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Soldering Station Repair

Discussion in 'Troubleshooting and Repair' started by Pejayuk, Dec 6, 2020.

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  1. Pejayuk


    Dec 6, 2020

    I'm 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. The voltage getting through to the iron is only 6 - 8V depending on temp setting.

    I need help to identify the resistor value because the colour code bands have burnt away. (I have marked the location of R3/R4 on the track side of PCB. The triac is a BT136 – 600E and the 2 caps are 2A683J. (They both measure 72nF).

    Thanks for reading this.

    Soldering Iron PCB_4.JPG

    Soldering Iron PCB_3.JPG
  2. 73's de Edd

    73's de Edd

    Aug 21, 2015

    Sir Pejayuk . . . . .

    I started on a prelminary rough schematic, but that thing has a whole bunch of wires coming into it that I need to know of their connection points to the board.
    Looking at the photos again, of which I have flipped the bottom photo over vertically to mirror the parts layout of the top boards components side.
    Right top corner I see incoming power wires as BLUE neutral 230, BROWN C Live and GREEN/YELLOW ? as equipment ground.
    Then I can see them move over to the set screw terminal strip where I carry forward my BLUE and BROWN SQUARES identifications of them.
    Actually that last GREEN / Yellow wire might actually be the sideview of a paired wire set. You can see 2 side by side wires at he center pic where I have two yellow lines drawn into the 2 YELLOW wires. Then on the bottom photo, you do not even see those wires, where I have them RED boxed, where they should appear.
    Fill me in on what they are and where they go to.
    Then in the bottom corner of the top photo I see a triple joined solder junction of DARK BLUE - MEDIUM BLUE - (AC NEUTRAL) and 1 small GREEN wire
    routed to a remote LED, then another small GREEN wire comes back from the LED and goes into a pad that then routes over to a lead of a 2W Metal Film power resistor. (That large resistor must be of a low ohmmic value. ) The other lead of that power resitor ends up at the corner of the board at the PINK circle.
    What is the wire color and end connection of that wire, at the pink corner ?
    I perceive of your 230VAC soldering iron having one its connections being made to ground thru that 2 W film resistor . . . . and its surfaces visible appearance shows of power consumption throughout the past..
    Next, look at the top boards bottom left corner for my YELLOW circling of a tinned wire stub in the center of 3 holes . . . . .then look a the bottom boards same area
    I see NO wire being in that hole but I did put in my to be expected , BROWN square.
    BECAUSE it does look like the LIVE AC would come in there and flow to the left of the AC power switch terminals being associated wih the temp adj pot.
    Aso I have two RED circles marking what I see as having no parts / wire connections coming to those pads. Are those just being vacant holes with copper surround pads that just had solder blobs placed atop them ?
    Thinking of it now . . . look at the CLOSE proximity of the 2W gray metal film resistor to the R3 assigned symbolization designator, is it posible it ( big grey) is being R3 and not your missing resistor that you have referred to as " R3 " ? . . . . or is another R numbered marking being at the BLUE diac side or at the other end of the big grey resistor for it ?
    In looking at the circuit design I actually believe the R4 metal film actually held up and is actually being a 1k unit.

    Preliminary thoughts . . .

    The interconnecting of the two pots is being in such a manner that the B500K linear pot is being transitioned into a quasi-logarithmic taper so that the resitance value can be either stretched or compressed to reflect with any calibration reference of the knobs pointer .
    There is being s o o o o much high resistance of the pots, who can see enough brute series power flow to be able to FRY those two small resistors and the DIAC .
    HOWEVER . . . if the temp pot was set with #2 terminal bottomed out against #3 and by chance trim pot was set with its #2 terminal bottomed out against #3 .There is a low ohms path.
    The later situation, also being possible if one op was diddling with that control, in thinking that more wattage could be available to the soldering iron in order to get more heat.
    Then it was being left in that setting.
    THEN if BT-136 fails with an opened MT-1 junction and a shorted MT-2 to gate. That leaves 230VAC into R4 and out to the low resistance end ranges of the two pots and into R3 and out R3 into Diac to the 40-50 ? watt soldering iron element and finally to line neutral .
    Should that be a possibiity, expect ohmmic testing to reveal open resiatance spots on the pots, when attemting a terminals 1 to 3 for a 500K ohms test.
    If they are both still compleely intact . . .it must be, that there is another fail situation.
    But it took some brute force power to FRY R3 and R4 . . . just look to the improbable circuitry tied into them ?
    A lightning strike getting to neutral thru C2, while plugged in ?

