Schematics for JCD 908S adjustable soldering iron

[digitalDNA]

Hello,

Does anybody have schematics for JCD 908S 220V 80W adjustable soldering iron ?

Mine lately stopped working, nothing is being displayed on the screen.

I was using it normally the day before, the next day, it just didn't work.

No component seems to be blown.

The heater resistance is around 140 ohms, measured off-circuit, is this value normal ?

On the board, there's an MCU, the MS51FB9AE by nuvoTon (the company is new to me), but pin 7 (Vss) is only connected to a capacitor, I couldn't find where it also leads!

There is also an invisible burnt trace which should connect the gate of a gate-sensitive triac (BT136) to the MCU.

The weird thing is that the MCU's VDD pin is fed from a voltage divider coming from the mains line, despite the existence of a voltage regulator (XC6206) on the board.

Thank you in advance,
Roger

[digitalDNA]

Re: Schematics for JCD 908S adjustable soldering iron

Also notable to say that the heater has only 2 pins.
There seems to be no temperature sensor anywhere, so how does such a device really control the temperature ?!

[Lisa Su]

Re: Schematics for JCD 908S adjustable soldering iron

schematic

[digitalDNA]

Re: Schematics for JCD 908S adjustable soldering iron

Hi Lisa, thank you for this diagram, however, it's not the complete circuit,
I also need the part that shows where the MCU's Vss pin is connected, as well as to which pin of the MCU the triac's gate is connected.

In fact, I've followed some of the traces, and the only possible ones for the gate are:

pin 4 P2.0
pin 6 P1.7 (connected through R4 56 k to the regulators's Vss pin)
pin 11 P1.4 has PWM capability (PWM0_CH1)

Other pins connect to the +/- buttons, the display, etc.

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

check c1 capacitance,
useing caps like that never lasts long.

[digitalDNA]

Re: Schematics for JCD 908S adjustable soldering iron

the cap is good, as the voltage regulator is getting voltage on its input, and outputing 3.3 V

[digitalDNA]

Re: Schematics for JCD 908S adjustable soldering iron

The information I gave regarding pin 6 P1.7 was not accurate.

pin 6 P1.7 (ADC_CH0) is connected to R4 (56K) to pin T1 of the triac (BT136) and to current sense resistor R1 (1 ohm). R1's other end is connected to (Neutral, Q1.Vss, let's call it GND). Q1 is a voltage regulator chip (XC6206P332, 3.3 V regulator).

To be able to perform A/D conversions, and measure the voltage across R1, the MCU's Vss (pin 7) should be connected to ground. This connection is gone on my board, I will fix it.

Another melted connection is between the triac's gate and, I guess, MCU.P1.4 (pin 11) because it has PWM capability (PWM0_CH1). The other left pin on the MCU is P2.0 (pin 4) (nReset), so I guess it's not the one used to PWM the gate of the triac.

I will try to fix these issues when I have time and post an update here.

By the way, the output of the regulator Q1.Vout (pin 2) is monitored using MCU.P0.4 (pin 20) (ADC_CH5), and is also used to maintain the screen's pins 6, 7, 8, and 9 at 1.65 V using equal 470K resistors in resistor divider configurations between Q1.Vout (3.3 V) and Q1.Vss (GND).

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

be more concerned about the fact that it's dead.
you need to get it displaying something, even if it's an error about the iron.
does the mcu have power and ground?
does it use a crystal or resonator?

[csengh]

Re: Schematics for JCD 908S adjustable soldering iron

Also notable to say that the heater has only 2 pins.
There seems to be no temperature sensor anywhere, so how does such a device really control the temperature ?!

So JCD 908S has self-regulating heater or No?

Please someone to answer because I want to buy one, but without a temperature regulator (in other words without a stable temperature) I don't need it!

I also want to ask how to turn it off, because I can't see any power button like the previous models.
.

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

Hello. My soldering iron began to greatly underestimate the temperature of the sting. For example, to warm up the sting to 250 degrees, I have to set the temperature to 480 degrees. Temperature correction does not work. It seems to me that the problem is in the firmware. Does anyone have a firmware for this soldering iron? Just in case, I attach a soldering iron circuit. The diagram does not show the display and its contacts.

P.S. I apologize for my English. I wrote with the help of a translator.

[eccerr0r]

Re: Schematics for JCD 908S adjustable soldering iron

I highly doubt it's the firmware. More likely your sensor is bad or incorrect, or your heater element is on its way out...

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

Visually, everything looks good.
The heater was tested in a similar soldering iron. He works fine with him.
The power supply voltage of the controller is 3.3 volts.
The resistors' resistances correspond to their nominal values.
Which sensor are you talking about? There is no temperature sensor in the soldering iron. When the heater is heated, its resistance increases. As a result, the voltage changes at the point between R24 and Q2. By measuring this voltage, the controller determines the temperature of the sting.

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

i think pin2 on the mcu is for zero-volt detection.
your sense input is the voltage divider R17,R18 and R19 feeding pin20

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

i think pin2 on the mcu is for zero-volt detection.
your sense input is the voltage divider R17,R18 and R19 feeding pin20

I think otherwise. The zero determinant is pin 20 of the chip. The chip determines the temperature on pin 2.
But I don't understand yet why you wrote this? How will this help me fix the soldering iron?

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

because if any of the resistors have changed in value it will read the element wrong

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

I agree with you, but I wrote above that all resistors correspond to their nominal values.

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

all thats left is Q2 having some type of resistance inside or bad mcu or maybe c7 being bad so Q2 is not turning on/off fully?

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

Yes, I agree with you. I see a possible malfunction in these details as well as you do. But first of all I think the problem is in the firmware, because I can't save the adjusted temperature values. I added a link to the video below to make it clearer what I'm talking about.
https://youtu.be/lHvObFBjQ1U

[stj]

Re: Schematics for JCD 908S adjustable soldering iron

you could try to read it - see if it's protected.
https://www.st.com/en/microcontrolle...tm8s003f3.html

but if you dont have an st8 compatable programmer you may find it cheaper buying a better iron based on T12 cartridges

[Pjatruha]

Re: Schematics for JCD 908S adjustable soldering iron

I don't have an ST-link programmer yet. Buying it is not a problem, but you will have to wait for delivery.
I doubt that the firmware in the MCU will not be protected. But I can check it out. In general, I was hoping to find enthusiasts who would write their own firmware for this soldering iron. There is a forum in Runet where guys discuss custom firmware for a similar soldering iron.
CXG E60WT