Just had a few minutes to reverse engineer the chip connections on my 898D clone (meta-clone?).
It's a 20 pin chip, no known data on it but it's at least a micro, not just a 7 seg display driver/digital voltmeter.
Vss is on pin 1, Vdd on pin 20, that's similar to the Microchip layout, I don't know about the AVR chips.
Pin 11 is ADC goes through a 10k to pin 6 of OP07 clone which would be the opamp output.
Pins 2, 3, 5, 7, 13-19 drive the 7seg display (SM310401k)
Pin 6 goes to the opto driver.
Pin 10 goes through SD8050 + S8050 -> TIP122 to fan.
Pin 8 goes to button up AND to SCL line of the EEPROM
Pin 9 goes to button down AND to SDA line of the EEPROM
Pin 4 goes to the hall sensor in the handpiece.
The eeprom is a 2k one, I assume it only holds the last set temperature but I have not read it yet.
If anyone has any idea what kind of microcontroller this would be please give me a shout: MK(?)1841D3 C1Y21319, 20 pin PDIP, VDD pin 20, Vss pin 1, 5V, ADC pin 11, possibly I2C on pins 8,9.
Drop-in replacements from Microchip, MSP430 or AVR would also be nice.