I did the teardown of the CSI 75W solder station as well as the Hakko 936. The Hakko of course had all analog circuits using operational amplifiers and potentiometers as well as a temperature sensor in the iron tip to signal back to a triac chip controller which would trigger the triac to push 24 V AC through the heater in the iron. The CSI 75W was all digitally controlled with an internal microcontroller which was reading the temp sense line and switching the triac on/off through an opto isolator IC. And because it had a microcontroller it was also able to have the programmable set points and programmable sleep and power off timers.
The workmanship found in both soldering stations showed quality construction, soldering and manufacturing. I did not see any sloppy messy joints or poor workmanship. Most items were quality parts with the exception of the JWCO electrolytic capacitors in the CSI 75W.
For a device that was trying to save cost at any expense I found it strange that they did not use the internal oscillator in the microcontroller. I don’t see the point of having an external crystal running at 8 MHz when you could use an internal 8 MHz oscillator in such a design. If you’re not doing data communication and other high-speed tasks then an internal oscillator is more than accurate enough timekeeper. Maybe they needed this because they knew they were going to have to program and calibrate the microcontroller after production and the accurate timekeeping was necessary for the programming communications.
Also, the S3F8S19 microcontroller they used was able to store data in a small section of flash so why they needed or felt they needed an external EEPROM is a mystery to me. And when you’re trying to save costs why would you include an ICSP plug header or any other such unused connector when simple pogo pin pads would be just fine for after production code updates or calibrations.
For more detail: CSI Premier 75W and Hakko 936 Solder Station Teardowns
