Powering sewing machine card
Since you asked for general situation advice, I will offer some general recommendations.

If your first impulse on troubleshooting an unknown board is &quot;let's just apply power and see what happens&quot;, I think you should reconsider your approach. You don't know what the appropriate input actually is, what the expected outputs are, or if there is any sequencing or fail-safe circuitry needed to make the board do anything. I would start by first asking more questions from the person who supplied the board:


Which product did this board come from? (Manufacturer, model number, etc.)
Have them provide a picture of any labels on the device for clues about input requirements (voltage, frequency, etc.)
Ask them to describe the fault in more detail.


Contact the manufacturer and ask if they have any schematics or repair resources available. Alternately, scour the Internet and see if you can find this from any third-party sources.

Continue your reverse-engineering activities to understand the basic flow and sequencing of the circuit. Draw your own block diagram. Identify test points (which you have already done) and voltage rails (based on capacitor ratings and ICs connected to those rails)

Conduct a visual inspection of the board for components which look damaged; probe and measure before applying power.

Try powering DC rails by connecting a current-limited DC source to the rail. Slowly walk the voltage up and watch the current meter, gently relax the current limit and bring the rail up to the expected voltage if it is possible to do so.

Once you are confident that the DC rails are OK, identify which parts of the board are primary (mains-connected) so that you will not incidentally contact or probe any dangerous areas. I would use a small variac with a conservative fuse rating (equal or lower than the fuse in the product) and walk the AC voltage up slowly to see if any on-board power supplies are operating correctly. (I didn't see any transformers on this board so it's possible the entire assembly is mains-connected.)


It is quite likely that without the complete sewing machine, it will be impossible to diagnose everything fully so there may be some back-and-forth with this particular customer if they are unable or unwilling to send the machine.