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PSU failure, initia...
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PSU failure, initial symptoms?


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Rbox-01 unit, over 5 years old but looked after, not thrashed.  Strange fault in that bedplate heats up ok and temp is reflected in on screen gauge.  Heating continues until filament heaters turn on, around 62c, on which the bed heating then struggles and starts to loose heat.  AM software now struggles to keep up and gets stuck in a cycle of waiting for the bed to heat to 70c but will never get there.  Meanwhile, filament purge then fails with no output at which point everything comes to a halt.

Now, it initially looks as though the PSU is managing to supply enough power to the bed heater, although not very fast to target, but then just totally gives up the ghost when asked to run the bed and the filament unit.  Thermostat is relaying temps back to the AM software, the numbers are there, but it is almost as if the PSU cannot provide enough power.  Attempted a purge of the head and that just fell over as well, no extrusion and heat not getting anywhere as hot as it should.

Anybody else seen this sort of thing, and a cure before it's back to CEL for a look?

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5 Answers

This is a symptom of a failing bed power cable. Stop using the printer until it is repaired. If you continue to use the printer and the cable fails, it may short, which will destroy the mainboard. 

If you replace the bed power cable and the head power cable and it continues, then you can look at the PSU, but the bed and head power cables are the first place to start as they are wear items. Power supply failures are rare if the printer is properly protected from dirty power.

Topic starter

Well a few days checking over the unit.  For the bed, the heater element reads on a test meter around the 1M range with no shorting between supply cables or to earth.  Wiring and plugs from control board for 24v supply checked, reseated and 24v at head with no print head fitted.  Head was then fitted and soak tested with fans and lights manually turned on but no filament and all was ok.

Then the fun started.  Installed filament and commenced a purge, for printer to stop with a Nozzle Control Error warning.  Heating of bed and print head behaved itself, no issues this time round so that fault may have been a red herring.


The heater should test not more than 70 ohms end to end. The printer I just checked that is known working reads 62 ohms at the connector into the mainboard, with the cable disconnected from the mainboard. I suspect you have break in your cable. 

Be very careful when heating the print head without filament moving. Always run a purge afterward to clear out any heat damaged filament. Sustained heat of the head without filament flow will bake the filament into the head and you can permanently block the nozzles. 

Topic starter

Fault has been tracked down to the print head.  Swapped print head with another known working unit and the fault transferred over to the good machine while the suspect machine behaved itself when the good print head was fitted.

So, new print head looking likely but at the moment they are currently out of stock at the CEL shop with a supposed availability date of late 2021 (yes, I know) so this could be a bit of a lay up period for the machine.


If your bed wires show a resistance of more than 70 ohms end to end you also have a problem with your bed heater. Likely a broken wire. If you continue to use the printer with this problem it can destroy the mainboard. Re-test your bed power cables!


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