Usually sparks are caused by shorting two terminals together with the probe itself. In other words, it is not a problem with the build.
Were your voltages correct on the power supply board testing earlier in the manual?
If so, you still need to do your final voltage checks. Proceed with caution, and never take your eyes off what you are doing. If you cannot safely use a probe, you can attach clip lead, power up, take one measurement, power down, wait, then move the lead, power up, measure, power down, and so on.