The stuff you show in the up close shot is flux. It will not cause any problems. If something was shorting there (the power plug) it would cause much bigger problems.
Checking the resistor should be easy. Write down the color bands and figure out what they should measure from this site.
Resistor_Codes
Measure the diode by measuring the resistance across them. Then swap your probe tips and measure again. You should see an open one way and short (or close) the other. You don't need power to test this.
Last in case of a cold solder joint, take a solder iron and melt each solder connection and let re-cool.
Last would be a connection issue. That would be a mechanical engineers job.
