KADA 936D+ soldering station repair.[successfully modded to work] - Page 1
Forum section: electronics / repair

author: Refrigerator
author_rank: Super Contributor
timestamp: January 19, 2015, 06:23:31 pm
content: So today i received the hakko (clone) heating element that i had ordered but putting it in my soldering station it doesn't seem to work.The thing is that the thermistor inside was a different value on the original, about 150R when hot vs 100R when hot on the new one.As i noticed there were three pots, after i located the calibration pot i tried the whole range but couldn't get it to work, there seems to be a resistor voltage divider of some sort that the thermistor is a part of.I guess i should replace the heater with a potentiometer and try dialing the resistance in and checking the voltage, the whole thing is a single chip solution with a LM7805 a TL431A and the triac with lots of passives around.As of now the heater glows red hot and the temperature won't go over 200oC (the minimum value that the display shows).Some in any help or advice would be appreciated.Ps: i only have a single DMM and a wimpy power supply ( LM317T based ) to troubleshoot this thing. I'll post the pics soon.

author: Refrigerator
author_rank: Super Contributor
timestamp: January 19, 2015, 10:05:33 pm
content: Took some pics:Solder side of the board.Component side of the board:Please note that that the brighter blue cap and the heatsink near the edge of the board are not original, the heatsink was fitted on the LM7805 because it was getting too warm and i had to replace the cap near it to a smaller size yet same value cap to make the heatsink fit.Also, the multiturn pot on the solder side seems to set the gain on the ( presumably ) op-amp near it, the multiturn pot near the TO-92 package sets the voltage reference for the set temperature display and the single turn pot near the big pot is your normal cal adjust.I tried playing around with a pot connected to the sensor contacts and it seems like i can get the full range ( 200-480oC ) by going from about 7.5K to 21K.

author: Refrigerator
author_rank: Super Contributor
timestamp: January 21, 2015, 12:31:19 pm
content: Well i traced the signal from the thermistor and i found out it goes in to the pin 10 of the MCU, I also found out that for every 1v on the pin 10 of the MCU coresponds to 100oC on the display.

author: Refrigerator
author_rank: Super Contributor
timestamp: January 24, 2015, 10:46:59 pm
content: I did it ! All i had to do was to replace the R11 resistor with a different value one.After some reverse engineering i found out that the op-amp was just to amplify the temp sensor signal to a more MCU friendly voltage, the 10-turn pot would set the gain from 85 to 200.I did some calculations and worked out that the resistor should be about 13K, i didn't find any 13k resistors so i put a 15k in and the station works perfecly fine, i have no idea about the accuracy of the display though, i might measure it sometime later.

author: Shock
author_rank: Super Contributor
timestamp: January 25, 2015, 07:47:40 am
content: A clone FG100 thermometer is about $10-$15 on ebay, would go nice with your repaired 936 copy to calibrate it.

author: Refrigerator
author_rank: Super Contributor
timestamp: January 25, 2015, 08:44:59 pm
content: Quote from: Shock on January 25, 2015, 07:47:40 amI've thought about that but i think i'll order a cheap 3 buck k-type thermometer, should be fine for some time.