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Hot Bed and Extruder Heating Issues

Posted by Supermec 
Hot Bed and Extruder Heating Issues
September 05, 2016 10:28AM
Hi All,

having assembled my Anet A8 I experienced issues with the Hot Bed and Extruder not heating and the message "def/190 def/50" displayed on the LCD.

This could have been either a faulty hot end thermistor or a faulty hot bed thermistor, either of which would disable heating but allow other functions. So with power off I unplugged and measured:
a. The extruder heater - all OK.
b. The hot bed heater - all OK.
c. The extruder thermistor - all OK.
d. The hot bed thermistor - this measured open circuit.

I then unplugged the cable from the hot bed and checked the continuity of the two wires to the thermistor - all OK. I then measured across the two thermistor pins on the hot bed - Open circuit.

Having removed the hot bed from the machine I then measured:
a. Across the thermistor - 100k OK.
b. From the right hand pin to the right hand solder pad (looking for a dry joint) - OK.
c. From the right hand pin to the thermistor on that track - OK.
d. From the left hand pin to the left hand solder pad (looking for dry loint) - OK.
e. From the left hand pin to the thermistor on that track - Open circuit - printed track faulty.

The two printed tracks leading to the thermistor are very thin. There was no sign of damage to the varnish layer over the faulty track. Examination of the printed track with a powerful magnifying glass revealed a break in the printed track about the thickness of a human hair. This break was covered by the varnish layer and thus obviously had taken place during manufacture before the varnish layer was applied. The faulty track is so thin that I don't fancy my chances of making a soldered repair without causing further damage.

This is one to look out for. I am currently corresponding with Anet concerning a replacement Hot Bed. Progress is being hampered by my lack of Chinese and the Chinese persons lack of technical English. I have had to resort to drawing up part of the board in CAD and then turning it into a PDF to send them as the fault is too small to show in a photograph, and the back of the board being covered in varnish means that the tracks don't show up at all well.

There is also another point I would like to draw everyone's attention to. While examining the hot bed I noticed that the printed tracks of the heater come almost right to the edge of the board. They are within the area where the four springs are located that press the bed against the adjusting screws. Any breakdown of the varnish in these areas (such as might occur from repeated adjustment) will result in a short across all or part of the bed heater (my version has the aluminium hot bed carrier - acrylic carriers would be ok). This will stop heating even if it doesn't destroy the power feed on the control board. When I re-install the hot bed I plan to use four thin fibre washers to keep the springs off the face of the bed.

I hope my ramblings are of use to any of you experiencing heating problems - particularly the faulty printed track.

Now I've just got to wait for the replacement hot bed.

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