User: "Loved the Printrbot Play but was having trouble getting first layer to stick so I go heated bed upgrade. Assembled everything to the letter of the guide, only variation is instead of using the sold ATX supply, I am running the power thru my current pc builds 760w ATX (I did the math and my computer is only using about 450w and I only use the Printrbot now and then so I'm not maxing out both) as a cost saving measure.
While the bed heats up it does so not on command, and can't be controlled by pronterface but hovers at 86-90 degrees.
I originally was having problems keeping the bed heating up, but I figured out it was a problem with the power cables (black and red) not staying in the green motherboard connector. Now that I fixed it am I getting the constant uncontrolled heating of it."
First, take a quick look at your Printrboard. If the fets that power the heat plate and hot end are touching, gently adjust them away from each other.
Often times, when I have heard of a heat plate 'taking off' on a user, it has been due to some type of metal-to-metal contact. Check that your the thermistor wiring is not making any contact to traces on the heat plate or the print bed. You can better ensure this by wrapping some Kapton tape around the thermistor itself. See a pic below.