Fried RAMPS?

Hello!

My heated bed is powering on as soon as i plug in RAMPS, even when the bed is disconnected D8 wont turn off. Ive replaced the MOSFET for the bed and Diode2 diode so far, but i am out of ideas.
now even though i have replaced Diode2, it fails the diode test on my multimeter, I get a small voltage on either anode or cathode. (short somewhere!?). any help is appreciated!

Edited 1 time(s). Last edit at 03/28/2014 07:19PM by dbuck.

Have you loaded firmware on to the Arduino Mega? It may be that existing code on the Mega is switching that pin on.

You also may have a damaged Mega, or there is a short on your RAMPS board in that part of the circuit for controlling the MOSFET.

D8 is powering even if i have the mega disconnected, so not firmware. so probably a short somwhere 'eh?

perhaps new RAMPS is the way to go...

When you say "D8 is powering even if i have the mega disconnected", I assume you mean the bed is still being heated- you can feel it?. Don't assume that because there is 12v on D8+ that it is supplying power, 12v is there permanent even with nothing switched on.

Anyway...
The only thing that can cause that is a Mosfet shorted internally or the middle Mosfet pin track circuit shorting to the ground plane (assuming the Mega is not fitted)
Is the Heat bed LED on? If so (Mega removed), then the circuit is definitely being decked. Solder splash?
Also, I'm not certain but I think the diodes play no part in the heat bed circuit.

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Waitaki 3D Printer

Waitaki, your assumption is correct, I can feel the bed heating up and the MOSFET LED is always on(with or without the mega). Changing out the diode(d2) was a bit of a long shot, I saw it mentioned on another forum somewhere, and figured it'd be worth a shot seeing how i had the spare parts.
I've already ordered a RAMPS, but this one is going to serve as an "educational project" for now.
thanks for the advice

Edited 1 time(s). Last edit at 03/29/2014 10:53AM by dbuck.

It's always a good idea to have a spare Ramps!
While waiting, you could remove the mosfet and check there is no short between the centre mosfet PCB hole and ground. If it's good then maybe its worth wacking a new Fet in

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Waitaki 3D Printer