It was working till now!


Although a very difficult area, the one of getting things to work again, it must be approached with the primary thought of "it was designed to work (or, it was working), all I've got to do is get it to work again". If one remembers that the design was proven then a lot of energy and time is saved by not embarking on "design improvements", unless they are absolutely called for. Remember, every bit added is another potential fault.

Before one can even attempt to start with a repair, the basic understanding of what to look for is of paramount importance. Situations have been mentioned where untrained technicians were shown to clutch at straws in the desperate hope that what they heard, saw, or were taught was the actual fault because they started out not knowing even the basics of radio telemetry, never mind the more complex aspects of radio or process control signalling. If the repairing of a radio telemetry system is a first then this section should be read in full before even leaving for site.

Separating Symptom from Cause:

Imagine the following fault report, "The system is working intermittently". Upon investigation it is determined that intermittent comms is to blame. Wrong thinking will classify this as the cause rather than still a symptom. If the reason for the intermittent comms is finally traced to a bad connector then the cause of the system failures is the connector, all other levels of the fault diagnosis identifying symptoms.

The levels described above are, however, important in determining a route towards and then locating the problem. What is being demonstrated is the ease with which the wrong diagnosis can be made. In the above example, once having ascertained that intermittent comms is the issue, the untrained individual could be led to believe that higher gain antennas are called for in the hope of 1) curing the fault and 2) impressing the customer. C'mon, a badly fitted connector would show up that he didn't do his job properly in the first!

Assembling the Correct Tools:

As stated in the opening lines of this book there are many disciplines and avenues of engineering involved with radio telemetry, and if that was not bad enough each discipline has its own set of tools and test instruments.

For openers, almost all telemetry modules are now programmed via a RS232 (COM) port. One of the most valuable tools one can have is a decent length lead (PC to module) with which to program the units. Many systems are mounted in almost inaccessible places and the danger of dropping a valuable laptop because of a dinky 1m programming lead does not bare thinking about. Whenever called to got to a site the 10m+ lead is packed. Seldom has this proved too short!

Nearly all of the signals surrounding a radio telemetry module can be tested and analyzed with a multimeter. The only one that proves to be an issue is SWR, the accepted measurement of the condition of an antenna system (although we do show later how a multimeter can be used to do this too). A "dummy load" (a device for presenting a pure 50W load to a radio) is useful especially when needing to take a module off-air but need to keep it powered-up (it can damage a radio when the antenna is unplugged and the radio transmits).

What has personally proved almost invaluable is an oscilloscope, preferably portable (battery driven even better). If this is not available a very simple yet effective emergency alternative is a telephone earpiece.

As process control signals (4-20mA) are used, current generators and limiters (commonly referred to as "calibrators") as well as a few calibrated "loop powered readouts" will prove to be extremely handy. Note the word "few" as there is often more than one loop that needs testing simultaneously.

Simple tools used by electricians, such as earth leakage testers and "power point fault testers" help to isolate mains problems, even if wiring in the supply does not fall within your jurisdiction.

During the following chapter there will be circuits for simple "non-commercially available" tools that can be constructed and kept in the toolbox - have at least a few components in the 'box for those faults requiring a bit of thinking.

Identifying the Cause:

In all the years of engineering and fault finding the principle of "start at the beginning and work your way through to the end" has never failed. The "divide in two" method of fault finding has one inherent flaw - potential faults get missed.

This method of fault finding appears to have 'developed' over recent years as this was definitely not the methods taught 10 years ago. An argument against this method is evident when faultfinding a 'private wire' or 'dedicated channel' between two locations of some distance.

Using the typical tone generator at the one side the ludicrous logic will be quickly seen of driving half the distance (which can be many kilometres) only to find that the fault is towards the side you have just left. In fact, after having divided the fault distance in two, and again, and again, the fault is finally discovered in the basement if the building you left about 4 hours ago!!

To reiterate:- start at the beginning and work you way towards the finish line. In some circumstances it is also acceptable, and indeed prudent, to begin at the end and work one's way to the start. This is especially so if the equipment at the end of the line can assist with the faultfinding (exceptionally true of radio telemetry systems). This saves having to re-connect it at the conclusion of the exercise.

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