RostockV2 hot end won't heat past 145C

[TimMcBrayer]

I have an essentially stock Rostock V2, with the old hot end. A few days ago, the last print was printing ABS at 220C. Today, I cannot get the hot end to heat past 140-145C.Watching the temp rise, it appears to act as if the set point was in the 140-145C range. That is, the temp rises more slowly as it approaches 140C, eventually plateauing out. It will stay at this temperature for several minutes, but will eventually start displaying "def" for both hot end and bed temps, and the bottom line of the display reads "Heater decoupled".

My research here makes me believe that the software has detected (correctly) that the hot end is not responding properly, and has shut the power to both hot end and bed down.

The thermistor seems to be behaving normally. I don't see any strange, out of range values (0 or 300C); it just never gets up to temp.

I tried the hot end PID tuning routine; it timed out, with the temp once again only making it to the ~140C range.

Could one of the heater resistors have died? This just occurred to me, and may be the case. I just now removed the hot end power wires from the RAMBO board. From that point, I am reading 9.2 Ohms. If the resistors are supposed to be 6.8 Ohms, with the two in parallel, the heater resistance should only be 3.4 Ohms. I don' t know where the other ~2.5 ohms is coming from. Nonetheless, this is a likely possibility.

I'd be happy to hear any other ideas as to what might be going wrong.

[IMBoring25]

It does sound like the increased resistance just isn't letting enough current through to overcome the heat loss to the atmosphere and pass 145. I'd check resistance at the resistor leads to rule out a problem with the wiring going down to the board. Check the leads of one resistor and the leads of the other resistor to see if there's a conductive path at the join.

If the resistance is still high at the leads, I'd figure one or both resistor(s) went bad. For what it's worth, I took failure of the wiring/components at the hot end as an opportunity to upgrade, but since you're already probably out of business with the printer you'd need to buy not just the hot end but also a mount unless you found a hot end compatible with the existing mount.

[Roberts_Clif]

I purchased some 12V 40 watt heaters and they sent me 24volt 40 watt heaters,


24V 40 watt heaters = 15 Ohms in parallel 7.5 ohms
12V 40 watt heaters = 3.8 Ohms in parallel 1.9 ohms
+ or - 10%tolerances
If they sent you 24v heaters this would account for the resistance you are seeing. It would also account for not being able to heat the Hotend.

So you have a two heater hotend where do you get them. I would like to use them on my 3D Printer.

[IMBoring25]

It's not a wrong resistor issue. These same resistors were working correctly before. Something has gone wrong.

The hot end in question is the stock v2 hot end. I don't see that you've ever said what machine you have but it would probably be a downgrade.

[TimMcBrayer]

I have ordered new resistors, which should hopefully make it here in a few days. In the meantime I'll tear down the hot end and examine it for what specifically may have failed. Thanks for the confirmation that I'm probably on the right track.

[geneb]

Chances are very high one of the resistors failed. They tend to fall into the "consumables" category.

g.

[TimMcBrayer]

I pulled the hot end apart, to get to the individual resistor leads. One is 8.4 Ohms, and the other is an open circuit. So, I have definitely burned out one of the heater resistors. 8.4 is about 1.25x the nominal 6.8 Ohms, so it's not _too_ far out of tolerance (assuming 20% tolerance parts). I know which one needs to be replaced now. I will replace both while it's apart, though.

Can the V2 hot end use the 12V 40W cartridge heaters, or are those only for the HE280 hot end? I've already ordered new resistors and crimp connectors, but it'd be good to know for next time.

[Xenocrates]

It can, but you will need to drill and thread a hole for a grub screw to hold it in place. Or you could try packing it in RTV (I wouldn't).