Hot-Air Rework Station Repairing

Last week, I switched on my hot-air station to do some soldering job. While I was doing other things, I smelled something. I noticed the smell is from hot-air nozzle. I switched off the station and checked that soldering nozzle is melted down due to overheat. Oop! what the hell is going on.

I quickly checked the problem. There is no air flow at the nozzle. But I still heard the air compressor  sound.

Hot air nozzle is melted down

Today, I tear down and find out the problem. My rework station is a cheap china made, LK852D brand. Here is what inside.

Hot Air rework station inside
Hot Air rework station inside

There are three boards, one step down transformer, one air compressor inside. There is nothing special, the right board is the controller board for hot-air nozzle and air pump controller. The left down side is the soldering gun controller. The left up side is a 7-segments display controller (3 digits display x 2 ).

hot air gun controller
hot air gun controller

This is hot nozzle controller and air pump controller. There are two Triacs, BTA136 , one for heat gun and one for air pump. There is also one IC, LM324 common opamp. The circuit is very simple. This is just Triac controller like a light dimmer. The Opamp LM324 is used to control Triac  with thermo-couple feedback, somehow comparator circuit like my previous soldering gun repair.

hot-airgun-repair (3)
LED Display and Soldering Iron controller

The upper circuit is 7-Segments display and the lower circuit is soldering iron driver. The 7-segment display driver IC is erased label. I don’t brother what’s inside these board for this moment.

The problem is the air compressor. There is no air flow. So, I removed the air pump outside the case and checked this. The controller of pump works well. The output voltage is measured and this can be controlled by the front panel knob. The noisy sound of compressor state that the driver is worked welled.

air pump take off



So, I open the pump by removing bottom side screw. This is diaphragm pump type. I quickly check that the both side rubber diaphragms are melted, leak  and damaged. Thus, the pump is not working well.

Air pump inside
Air pump inside


Damaged diaphrams
Damaged diaphragms


I cannot source locally the replacement part for this pump. But, I saw a guy is using aquarium pump in DIY hot-air gun.  The aquarium pump are also the same principle as the hot-air pump. Luckily,  I have this type of pump in hand. I bought this pump long ago to use with PCB etching tank but never use it. Good, now I have a good reason to use this.

aqurium pump


Opened the pump from down side, remove original AC cord and connected with driver output wires. This pump has dual output. I combine the two outputs easily to nozzle pipe like that. There is a air flow control switch beside the pump. I simply switch to high position.

Aquarium pump inside
Aquarium pump inside
replaced original AC cord and solder two wires
replaced original AC cord and solder two wires


Connect air pipes
Connect air pipes


Then sealed the two air output pipes inside the nozzle pipe with hot glue gun.

hot-airgun-repair (17)



hot-airgun-repair (18)

Attached the pump inside the case by cable ties.

Then, switched on and test the hot-air flow. It’s work. The hot air is flow out from heat element nozzle again.

hot-airgun-repair (19)



Finally results and  thought,

– The air flow is not strong as original compressor but it still work

– In most soldering case, the pump controller knob is required to set at 3/4 or 4/5 position. The air flow setting adjustment clearance is not so good.




Fixing and Reverse Engineering Cheap Temperature Controlled Soldering Iron

Last year, I bought a temperature controlled soldering iron from local supplier. I bought a hot air rework station soon and I seldom used this iron because new rework station also has a temperature controlled iron. I like the idea of this iron because of its design. It is not like others temp-controlled soldering stations. See the photo.

Here is close up. You can see inside circuitry.

It is just an iron and temp-control circuit is inside it’s handle.  No dedicated controlled station like famous Weller gun. There is a knob at handle to adjust temperature from 100~400 C.  Yes, it is made  in china. The price is very cheap, about 5$ (do you believe it or not). But the quality and performance is quite good.

One day, I used this iron and it was not working. So, I opened fixed it. Fixing is quite simple and I checked the fault within minute.

– Removed the screw and knob first.

– I checked circuit.You can see an IC, components and a Triac. There is no SMD devices or expensive, complex thing. I noticed that there are 4 wires output to heat element tube. Interesting, I first thought there will be no feedback circuit or sensor in such cheap tool. I was wrong. The red-blue pair is a nichrome coil and the white pair is a thermo-couple. It is a closed loop control circuit, unbelievable.

I started to examine the circuit.

The fault was nothing. The soldering joint of heat coil was dry. Soldering directly to a nichrome coil is not a good idea in such circuit. It should be crimped. But this is no room for this and I just do re-soldering the joint with acid paste flux (which can help to solder metal joint like nichrome, tin sheet ect.. Don’t forget to clean the joint after soldering, acid from flux may conduct your circuit lines). Then, the iron worked again.

I started checking the circuit detailed. The IC is simple common Op-Amp, LM358, low power dual operational amplifier. The Triac is MAC97A6, logic level triac. This means that this Triac can be directly switched with logic level, 5V.

The close up,

As usual, I tried to trace the circuit with the help of back light.

This is final circuit.


# Updated 21 July 2015 : Added the value of C and corrected polarity according to Nguyen Ngoc comment, Thanks.

– The principle of circuit operation is simple. The Op-Amps are configured as comparator. The Triac drive directly by Op-Amp2 via C and LED1. The thermo couple is connected to U1, non-inverting input. The preset VR1 is used to adjust the comparator reference voltage. When temperature is lower than preset, the output of U1 will be lowered than U2 non-inverting input, The output of U2 will be high state and switch on Triac to heat up iron. When temperature is reached to set-point and U1 output will changed to high and U2 output will set to low to switch of Triac.

– The C1,ZD1, D2 and R7 are a simple transformeless zener regulator supply.

-Although it is not a linear feedback controlled circuit, it works, at least for this type of iron. Quite a simple and working design. You cannot defeat china 🙂 .

– I haven’t construct this circuit yet. If you want to construct, I cannot grantee it will work. Do it at your own.  If any one have experience on this, pls let me know.

#Updated August, 2015.

“who care”(may be someone 🙂 ) shared this schematic with some useful calibration tips. Thank a lot. If someone has experience in these tips, pls share your experience also.