When faced with a system that is not performing as intended, not being able to see the wood for the trees is frustrating to say the least. This chapter acts as a starting point when things go wrong and a guide to fault finding and repair.
Before we start a warning to those who are looking for a fault to be very careful of the pitfall called 'overeagerness'. It is so easy to be steered, when reading fault finding guides, into believing what you read is the problem rather than accepting that all that is wrong is a little bit of bad design.
This comment was prompted after having heard of a system which consisting of 8 moving cranes, each connecting back to the control panel but each crane required permanent RS232 comms between it and the control panel. The hardwire lines were breaking and they were replaced with radio modems but obviously as soon as two cranes were in service the system 'fell over'. An engineer was brought in to repair this project but found himself believing the fault to be all sorts of things such as intermod, antenna phasing, fresnel zones, and anything else he heard. The moment the next buzz word was mentioned his problem became it. The fact was, it was bad design. Always start with the basics, ruling out the obvious, and then continue up the ladder toward the real problem.
Curing problematic systems is a two-tier process. The first is identifying the symptoms. "Does the system match what was planned" deals with new systems while "It was working till now!" with systems that were, as the title suggests, working fine and then.....
Armed with the symptoms "Isolating the problem" deals with the second leg of the process by showing techniques and methods for identifying and tracking down the cause.
Ending off this chapter is a section "Replanning with what is available" being a guide to taking a badly designed system and making it work as best as is possible.
Before we move onto the serious part of fault-finding, a little bit I just had to put in! I had tears rolling down my face I laughed so much. It's a little story to prove that when looking for faults, always expect the unexpected. It came while supporting someone with a system that had become unreliable. The question was asked if the antenna may be at fault, specifically a bad mismatch. The reply was better than had ever been expected.....
"You also hit on another very salient point - one of which I was unaware could have been salient until I visited the site again:- in the interim we had supplied another antenna of the Collinear type, to the Systems Integrators taking care of the installation. They had found a good vantage point - a high area on a raised steel walkway - superb. They then mounted the antenna upside down 'to reduce the possibility of lightning strike'. Water ingress of course stuffed it quite effectively resulting in a good mismatch and sure will have stressed the output amplifier stages. Customers, eh , who would have them!!!"
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