DC power not working, USB does

Dug out an old Uno board to use for a prototype I am working on. Wanting to power it with a DIY'd benchtop PSU however the DC power isn't working, and I am not sure how to trouble shoot it.

Powered by USB all is fine, and thinking back I do recall this being the case. I think at some point I may have plugged both USB and DC power in at the same time.

So, question is, where do I start looking for how to rectify this issue? I am not worried about replacing the dc jack of anything as this board is likely to be hardwired into a powersource long term.

Any advice is welcomed.

Thanks

What voltage are you plugging into the DC jack?

12v

OK... I suspect the regulator is fried. I take it the board works if you put 5 V on the 5V line. The only thing you can check is if you get 5V there when you power from the DC jack.

Do you have + going to the center of the power jack? With the usb disconnected, and 12 on the jack, what voltage do you measure on the Vin and +5 volt header pins with respect to the GND header pin?

Could have zapped the reverse polarity diode too. With 12V applied, do you measure 12v on the anode side of the diode (back of the barrel jack) and ~0.5V to 0.7V less on the cathode side (Vin pin)?

ChilliTronix: OK... I suspect the regulator is fried. I take it the board works if you put 5 V on the 5V line. The only thing you can check is if you get 5V there when you power from the DC jack.

Yes.

5v pin to ground gives 1v with 5v DC jack

LarryD: Do you have + going to the center of the power jack? With the usb disconnected, and 12 on the jack, what voltage do you measure on the Vin and +5 volt header pins with respect to the GND header pin?

yes

11.69 at Vin, 1.7 at 5+ pin.

CrossRoads: Could have zapped the reverse polarity diode too. With 12V applied, do you measure 12v on the anode side of the diode (back of the barrel jack) and ~0.5V to 0.7V less on the cathode side (Vin pin)?

Yes, anode side 12.4v, cathode side 11.7v

Still not quite sure what the damage is.

11.69 at Vin, 1.7 at 5+ pin.

If the board works with USB, looks like you need to replace the voltage regulator.

I have replaced the power regulator with a (rather large but oh well) LM7805A. Board still is not functioning correctly.

Vin = 12v, 3.3v pin = 3.1v, 5v pin = 1.5v

Not sure what to do at this point or what I may have done wrong.

Guys, having similar problems, my Mega2560 works fine when powered up via USB cable and switched over to 12V still runs fine.

However, when powered with 12V directly into the Vin header, it does not boot (i.e does not go beyond setup).

What could be happening?