Having presented a series of case studies, we're taking a small turn in this issue's Our Home Studio section. Today we'll tackle MIDI technology, looking at the possible problems that can keep our gear from working properly.
Whether we're just starting out or have been working with MIDI systems for a while, we're likely to run into problems that make our machines not work the way we want, behave unpredictably or, most commonly, leave the MIDI instrument doing absolutely nothing. The most frequent causes will be:
1. Problems with our cables, or badly made connections.
2. Configuration errors on the instruments or modules.
3. Faults in the machine itself.

With a logical, planned search, it won't be hard to track down the problem we've run into. If, after the analysis, we detect physical faults in the "instrument," it isn't advisable to try to open or repair it, since these tend to be extremely complex devices and, besides, finding the right replacement parts outside the proper service center is a hard task. We would therefore most likely cause more damage to the unit, and it's undoubtedly best to have a professional do the repair.

Cabling
Cables are in fact the most sensitive and problematic part of our system. Although it seems logical to assume that once they're installed and give us the result we want they shouldn't fail, we have to remember that most of the time the cabling ends up on the floor, gets tripped over or, quite simply, can deteriorate when we reconfigure the rig by plugging and unplugging connectors.

It's often handy to have a continuity tester (something that will also be useful when checking our audio cabling). A simple setup could be based on a lamp and a battery. That way we can test the cable in question, checking whether current flows between one pin and another.
MIDI cables have no crossed connections, so pin 2 at one end connects to pin 2 at the other, and the same goes for pins 4 and 5. MIDI devices don't use pins one or three.

Such simplicity means that, with a bit of skill with a soldering iron, we can make our own MIDI cable in exactly the length we need.
Another way to check, which I imagine you've tried on various occasions, is to connect two units we know work perfectly with a new and/or known-good cable. Then we swap the "good" cable for the "problem" one to confirm whether the cable really has faults.

A common anomaly that a bad cable can cause is intermittent transmission. The cable may be half broken and only make contact now and then; this leads to erratic behavior, since sometimes the systems work perfectly and other times nothing responds as expected. Many faults, such as a stuck note or even a lost one, can be due to these same cable errors.
Finding the "culprit" (the faulty cable) shouldn't be a complicated task. Imagine that in a star configuration, if one unit fails, the most likely culprit is a damaged cable connecting that device to the THRU box. The unit itself could be the one that's broken, but we would surely see clear signs of malfunction, such as the startup screen not appearing or some similar problem that strongly suggests a fault.

If nothing at all works in our star setup, we should assume the faulty one is the cable running from the controller to the THRU box.

In short, depending on how our cabling is organized, we need to work out which cable may be driving us crazy. For example, we can deduce that if our setup is daisy-chained, a fault in an early cable can disrupt the signals of every unit after it.
As a tip, I recommend (especially if our system has as many cables as a braided head of hair) mentally summarizing our studio setup so we have the possible faults clear before acting. If we don't keep a logical order, we'll end up going mad without knowing what we've checked and what we haven't.
Finally, a warning: depending on the quality of our cable (some cables let you open the connectors, generally metal ones, and therefore repair them, unlike cables with soldered plastic connectors that offer no possibility of repair), we should open it to test and/or solder it, or throw it in the trash and replace it outright.

Checking Signals
Once we're sure all the cables work properly, and as long as the faults persist, we should start to think there may be configuration problems. Given the complexity of ever more sophisticated electronic instruments, it's easy, even for experts, to make a configuration mistake at first.

In any case, it's always a good idea to check every possibility before sending the device to the service center. Only if we see clear signs of a fault, such as random characters on the startup screen, power problems or clear deficiencies in the audio signals (having made sure the cable works correctly), can we conclude that the instrument, module and so on may be malfunctioning.
There are various signal analyzers (even software ones) that can help with the different verification tasks. The simplest analysis units consist of several LEDs that indicate the types of messages and the channels used.
On the other hand, it is sometimes easier to connect our "problem" unit to another unit we know works perfectly, preferably a simple one, such as a keyboard we know inside and out. That way we can try setting up the most common options and find out whether there are really problems.

Still Having Problems?
If after following all these steps we still have problems...
Perhaps the best thing is to take a breath, keep our patience and make ourselves some herbal tea while we carefully go over the "suspect's" manual.
Sometimes the most insignificant detail can be the cause of hours of troubleshooting. For example:

a) Some devices include a MIDI disable switch, and we may have pressed it inadvertently.
b) We're operating on the wrong channel.
c) We've gone over the connection a hundred times and accept one as good that we finally find is wrong (banging your head against the wall is not recommended in this case).
d) Certain MIDI instruments have a battery that, even though it's designed to last a very long time, may have run down, thereby upsetting the operation of the unit.
e) We forgot to check whether the MIDI filters are on or off. This is a reason that will surely drive us crazy.
f) We haven't read the manual and expect the unit to perform a function it really can't do.

Conclusions
To wrap up this chapter of Our Home Studio, I would sum up by advising a careful reading of the unit's instruction manual (which, by the way, should be demanded in your own language), slow and thorough testing of all our cables (replacing any that have cuts, breaks or homemade hack jobs), a review and check of the settings (starting with the silliest or most basic things and moving on to the more complex ones) and, finally, if the faulty unit still doesn't work "for love or money," going to the service center, the warranty or the store where we bought it.
Although I'm absolutely sure that every one of you has had or will have a desperate morning hunting for the wretched parameter that makes your inspiration vanish, I leave this summary of experiences here in case it gives you a clue to the fault you can't pin down.