The 7603 is most definitely worth $100 if it's in good working order and has a full bay of plug-ins. Without the plug-ins, it's useless and they are commonly sold without them.
There are several ways to try to find the defective component. The simplest way would be to apply voltage to the board (between pad 18 and ground) and look for component that is heating up slightly. Sometimes nothing heats up enough to feel the difference in temperature so freezing spray must be used. The component that defrosts the quickest is likely the one that's defective. If that doesn't work, you can measure the voltage along lengths of traces to see where current is flowing.
If you use something larger than AA batteries, you need to insert a 1 amp fuse in series with the power source to prevent damaging the board.
There are several ways to try to find the defective component. The simplest way would be to apply voltage to the board (between pad 18 and ground) and look for component that is heating up slightly. Sometimes nothing heats up enough to feel the difference in temperature so freezing spray must be used. The component that defrosts the quickest is likely the one that's defective. If that doesn't work, you can measure the voltage along lengths of traces to see where current is flowing.
If you use something larger than AA batteries, you need to insert a 1 amp fuse in series with the power source to prevent damaging the board.
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