    My last thought would be that the soldering iron power connection are made from the not yet confirmed PINK circle corner to the DARK BLUE wire of the center board soldered trio of Blu-Blu-Sm Grn wires. Can you confirm that ?

    Thaaaaaassssit . . . . . . .talk to me . . .



    ( So . . .MODERATORS , do not host image on site, as of just yet . . . for now, I will hot link host . . .
    in order to be able to make any feed back corrections later. )

    73's de Edd . . . . .

    People who complain about the way the ball bounces, are usually being the ones who dropped it.

    Pejayuk likes this.
  3. Pejayuk


    Dec 6, 2020
    Many thanks for your detailed response to my question, all very interesting to someone keen to learn.

    I have uploaded another photo that shows the internal wiring. But I think you had already pretty much worked it out correctly. I should have explained that the live coming in from the mains cord to the board had broken off whilst I was investigating and needs to be re-soldered.

    The incoming live enters the casing and goes through an external 1A glass fuse. It does go to the PCB bottom left, where you have marked with a brown square. Another brown wire goes from your pink circle to the connector block, where it is joined to the live feed to soldering iron.

    Neutral in is connected to the point on the left center of board you have marked with blue squares. The second blue neutral wire goes to the connector block, then up to the iron.

    The two yellow earth wires are slightly different. One from the mains cord goes straight to the connector block. In the same terminal is the other short yellow wire which goes to an external socket, where for example, an earth wrist strap can be plugged. On the other side of the connector block, the earth wire for the iron is joined.

    The large power resistor measures 5.7K. (In circuit). I'm probably wrong, but I thought it was just for powering the LED power indicator. I can see underneath it is marked R5, so R3 is definitely the one I removed. (I have now included a photo of R3). R3/R4 measure 5mm in length, don't know if they are 1/4W.

    I have checked the track side and can confirm the two red circles you have marked never had anything connected to them.

    I don't completely understand what you said about the relationship between the two pots. However, I have had this item from new and this is the first time I have ever opened it up - so no meddling with the trimmer! Strangely enough though, I did measure the total power consumption not so long ago, as it didn't seem to be getting as hot as it used to. It came out at 36W as opposed to the stated 50W. Perhaps an early sign of failure.

    I can also definitely rule out a lightening strike. It's never been left plugged in.

    I have tried measuring the resistance of the temperature control pot.
    From terminal nearest trimmer - center terminal 1.2ohms - 141K.
    From terminal nearest trimmer - bottom terminal 1.2ohms - 141K.
    From bottom terminal - center terminal 0.06ohms (Adjustment made no difference).
    Also, non of these values changed if pot switch was open/closed.

    I also tried checking the diac and triac with multimeter in diode mode.
    I get OL both ways for diac.
    triac MT1 - MT2 = OL.
    MT1 - Gate =0.122V (340ohms)
    MT2 - Gate = OL

    Thanks once again for your help so far. If I have missed anything, please let me know.

    Soldering Iron Wiring.JPG

    Soldering Iron Resistor.JPG
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Have you checked the soldering iron for a short circuit or an open circuit? These don't last forever.
  5. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir Pejay in the uk . . . .

    Who could ask for more now . . . what with that all explanatory follow up photo !
    (Why fo' the sizeable molded in / purposefull hole in the very bottom of the case . . . . .possible . . . FLUX CAPACITOR overcharge drainage ?)
    NOW . . .are you being " electronically endowed ", enough to round up / scavenge / cannabalize some hook up wire . . . . even CAT-5 cable with
    its insulated copper wire strand sets.
    ( All it takes is a dark, moonless night, diagonal cutters, and a walk in the neighbourhood to come up with a sizeable chunk of same. )
    Und ve vill also be needink for und ~ 1 kohm axial leaded resistor, approximating the ~ wattage of your burn out unit .
    Inspection of your burnups photo of its spiral laser vaporization path relates to it as being of a low resistance value, as well as its flaky appearance, as to the instant voltage / wattage stress that it was once subjected to.

    from Sir / Herr Harald . . . der Kappster . . . he sez . . . .

