No, it's not the power supply; it's fine.
When electricity is flowing, it will cause heat to be created wherever the resistance to the flow is relatively high. In some cases, that's what you want, e.g. the heated bed and the heater for the hot end (extruder). In other cases, you want the resistance to be as close to zero as you can get it, so that little to no heat is generated. Since the excess heat is right at the connector where the wires plug into the board, that is where your problem is. For some reason, there is excess resistance there. The more current, the more heat - so when you turn on the heated bed you are pulling much more current from the power supply and therefore making the small but significant excess resistance at the connector generate more heat.
There are several possible causes:
(1) The connector is not fully seated, so the metal parts are not making good contact.
- Try unplugging and re-seating the connector, making sure it's pushed all the way down.
(2) There is corrosion or contamination preventing good clean metal-to-metal contact inside the connector.
- This is unlikely unless the connectors have been exposed to excessively moist conditions or there's lots of dust or dirt around.
(3) One or more of the wires going to the connector is not properly crimped to the metal part that's supposed to make contact with the pin on the board.
- You can test for this by pulling fairly firmly on individually on each wire. If any wire pulls out of the connector, then you know it was not crimped properly. In that case, there are different avenues for repair, but I won't go into all of them because that's also fairly unlikely.
(4) One or more of the mating parts of one of the connector halves is bent and not engaging properly with its mate.
- This could be either on the board or in the power supply connector.
You are wise to be concerned. There's a whole long thread in the forum about "Printrbot on fire" where a heated bed connector was overheating.
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick