This is all part of learning the trade, mate. Fault finding is a logical process you need to go through. Just follow the whole circuit, consider what is attached at each point, and apply appropriate tests. And don't put 500v across electronic components! Some faults you find easily, others can and do take hours. Just do your best and learn from the experience.
Another important aspect is to keep the customer reassured. If you are going to spend several hours tracing a fault, keep them informed. Blind them with technical detail if necessary. Just don't let them know you are cacking yourself because you don't know what the problem is. Because you will identify it eventually.
You will find that some customers are ungrateful, but others will be impressed with your professionalism, dedication and attention to detail. You may get paid or you may have to write off a day's earnings, but either way you will gain considerable experience and next time it will be easier. You may well also gain lucrative contracts from the customer and their friends. A good reputation is a very valuable thing to have, but it needs some investment from yourself.
Come on mate, you CAN do this.
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