4. Its blindingly obvious

A re-statement of point 1 really, but keep telling yourself it is probably obvious. Switch in the wrong position,  loose connector etc.

 

5. Its dirty contacts

It is amazing how often multi-card systems stop working and then start again if you take all the cards out, clean the connectors and then replace them! This is particularly true of gold-edge connectors, which are thankfully becoming rarer.

 

6. Beware the dreaded solder crack

If the connectors aren’t dirty they can come loose. A good plan is to gently waggle everything (on the equipment!) with power on and see if it bursts into life, squeaks or flashes or does more what it should. Heavy components, mechanically strained components and components subject to vibration and heat (transformers, relays and connectors mainly) are prone to solder failure. A jeweller’s X10 loupe is a good thing to carry. The cracks are often hardly visible to the naked eye.

 

7. Talk to as many people as possible about the fault

Talking to machine operators is usually very productive. They may not know the first thing about electronics, but they do know how the machine normally works, and what has changed. They also may admit to things that give a good pointer as to the cause. Since these events may not always be defined in the operating instructions it is most productive to adopt a friendly non-accusational manner. If people think (often know) that they have done something to break a machine they will develop amnesia. The story you got from the supervisor who rang you up is usually pretty wild - and he usually has an interest in faults being a result of outside influences. Well, sometimes they are!