858D v8 temp scale completely wrong

[pmars]

Just received a generic 858D (board says YH858D v8) rework station and first thing I noticed is the actual temp scale is always around 638 to 700. Temp set buttons seem to work, but the heater is all over the place. sometimes hot sometimes cold. I have tried the calibration pot on front panel but little to no change. Sometimes display shows "5-E" I have read many posts here about this model and realize this could be a hopeless battle as this one I have received is even crazier than most.

I have compared wiring diagrams and all seems fine except I cannot confirm the polarity of the thermocouple and I'm not sure if this would make any difference. I am thinking about reversing the red and blue thermo wires on the handle circuit board and see what happens. Any suggestions other than "trash it" ? Only cost $29 US and ebay seller has already refunded my purchase price. He doesn't want it back!!! Thanks

[pmars]

OMG... UPDATE: I just kept fiddling with this and found the problem!! Dumb luck. After much inspection with a magnifier and after rewiring the black lead to be fused and switched (instead of the white wire as manufactured) I plugged it back in to check it one more time and since the board was still exposed I wiggled the calibration pot and the up/down temp set buttons and noticed the display jumping between really low numbers like 026 and 680 or higher. Hmmmmm... I settled on the Down Arrow button as the culprit. broke it open, cleaned with an eraser, melted it back together with small soldering tip and added some fresh solder to the 4 leads.. IT WORKS!!!!!!!!!! Maybe the "pop-disc" was in crooked or a bad solder joint?

Now I will get out the k-sensor for my multimeter and recalibrate this thing. YES!!!!!!

[bitseeker]

Good sleuthing. Probably bad solder job.

[pmars]

Calibration is complete, but only in the sense that it's basically a loose averag. I set the temp for 250 and get a swing from 225 to 275 as the heater kicks in and out roughly every 10 seconds. Not sure if this is normal. Seems like the circuitry could be a lot more sensitive. Wonder if there is a sampling rate that can be adjusted. The K sensor of my meter responds instantly and continuously.

[bitseeker]

The default firmware of earlier versions doesn't regulate the temperature all that well and is notorious for overshoot. I don't know if it's improved since. There's another thread about replacing the firmware/microcontroller.