TENMA Rework Station Teardown

In a previous post I added a control knob to my Tenma 21-10130 Rework Station, but now I am taking a more detailed look at the controller board hardware inside paying particular attention to the microcontroller connections.

Removing the board from the rework station was a bit of a hassle, the screws at the bottom are particularly difficult to access. Eventually I had to unbolt the transformer from the case so I could get the screwdriver in, the transformer bracket catches up against a heatsink and capacitor so it cannot be completely gotten out of the way.

The board can be divided up into five sections, on the left is the mains power supply, with connections to the power switch and rework heater, along the bottom left is the 5V DC power supply for the microcontroller, top right is the control circuitry and connections for the rework heater and hot air, and bottom right those for the soldering iron. In the centre is the microcontroller and associated circuitry.

The rework heater, air pump and soldering iron are all controlled using triacs, these in turn are connected back to the microcontroller through optocouplers. The rework heater and air pump operate at mains voltage, 220V, while the soldering iron works at 24V, these are all using Alternating Current. Essentially the station is a collection of variable dimmer switches controlled by the microcontroller.

Wires Everywhere

There are ten connections to the controller board

Conn	Purpose
CN1	Mains in – from power connector
CN2	Power Switch
CN3	220VAC out to transformer
CN4	Rework Heater
CN5	Rework Air Pump power
CN6	Soldering Iron Temperature
CN7	Hand Key – Controls for Rework Wand
CN8	AC 9V input
CN9	Soldering Iron Power
CN10	AC 24V input

The connectors CN5 and CN6 are used to provide sensing for the microcontroller; one for the soldering iron temperature and another from the rework wand with the button controls, in cradle detect, and temperature sensing, there is also a row of five onboard button switches.

Microcontroller Control

With the multimeter in beep mode, tracing back the connections to the microcontroller took a couple of days.

The PIC19F916 microcontroller has 24 digital Input/Output pins which are divided into three ports of eight; RA0-RA7, RB0-RB7 and RC0-RC8. In the lists below I have shown the physical connection as well as the I/O port used.

The power control connections are to an optocoupler which in turn switches a triac:

PIC Pin		Optocoupler	Triac	Purpose	Conn
11	RC0	U6: MOC3083	TR1: BT136	220VAC	CN3
6	RA4	U5: MOC3083	TR2: JST139F	Rework Heater	CN4
16	RC5	U4: MOC3023	TR3: BT136	Rework Air Pump	CN5
7	RA5	U7: MOC3041	TR4: BT137	Soldering Iron	CN9

After much tracing of circuitry I found the triacs to be connected to the optocouplers much as shown below. Resistor values vary and on the microcontroller connection side the current limiting resistor is on the low side, pin two, rather than on the 5V line.

Optocoupler – Triac switch (from the MOC3043 datasheet)

There are five front panel control buttons which go low when pressed. Internal pullup resistors appear to have been used in the microcontroller.

Key	PIC Pin		Purpose
K3	24	RB3	Rework Power
K1	22	RB1	Up Button
K2	23	RB2	Down Button
K5	26	RB5	Set Button
K4	25	RB4	Soldering Iron Power

The two LCD displays, both are the same with seven connection pins with pin one at the top. The rework stations designers have not used the PIC’s built in LCD display functionality. The LCD panels are marked JRD90601A on the underside, I couldn’t find anything about this on Google.

	LCD1 Rework		LCD2 Soldering
Conn	PIC Pin		PIC Pin		Purpose
1	n/a				5V
2	n/a				GND
3	13	RC2	13	RC2	Data 1
4	12	RC1	12	RC1	Data 2
5	15	RC4	14	RC3	Data 3 – LCD Select
6	n/a				Backlight 5V
7	n/a				Backlight GND

At present I have no information about the LCD data pins, I’m thinking that RC1 and RC2 could be Data/Clock while RC3 and RC4 is for selecting the LCD to send data to.

The Hand Key connector CN7 for the Rework Wand with pin one to the left when looking at the component side with the key notches uppermost. I have not opened the wand as it is sealed closed with glue and I did not want to damage it.

CN7	PIC Pin		Purpose
1	n/a		GND for temperature
2	2	RA0	Rework Temperature – through OP07C op-amp
3	n/a		GND
4	27	RB8	unknown – no connection?
5	18	RC7	Up Button
6	17	RC6	Down Button
7	28	RB7	Select Button
8	n/a		GND for button controls

Other Connections

PIC Pin		Purpose
10	RA6	Buzzer – through Q2 (possibly a SS8550 PNP transistor)
9	RA7	U3: PC817 photocoupler – some kind of mains frequency monitor?
3	RA1	CN6: Soldering Iron Temperature – through op-amp OP07C
1	MCLR	Pulled high through 10K resistor – Master Clear Pin External Reset
21	RB0	9v AC monitor?

This concludes the examination of the hardware connected to the microcontroller, further work needs to be done through software and oscilloscope observations to see how the LCD displays, power controls (probably PWM), and temperature sensors work and what the 9V AC and 220V AC monitors are doing.

Circuits

Here are a couple of diagrams I drew up of the more involved sensor circuits while tracing things out. Values for the ceramic capacitors have been omitted as they are not marked on the SMD package. Both the rework and iron temperature sensors have similar op-amp circuits.