Heated bed not heating

[jhodges]

I'm having some trouble with my heated bed (Phebe I on a RostockMax kit with RAMBO). Sending the M190 command to heat the bed or using the manual button in Repetier starts the heating sequence, but the bed never reaches temperature. From a cold start, the thermistor on the hot end and heated bed are the same value, so I know the bed is getting a reading. If I heat the hot end, it can reach the set temperature. LED2 on the RAMBO illuminates, indicating that I can get power on the board and measure 12V at the output to the first extruder hotend. However, if I try to heat the bed, the corresponding LED (LED6) for the hot end does not come on.

I understand that the hot end and extruders are on different isolated power supplies. Could it be that this is the issue? I checked all the fuses (F2,F3 and F4) and none are broken. The 3 pairs of input voltage to the board from the PSU each measure 12V as well.

Is there a software setting that perhaps got mangled? I defined MOTHERBOARD = 301 in the Marlin Configuration.h file. Is there something else?

Stumped.
JH

[Polygonhell]

Where did you get your version of Marlin?
There was a released version with a partially incorrect pin mapping for the RAMBO board.

[jhodges]

I downloaded the latest firmware from github:
https://github.com/johnoly99/Marlin-for ... kmax-rambo.

Do you know what the PIN mapping is for the RAMBO? What is the variable? If this is all good, any other possibilities?

[Polygonhell]

I downloaded the latest firmware from github:
https://github.com/johnoly99/Marlin-for ... kmax-rambo.

Do you know what the PIN mapping is for the RAMBO? What is the variable? If this is all good, any other possibilities?

That version as far as I'm aware has always been correct.

[mhackney]

Hmm, I'm having the same issue. I first tried the default Marlin installed on my RAMBo. No bed heat. I then switched to the Repetier configured for the Max, again, I don't get voltage output to the heated bed. today is the first day of powering up. Steppers work fine. I do not have my hotend or hotend thermistor hooked up yet so perhaps the heater circuitry is disabled in that case?

My pins.h has the heated bed on pin 3 for RAMBo. I have measured 12V at the heated bed power input block.

Did you figure your problem out yet jhodges?

[mhackney]

Ok, I discovered that the hotend thermistor MUST be connected and functioning for the heated bed circuitry to work. I posted about it on my build thread. Perhaps this is your issue too?

[jhodges]

I actually got mine to work, but for a completely different reason. My thermistor was always connected and reading out the true value. For me, the culprit was a loose connection on the 6-pin input from the power supply. The short leads must have pulled when I opened the door completely. Truly a noob problem.

Thank you everyone for the assistance.

[Craftgizmos]

I'm having a related issue. My Rostock Pheobe beds heats, but it doesn't get above about 88 centigrade, and the red light from RAMBo is always on. It just doesn't get hot enough, so if I really need to get to 100 I can't now. My house is at 65 fahrenheit FWIW.

[mhackney]

As Scotty would say "we need more power, Captain". It sounds like you don't have enough amperage to heat the plate. Do you have an amp meter to test it?

[Craftgizmos]

Yes, but I'm not sure exactly where to measure what on which scale. I did do the tree yellow and three black wires from the power supply to the RAMBo, there's a picture on my build zone thread.

[mhackney]

You need an Amp Meter - many multimeters have this feature. However, make sure that it can handle the amps you intend to measure. 10 amps is probably ok. Multimeters usually have a separate plug for the probe when measuring amps. Scroll down to measuring current in this article: http://www.sparkfun.com/tutorials/202

You basically insert the multimeter between the heated bed (+) terminal and the power source - in this case the RAMBo connector. Since you have 3 yellow wires for (+) I would disconnect those at the RAMBo and then connect them to one of the multimeter leads and connect the other lead to this terminal on the RAMBo. Please make sure you know what you are doing and read the article! These devices take a lot of current - the RAMBo has a 15amp fuse to protect the hot end circuitry, so if your ammeter can handle 10 amps you should probably be good.

[Polygonhell]

You need an Amp Meter - many multimeters have this feature. However, make sure that it can handle the amps you intend to measure. 10 amps is probably ok. Multimeters usually have a separate plug for the probe when measuring amps. Scroll down to measuring current in this article: http://www.sparkfun.com/tutorials/202

You basically insert the multimeter between the heated bed (+) terminal and the power source - in this case the RAMBo connector. Since you have 3 yellow wires for (+) I would disconnect those at the RAMBo and then connect them to one of the multimeter leads and connect the other lead to this terminal on the RAMBo. Please make sure you know what you are doing and read the article! These devices take a lot of current - the RAMBo has a 15amp fuse to protect the hot end circuitry, so if your ammeter can handle 10 amps you should probably be good.

Or you could just measure the resistance of the bed disconnected from the board, measure the voltage being supplied to the board when the heater is on and use I = V/R to calculate it.
Note the voltage at the output will be less than you expect because of the loss from the Mosfet.
Generally if you are not seeing a significant voltage drop at the input to the RAMBO board when the heated bed is on, you probably don't have a PSU issue.