Hot end temperature problems  

Hot end temperature problems

I recently received and built an MKS3+ from the kit.  While I was waiting for the printer to ship, I ordered a Revo 6 hot end thinking that the flexibility to change nozzles would be nice and also it would make sense to build it directly with the Revo 6 than change it later.  It worked fine for nearly a full spool of PETG (I was printing files for the Lack enclosure), but then one of the prints I had running overnight failed.  When I checked it in the morning, it looked as though parts of the print had come off the bed and accumulated masses of melted plastic and there was a lot of plastic melted around the nozzle.  I heated it up and removed the plastic  around the nozzle as best I could with a rag and then and used a brass brush to clean the nozzle more.  I restarted the print and that one finished but then the next one failed in a similar way to the previous failure.  Here's where things  became more difficult.

After removing the plastic melted around the nozzle, I could not get the printer to print well again.  Any first layer calibration attempts printed the lines very uneven, like the filament was not extruding smoothly.  I tried adjusting the Z offset in case it was too close to the bed but it did not help.  I tried using a needle to check the nozzle for blockage and clear it, but it seemed clean.  I noticed that when I tried running the first layer calibration the hot end took much longer to heat up, or so I thought:  I saw it go from around 150 degrees immediately to around 220 degrees at one point, then back down and it stayed there.  Eventually the printer gave me a thermal runaway error.  I tried running a PID tune and it did not help.  At one point during another first layer calibration attempt the temperature read low while heating, but smoke started coming out of the nozzle.  Doing some research online I saw that others have experienced problems with the Revo 6 heater block and thermistor from the look of things.  So, last weekend I decided to go through and swap that out for the default hot end that came with the printer.

The first print was fine, but I neglected to add supports so it didn't come out right and I wanted to print it again.  On the second attempt, the same type of thing happened where the plastic melted around the hot end while printing and parts of the first couple layers were being dragged around as the print head moved.  I caught it early enough before it turned into a big mess, but I still heated the nozzle and cleaned with a brass brush again since plastic was melted around the heater block again.  I tried doing the first layer calibration again in case it was a problem with my Z offset, but I observed the same behavior described above for the Revo 6:  initially uneven extrusion, then temps that jumped around then smoke coming out print head and finally a MAXTEMP error.  I also noticed a 0 reading for the bed temperature right before I turned it off.

I checked the cables ties are not overly tight, I checked to make sure the thermistor connector is seated...  Not sure what else I should look at so thought I'd see if anyone has any advice.  Thanks.