Let them just sit there loosely.

Below are some pictures from the process.

Using these tricks, my third attempt was successful and the fan is now back in working condition.

It cost me two thermal fuses, but I think I have got the lesson about what not to do to this (for me at least) somewhat unusual component.