24v Heat Bed Port Only Outputs 3-5v. Dead board?

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JAGreene
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24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by JAGreene » Mon Aug 21, 2017 11:57 pm

So I have a few 24v M2's, and my oldest one's heatbed has stopped powering up. For reference, I only ever print ABS and heat the bed to 93C for every job.

I checked all of the wires, fuses, power cord connectors for breaks; everything is in fine working order. Swapped out the heatbed, not the problem. The bed is working fine when I put it in my other identical M2, so it's not that.

So the problem is definitely with the board. LED6 lights up when I manually set the heatbed to heat up, and the port outputs voltage, but only when the bed is UNPLUGGED. When I plug the (perfectly working) heatbed in, LED6 turns off and I get zero volts out. When I unplug the bed, LED6 turns on again, but when I measure the output voltage, it's never 24v, only around 3-5 volts, which I know is bad.

Is there a particular failure point on the board that could be repaired or re-soldered, or am I better off buying a new board?

I just want my baby to get better :(

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jimc
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by jimc » Wed Aug 23, 2017 3:15 am

have you checked the connection where the bed power plugs into the board. this was a big issue on 12v beds. we see alot less of it on 24v but it still has alot of power running through that small connection. the pin or socket can get a little bit of a weak connection over time. a weak connection would give you low or normal voltage with bed disconnected then when you plug the bed in it would drop to zero or almost zero.

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ednisley
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by ednisley » Wed Aug 23, 2017 4:21 pm

JAGreene wrote:when I measure the output voltage, it's never 24v, only around 3-5 volts, which I know is bad
The RAMBo connects +24 V and the MOSFET output to the heater: the MOSFET connects the heater terminal to ground, not the power supply, when it's turned on.

With the voltmeter's black (negative) probe on a RAMBo / power supply ground terminal, the red (positive) probe should measure a constant +24 V on one heater terminal and +24 V (heater off) or nearly 0 V (heater on) on the other.

With the voltmeter probes on the heater terminals, you should measure about 0 V with the heater off and about 24 V with the heater on.

Never, ever connect or disconnect the heater when the power supply is turned on: always unplug the supply before changing the connections.

JAGreene
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by JAGreene » Mon Aug 28, 2017 11:16 am

jimc wrote:have you checked the connection where the bed power plugs into the board. this was a big issue on 12v beds. we see alot less of it on 24v but it still has alot of power running through that small connection. the pin or socket can get a little bit of a weak connection over time. a weak connection would give you low or normal voltage with bed disconnected then when you plug the bed in it would drop to zero or almost zero.
I've checked the wires and even tested the voltage of the connector pins directly at the board, and there doesn't seem to be a weak connection. The voltage does drop to zero with the bed connected though, and LED6 will not turn on with the bed connected. With the bed disconnected, LED6 will light up for a second when I first turn on the heater, then it dims and I measure 2.8v on the heater pins.
ednisley wrote:
With the voltmeter probes on the heater terminals, you should measure about 0 V with the heater off and about 24 V with the heater on.

/quote]

Here's what I'm measuring-

Heater Connected: +24v and -24v while on, +24 and 0v while off.
Heater Disconnected: +2.8v and 0v when on, +23.4v and +22v while off.

Could something be wrong with the MOSFET?

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jimc
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by jimc » Mon Aug 28, 2017 2:22 pm

It can be the mosfet yes. You are checking voltage though and that doesnt mean anything if you have a weak connection. You can test and get 24v but when you connect the heater there is more draw than can be supplied or passed through the weak connection give you a low or no voltage situation. Generally when this happen you start generating alot of heat at that connection. The bad connection could be a wire broken internally, a plug or connection point or the mosfet

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ednisley
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by ednisley » Mon Aug 28, 2017 2:55 pm

JAGreene wrote:Could something be wrong with the MOSFET?
The typical MOSFET failure is a dead short from drain to source, which would force the heater on permanently. Failing open, which is much less common, wouldn't change the LED.

Have you measured the fuse resistance?

Sometimes blade fuses fail with a (relatively) high resistance, rather than blowing clear, which would explain the symptoms:
  • The low-resistance heater pulls the supply down, because the fuse is a resistor, so the LED stays dark
  • Fade-out LED with no heater, because the caps downstream of the fuse charge through the fuse resistance
Unplug the power supply and let the voltage drop to zero.

Pull the fuse and measure its resistance. If it's more than a small fraction of an ohm, it's bad.

With the fuse out, measure the resistance across every junction from the heater's +24 V power supply wire through the heater and the PCB back to the heater's common wire:
  • Screw crunching the +24 V power supply wire -> male pin inside connector shell
  • Female pin inside PCB connector shell from +24 V supply -> incoming fuse clip
  • Outgoing fuse clip -> female connector inside PCB shell to heater
  • Male pin inside shell -> screw crunching heater wire
  • ... and so forth and so on ...
  • Male pin inside connector shell -> screw crunching power supply common wire
Tedious, but if the fuse is good, it's most likely a well-concealed poor connection. Consulting the schematic can help with the search:

http://reprap.org/wiki/Rambo

JAGreene
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Re: 24v Heat Bed Port Only Outputs 3-5v. Dead board?

Post by JAGreene » Mon Sep 11, 2017 12:35 am

So I checked every connection on the board. It looks like it was the power supply connector after all. The positive connection wire was charred, and the male end port was melted a slight bit. Whenever I check the voltage from any part of the circuit to ground, it would start at 24 and slowly decrease. I assume when the heat bed turns on, that voltage drop is extremely fast, which would explain the indicator LED turning on full brightness, then dimming, and the heat bed port only outputing a small amount of voltage.

Anybody know a convenient source to get a replacement male/female power supply MOLEX connector?

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