logical fault finding – ten tips for when things go wrong

When everything works there are no worries, but when
things go wrong where do you start? When you are under pressure and the screens
are black, and everyone is looking at you, it’s tough to think straight and the
danger is you just grab stuff and start unplugging, and make things work. Here
are some possible tips.

There are some basic rules about fault finding that
will make it more likely that you will find the fault quickly and be able to
work around it in a hurry.

1. Think
before acting. The temptation is to dive in and do something, but pausing for a
moment to think may well allow you to dive in and do the RIGHT
thing.

2. Question your assumptions –
there’s no point in saying “Well, I KNOW that DVD player is OK” when you have
selected the DVD input and are getting a blank screen. Test every assumption by
experiment.

Try connecting outputs
directly to monitors to see if you get a picture, try swapping inputs – does the
fault follow the swap or not?

3. What
did you just do? Usually the problem will be that you did something that caused
an unexpected effect. Retrace your steps and undo whatever you just
did.

4. Actual equipment failure is
quite rare – operator error or misconnection is much more
common.

5. This is when you are going
to REALLY wish you had labelled all your main cables, especially the long
ones.

6. If you have actually lost live
video during a performance, and need to get something up on the screen quick,
then consider removing things in the chain till you get the most direct
connection. Ultimately this may mean connecting your VCR output directly to the
projector input, at least until you get a chance to do something about
it.

7. OK, now down to the logical bit
– You start at one end of the chain and work back along checking every
connection, every input and output. Once you STOP getting a signal (or START
getting one) you know you have found the problem item or cable, and can swap it
out or bypass it.

You can either start at the
end (projector) and work back, or start at the input (camera) and work
forward.

What is especially useful for this
is a battery powered LCD monitor that you can plug in as you go down the chain,
to check where the signal is getting
to.

8. Remember to mark suspect cables
or connectors IMMEDIATELY otherwise you will waste time retrying stuff you have
already tried.

9. Swapping out gear or
cables is the quickest way to bypass any fault. Always make sure that you have
spare cables, or have worked out some alternate way of connecting your critical
path items. Design your system, then think “What would I do if this gear/cable
failed?”

10. Try to only change one
thing at a time then test – if you change too many things you won’t know what
fixed it, or you might fix it, but then unfix it without realising.

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