Another bed heating Problem

Charly3

My A5 bed thermistor is 87,1k ohm and printer display as well as pronterface report bed temperature to be 111° Celsius even if it is totally cold. For 110 is the highest temperature to set up, bed won´t be heated at all for it belives to be already matching desired temperature .

Nozzle (it reads 31° Celsius) won´t heat up too. For normal printing start this would be OK, for bed is heatened first, nozzle second. But even from heating menu that allows indivudual temperatures for bed and nozzle without that order it doesn´t power up.

Don´t knew if it´s worth to be mentioned: Usually the heating was corrected when it was 1° below programmed temperature. The last day it often felt down -2° before it restarted heating.

By the way - is it possible to solder in a new thermistor to the bed even if it comes preconfigured for uns in nozzle heating block? For example could this work beeing soldered into the bed?

https://www.amazon.de/Aibecy-Thermische-Widerstand-Thermistor-Ersatzteile/dp/B06ZZFY8MK/ref=cm_cr_arp_d_product_top?ie=UTF8

Samuel Pinches

Hi @Charly3, welcome to the forum - I'm sorry I missed your message! Have you sorted this out yet, or do you still need assistance?

Charly3

Hi Samuel,

Thanks for your welcome.

Support ist sending a new Mainboard. Seems both thermistor ports are broken. Not 100% Sure how this could happen. Only found that cabeling running into hotbed was sliced twice, one time while scratching the housing, second time while beeing mounted rather tight with a tiny strap to one of the motherboard feets. 

So if

a.  constant same wrong temperatures are reported

b. Ohm readings on thermistors are Ok

 broken ports are probably the cause.

Not heating nozzle in this case is caused by safety function that calls a kill routine if temperature measurements doesn't show temperature raise when nozzle is powered.

Not heating hotbed in this case might be caused by wrong temperature readings the software recognizes as having reached max temp already. This will take place if wrong temperature is about 110 celsius.

Samuel Pinches

Ok, hope that replacing the board solves that issue!

Maybe you want to also check out these pages on the wiki:

https://jgaurorawiki.com/not-heating

https://jgaurorawiki.com/a5/faulty-temperature-readings

You may want to also check that the heater wires from the bed and nozzle are not contacting the metal frame. This can pump 24V into the frame, which can damage the thermistor inputs.

The other thing to watch out for is if you have bed clips holding the bed down - you need to keep the blue tape to electrically insulate the metal clips, as otherwise they can scratch the bed heater (underside of bed) causing a short circuit.

Charly3

There seems to be quite a couple of people out there facing the same problem with broken thermistor ports on that board. Often a circuit of heating element is reported. So how to get better protection. Will external mosfet - even if not needed for powerconsumtion reason on A5 - help to keet shurtcut damage away from the board? Is there a way to have thermistors galvanically separated from the board?

Samuel Pinches

The thermistors are definitely vulnerable, but they are almost directly connected to the analog interface on the main processor. Unfortunately, these analog interface inputs are very fragile, and because they are analog it is difficult to isolate them in a cost effective way. All I can say is: at least the motherboard is dirt cheap to replace (https://sampin.ch/mksgenL), you might be wise to keep one on hand as a spare part.

Charly3

Bought a new one while still witing for the jgaurora substitution board. Working fine with comunity firmware as far i can tell now. There is something wondering me a bit. Whilst hotbed is constantly powered for a couple of sec hotend is powered in real small amounds. Led is flickering all the time. Is it normal behavior?

Samuel Pinches

Yes, the bed is “bang bang” controlled, while the hotend is PID controlled. The flickering is normal

Godzilla32

I just realised ive got the metal clips and no tape. Also ive had small zaps from touching the bare metal screws and parts generally when the printer is off. Im still having an issue with the heating on my bed effecting the nozzle clearance. So much so that the first layer ends up having a pattern through it if its a wide layer. Maybe its all connected..

Charly3

Zaps might occure if you're using a plug with only two wires. Or if false power recognition is't avalable in your power supply. There are some videos about it on youtube.

Got a second one A5. Halve price, from gearbest,but somewhat different. Many plastic parts in the mechanism where my first has metal guides. Will see how i can stabilize this.

The hotbed is clamped not glued. Any idea where to buy heat resistand double sided tape in the size of the full bed?

Samuel Pinches

@Charly3
Here's a link on aliexpress for a 300mm square of high temp 3M 468MP double-sided tape ...and comes with a sheet of pei too!!

http://s.click.aliexpress.com/e/cNf5Er88

Also available by itself from Amazon USA if you need it sooner.
https://amzn.to/2op5mr3