Sega Master System / Mark III / Game Gear
You've finished building the SMSReader; before we insert the chips and hook it up to the PC, however, we need to go back to the beginning and check all of the wire links. Use your multimeter (or continuity tester) to ensure that the links are correct. Also, check that the components are in the correct positions, and that the tracks have been cut properly where specified. (Again, use your multimeter to ensure the circuit connection has been broken).
It's especially important that you check the electrolytic and tantalum capacitors are correctly oriented! If you're then injured by an exploding capacitor, you can't claim that I didn't warn you :-)
Going over all the circuit connections is a chore, I'll admit, but if you do it now it can save obscure troubleshooting problems later.
The final safety check is to ensure that the resistance between negative and positive pins of the electrolytic capacitor is greater than zero (should be a few hundred or thousand ohms at least), and that the same is true for the tantalum capacitor. A short-circuit here could potentially wreck your AC adaptor. A short between the 5 volt line and Ground is also a safety hazard - the 7805 regulator will get incredibly hot if short-circuited (I still have the scar to prove it!), so you must make sure there's no short here before applying power.
Once the above checks have been carried out, we can power up the SMSReader for the first time in order to check board voltages. Touch a radiator pipe or similar to ground yourself (static precautions). Then, remove the 74HC00 IC from its protective packaging and carefully insert it into its 14-pin socket, ensuring that the notch on the chip lines up with the notch on the socket. Don't insert the 4040s or connect the parallel cable yet.
Now, take your SMS or Mega Drive AC adaptor and plug it into the power connector. Connect the adaptor itself to a mains socket and (with fingers crossed!) switch it on.
Assuming nothing untoward happened, we can now begin to measure voltages on the board. Use the metal tag on the 7805 regulator as a Ground point for your multimeter, and ensure that the voltages are as indicated in this picture:
If the voltages check out OK, it's time to connect up to the PC. Remove the AC adaptor cable, then insert the two 4040 ICs, following the same procedure you did for the 74HC00. Connect the parallel cable between the SMSReader and the PC's printer port (note: the PC should really be switched off when you do this), reconnect the AC adaptor, then move on to the next section.