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RAMPS 1.4 Wierid bed temperature behaivour.

Posted by BortEngineer 
RAMPS 1.4 Wierid bed temperature behaivour.
June 29, 2017 01:59AM
I have RAMPS 1.4/MEGA 2560 machine, which worked flawlessly for almost 6 years until today. I powered it up today and instead of expected "3D printer ready" message i've got "ERR: MAXTEMP_BED".

So I've started diagnostics. First thing i did is thermistor check: both extruder's and bed showed around 115k resistance with room temperature of 20 degrees celcius.
I've checked this values on the board side connectors, with this values i can assume that wiring and termistors are fine.

Then I've pull arduino off and test analog pins responsible for temperature readings(A13-A15) with this simple sketch:
int a = 0;
int b = 0;
int c = 0;

void setup() {

void loop() {
  a = analogRead(13);
  b = analogRead(14);
  c = analogRead(15);

  Serial.print("A13: ");
  Serial.print("; ");
  Serial.print("A14: ");
  Serial.print("; ");
  Serial.print("A15: ");

Plugging all 3 pins on different external voltage sources shows that's arduino is ok(readings from pins are ~0 for GND, ~690 for 3.3 volts and ~1020 for 5 volts).

Moving to RAMPS board: all three thermistor inputs has correct pull up resistanse of 4.7k on the one side and straight path to the ground on the other side.

One diference i have found is the voltage on the thermistors itself. On T0 which is still intact, voltage is 5v no matter what thermistor is plugged in, on the T1 and T2 there is 0.5v in all cases tested(yes, tried to remap bed to another pin with no luck).

Is there any quick fix for that issue or it would be simplier to buy new RAMPS board?
Re: RAMPS 1.4 Wierid bed temperature behaivour.
June 29, 2017 03:38AM
poss leaky caps C8, C1. these are connected across the thermistors
open | download - RAMPS1.4schematic.png (444.2 KB)
Re: RAMPS 1.4 Wierid bed temperature behaivour.
July 02, 2017 01:39AM
Unsoldered one of 'failed' capacitors and tested it, it's should be fine.

Any suggestions?

Edited 1 time(s). Last edit at 07/02/2017 01:42AM by BortEngineer.
open | download - IMG_20170702_112911~01~01.jpg (569.4 KB)
Re: RAMPS 1.4 Wierid bed temperature behaivour.
July 02, 2017 03:25AM
Nevermind, i found the issue. Rewinding all stuff that were done, I've came to conclusion that I have toasted A14 and A15(somehow confused smiley) on Arduino when i was cleaning heatbed from hairspray residue with razor blade, shorted 12 volts to thermistor terminals, which altered input impedance on that inputs.

To fix this i unsoldered A14 pin and add wire to A10 pin on AUX2 port, and remapped heatbed thermistor in pins_RAMPS_14.h corresponding to new wiring.

Edited 1 time(s). Last edit at 07/02/2017 03:28AM by BortEngineer.
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