Soldering Station Repair - Identifying Burnt Resistor

Thread Starter

pejayuk

Joined Mar 28, 2020
12
Trying to fix my Maplin N78AR soldering station. It’s pretty basic and uses a ‘dimmer’ type circuit to adjust the temperature. I first noticed that the LED power indicator was only on when the temperature was up to maximum, even then it was flickering. After running it on maximum for a few minutes it stopped working altogether.

As can be seen in the photos, the 2 resistors are burnt. The one connected to the caps is measuring about 26 meg ohm out of circuit. The other (R4) measures 1K in circuit.

I appreciate this is a very common and simple circuit, but being a beginner, I need help to identify the resistor value because the colour code bands have burnt away. The triac is a BT136 – 600E and the 2 caps are 2A683J. (They both measure 72nF).

Soldering Iron PCB.JPGSoldering Iron PCB_2.JPGSoldering Iron Resistor.JPG
 

Dodgydave

Joined Jun 22, 2012
9,951
Can't see the other resistor, better pictures of the component side would be better, also can you mark out where it goes on the track side, it looks like it comes off the MT2 terminal.
 

Thread Starter

pejayuk

Joined Mar 28, 2020
12
Thanks for the reply. The resistor you are looking at (R4) yes I think it goes between the track going to a terminal on the POT (Shown bottom in photo) and MT2. That one possibly seems like it might be okay as it reads about 1K, though I haven't checked it out of circuit.

The other one that I need help with can just be seen in the photo, it's tucked up tight to the cap on the right.
It is connected to one leg of each cap.

I will try to post some clearer photos and show resistors on track side.
 

Thread Starter

pejayuk

Joined Mar 28, 2020
12
This new photo shows the resistor locations better. (R3 is removed). I have marked them on the track side too. The AC live wire to board needs soldering back on.

Am I right in thinking that the purpose of R3 is to charge the 2 caps?

Soldering Iron PCB_4.JPG

Soldering Iron PCB_3.JPG
 

Thread Starter

pejayuk

Joined Mar 28, 2020
12
Hi,

I'm still looking for some help with this repair if anyone has any idea about what replacement resistor I should try. I've seen several similar dimmer circuits use a 4.7k resistor but I'm not sure about trying one incase it destroys the caps if it's wrong. Before I took the burnt resistor out, I measured 240V between one side of resistor and the neutral and between 6 - 8V on the other side. (Depending on temp adjustment). The iron itself was only getting 8V.

Thanks
 

Dodgydave

Joined Jun 22, 2012
9,951
Can you mark out where the Live , Neutral and heater goes on the circuit, then we can see how it functions, it looks like a dimmer circuit with the resistor in series with the pot.
 
Last edited:

Thread Starter

pejayuk

Joined Mar 28, 2020
12
The live comes into the casing and through a 1A glass fuse. It goes to the PCB top left (Marked AC Live). Where I have marked Live out, it goes from that point on the board via a connector block to the iron. Neutral in and out is both connected to the point I've marked and again goes out via the connector block to iron. I have also marked the wires connecting the power indicator.

Thanks again.

Soldering Iron PCB_3.JPG
 

Dodgydave

Joined Jun 22, 2012
9,951
This is the circuit diagram I can make out, looks like R4 is in series with the potentiometer , so try a 10K and for R3 try a 1K both 1/2W.
There may be a diode in series with the led.


IMG_20201206_200230_BURST1.jpg
 

Thread Starter

pejayuk

Joined Mar 28, 2020
12
Many thanks for the diagram. I only have some 1/4W resistors that size so I'll order some 1/2W and give it a try. Will post back with the outcome.
 

Thread Starter

pejayuk

Joined Mar 28, 2020
12
Checked diac and triac like this. DMM in diode mode. I get OL both ways for diac. Triac MT1-MT2 = OL.
MT1-Gate =0.122V (340ohms)
MT2-Gate = OL
 
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