no heat on bed or hotend  

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Bruster999
(@bruster999)
Active Member

I have an MK3 that I use on a regular basis. I stopped heating recently on both the hot end and the bed. I saw a post about two black and white wires to disconnect them but that did not help. The thermistors respond to heat from a hair dryer and I checked all of the connections and fuses. Any suggestions would be appreciated.

Posted : 26/04/2021 6:22 pm
Diem
 Diem
(@diem)
Honorable Member

Both going down at once is unusual. Quite often a wire will work-harden and break but both at once..?

First pull and replug all the connectors, not just the heating circuits.  Let's make sure nothing is loose or masked by corrosion.

No help?

Use a multimeter to check resistance of both heating circuits - open circuit means broken wiring.

Contact Prusa by logging in to your Eshop account, go to chat at bottom right (you may need to temporarily disable ad-blockers) and give them the values.

Cheerio,

Posted : 27/04/2021 1:33 am
gkozminske
(@gkozminske)
New Member

I have another different issue with my heatbed. I'm just starting up my MK3 which I built from a kit.  Upon startup the heatbed begins heating uncontrollaby. I've checked the power supply wiring and the thermister wiring. Both seem correct. I've been able to run the Self Test, if I do it quickly before the bed heats up. The Self Test Fails and reports "Selftest Error" Please check Bed/Heater wiring error. Any suggestions?

Posted : 27/04/2021 5:28 pm
Dan Rogers
(@dan-rogers)
Prominent Member

@gkozminske

New machine still under warrantee - do log into the E shop and chat with support.

Posted : 27/04/2021 9:39 pm
Bruster999
(@bruster999)
Active Member

It's very odd issue. I rant the self-test but it fails when trying to heat the heat bed. I measured no voltage at the screw terminals during that time, it should have read around 24vDC. The resistance of the heat bed was around 2.8 ohm. It did not change if I moved the cables. The resistance of the heat bed thermistor was about 108 ohm. The resistance of the hot end heater and thermistor are 15 ohm and 111 ohm respectively. These are all within normal parameters. It's like the relay isn't engaging but I don't see any relays on the main board.

Posted : 28/04/2021 2:11 am
Bruster999
(@bruster999)
Active Member

@diem

Thanks for the suggestions. I tried pulling and inspecting all connections but everything looks normal. No hot spots, no burned or loose wires. As I said earlier, it's as though the the relay's for the hot bed or hot end are not engaging but I don't think relays are used.

Posted : 28/04/2021 2:14 am
JoanTabb
(@joantabb)
Moderator

@bruster999

thermistors on Prusa's are normally 100,000 ohms at 25centigrade  (100Kohm)

your heatbed and heater are controlled by Mosfets,
a bit like the image above. there are fuses in the power feed, on the controller board and on the powersupply, and in the UK, ther is a fuse in the mains plug as well
regards, Joan

 

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 28/04/2021 3:38 am
Diem
 Diem
(@diem)
Honorable Member

@bruster999

Contact Prusa by logging in to your Eshop account, go to chat at bottom right (you may need to temporarily disable ad-blockers) and give them the values.

Cheerio,

Posted : 28/04/2021 6:15 pm
Bruster999
(@bruster999)
Active Member

Prusa says the PSU is bad but how do I test it? I read 24v output from the unit, even under load. I read no voltage on the power panic wires but I'm not sure exactly what to expect on these.

Posted : 04/05/2021 3:30 pm
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 --
(@)
Illustrious Member

Verify the amber LEDs on the Einsy are turning on when you expect the heaters to be working.  If you don't see these LEDs turning on, you can check the voltage at both sides of the 5a and 15a fuses. But generally speaking, the 15a can't be blown by the 200w power supply.

Also - not sure what Prusa had you measure: but did you measure BOTH 24v pairs coming into the Einsy?

Posted : 04/05/2021 6:37 pm
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 --
(@)
Illustrious Member

@gkozminske

You probably reversed the heat bed wires with one of the incoming power wire pairs.

Posted : 04/05/2021 6:39 pm
Bruster999
(@bruster999)
Active Member

@gkozminske Good thought but no, I didn't reverse any wires as I didn't remove any, save one so I could measure the heat bed resistance. 

 

Posted : 05/05/2021 12:15 pm
Bruster999
(@bruster999)
Active Member

@tim-2

Yes I measured both pairs coming in at 24v.

Anyone know what the black and white pair should read? 24v I assume but I can't find an answer anywhere.

Posted : 05/05/2021 12:17 pm
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(@)
Illustrious Member

I think Black/White is power panic, and should normally be high (at Vcc, ~5v). 

This post was modified 2 months ago by --
Posted : 05/05/2021 4:47 pm
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