I have just replaced a partially fried control board on my K8200. Through my negligence, I allowed a bare hotend thermistor wire to contact the heat bed power (a 24V separate power supply). Somehow, this did not completely kill the board but only it seems THERM1 and THERM2 are dead. I replaced the 104GT2 hotend thermistor but the hotend still indicated a constant 94.9C (and the bed 0.00C). Resistance across THERM1 and THERM2 pins read 1.72K and 4.74K ohms, so it appears that only THERM1 may have fried. However, control through Repetier and the steppers still worked fine. I went ahead and purchased a replacement board that was sent from Velleman USA in Fort Worth TX. Thinking I could troubleshoot the fried component on the old board with hopes of repairing it one day, before installing the new board I compared the new one out of the bag to the old board. I noticed right away on the NEW board that the T7 and T8 4701 resistors on the board meter at 7K ohms when measuring directly across the two sides of the resistor, rather than 4.7K (even though they are both stamped 4701). I went ahead and installed the board, autotuned PID, adjusted stepper voltage, and installed firmware to match my inverted Z configuration. The board works but my print quality has suffered greatly. I’m printing Verbatim PLA (black) and had been getting fantastic results. But now I continually get lots of messy blebs, zits and strings even when I use old settings that had given fantastic prints. I reduced temp from 205 to 200C and have also adjusted speed and retraction, all to no avail. My question is, would incorrect 4.7K pull-up resistors (that are actually 7K resistors) cause THERM1 and THERM2 to read incorrect temperatures? Should I return this board or do I just need to continue tweaking settings?
One other minor issue is that the new board will only intermittently connect to Marlin, even when the JPROG pins are jumped. Sometimes I have to try multiple times to get the firmware to load, but I have not had issues connecting to Repetier Host. I have never had that issue in the past.
came across your post while trying to get my hands onto a 8200 myself. So,yet i can’t help with printing settings or other things but may help solving pieces of your mystery.
You measure your fried board and find readings that may fit, and measure the new board and the readings don’t fit. So i guess you measure the components in the circuit. Doing that needs a schematic and years of experience.
Righty, so as i don’t have the 8200 and thus no beard as well, i relate to velleman.eu/images/tmp/K8200diagram.jpg for reference.
Measuring components soldered into a board means that the readings are influenced by all the other components in that board. So when you measure across R7 or R8 you also measure ways through C7, C8, U1 and also components connected to the circuitry labelled “+5V”. So if all the capacitors “near” the points you measure are not completely discharged, they make you meter show anything but the resistance of those two components. Having them discharged might get you closer to a relatively correct reading, but only closer. A true reading can only be accomplished by removing the components from the circuit - or at least one end of them to disrupt the circuit and make the component seem not to be in an electric circuit.
I guess your fried board hasn’t been powered up for some time what makes the capacitors pretty discharged and thus the reading across R8 fit pretty well - R7’s reading does not. Now, the resistors and the capacitors around THERM1/2 are not so sensitive as U1 is so I believe they are in good shape but 1K7 measured across R7 pretty sure says U1 has been damaged. To verify your reading you may check the inputs of U1 for THERM like this: Connect positive wire of your meter to THERM positive and negative wire to THERM ground. Watch your meter spool up from a few tens of Ohms up to a value near 4K7 or higher and you may be sure, the input circuitry of U1 is quite in shape. I think for “fried” R8 this may work, but for “fried” R7 i’m sure the reading will be clearly below 4K7 - stating U1 has been damaged and also renders THERM2 unreliable. Replacing U1 will pretty sure heal the fried one.
Now for the new one, since you used it in your printing attempts has charged capacitors that make your meter go wild. So if you want to measure a resistance in a circuit, first be sure you don’t have residual voltages (literally millivolts) in there.
If you haven’t mistreated THERM1 or 2 inputs, they should be all right and i believe your new board’s behaviour is much more related to the connection problems to Marlin.