    Have you checked the soldering iron for a short circuit or an open circuit?

    In AL L L L L L of my 80 years of encountering soldering irons failures, ( I dribbled my first solder droplets at ~ age 12 ) . . .they have all just gone open heating element, and not a one of them have developed broken internel wires within their insulation, from constant flexures metal fatigue ******. Nor, have I had any of them SHORTED out. In the emphasizing of SHORTED, note that you have a series fuse holder being inserted within YOUR soldering irons power loop . . . was it even blown ?
    ****** ( NOT so for Mamma Cass's constantly swinging Ironing cord(s) !) ( It has always been a gut splitting side show at that cords wires final intenal breakages and first occurance of ARCYS- SPARKYS AND BIG BANGS and the forthcoming shrieks and jumping around of the little lady ! ).
    (Seems likes . . . . . I initially married meselfs a "TWIGGY" . . . . . .now I gots me a "Mamma Cass" . . .sigh . . . .alas . . . . .) ( Dave ? do Philipinas seem to have that same proclivity . . .even a leeeeeeeetle bits ? )


    Start by cutting / taking the BROWN and BLUE wires of your soldering iron free from their presently soldered connections to the Pee Cee Board .
    Then use an ohmmeter to be sure that no short or low leakage path is being between the shared bare metal of the iron and the yelllow green ground wire . . . .over to the BLUE and then the BROWN power handling wires of the soldering iron.

    Then you bare enough copper from each of their ends to then be able to loosen, and take free, the presently installed PCB's BLUE and BROWN wires from the left two set screw Euro terminals strip connections, and install your soldering irons BLUE and BROWN wires in their place.
    Then you set up to be able to safely plug the power cord of the station into a live AC power socket . . . BUT . . . ONLY for just an initial quick in and out contact time of eggzackly one hunna and 'lebeneen milliseconds.
    ¿ ¿ ¿ It no makee FLASHEE-FLASHEE ? ? ?
    If not, GOOD ! then plug in and check to see if you don't have yourself a conventional full 50 watt iron again ? I am expecting the same.

    Next you track down a quite low wattage 230-240ACV INCANDESCENT lamp such as you will / might / probably find being used in/as your refrigerators internal lamp as the door opens.
    That should let you then take it and tightly wrap bared copper wire around its base and twist the copper tightly around itself at the end to make a "socket" connection for one hook up wire.
    The second hook up wire should solder onto the already present solder tit / blob being found on the lamps other connection.
    Solder a replacement 1k guesstimated value of resistor in place of your now missing / burn up resistor. Leave the " still hanging in there " resistor as it is.
    Now you have mere 15-25 watt variable load, instead of the somewhat heavy 50 watts loading of the previous soldering iron. PLUS it can give us a visual indication of what is going on.
    You solder the now " leaded lamp " onto the points of the PCB that the soldering irons two BLUE and BROWN wires were previously connected to.
    Then you can lift away and tape up the soldering irons BROWN wire UNLESS you are needing to use the iron in its limited JUST unplugged cool down, heat retention timeframe. (If you just don't happen to have another back up soldering iron to use . )

    MAKEE -TESTEE-TESTEE . . . . . .

    Turn the soldering station temp pots control knob to its power switches fully CCW . . . off position.
    Blip the stations power plug in and out of the AC power socket, with nothing being expected to happen.
    Then plug into power, to be able to quickly flip the knobs power to on position just for an instant. Then observe if the lamp comes on at full brightness . . . . for having a ZAPPED . . .TRIAC.
    Or no lamp brightness for an opened TRIAC . . . . . OR. . . . TOTALLY-TOTALLY-TOTALLY unexpected . . . . is low brightness at turn on and and then increasing lamp brightness as the temp pot is then progressively being rotated to its full CW position.

    Come back with your findings and we should then be able to attack the TRIAC and DIAC aspects of the circuits design.

    73's de Edd . . . . .

    Correct a wise man and you gain a friend . . . . even attempt to correct a fool and he'll bloomin' bloody your nose.

    Last edited: Dec 9, 2020
    Pejayuk likes this.
  6. Pejayuk


    Dec 6, 2020
    Hello 73's de Edd,

    Thank you for your step-by-step guide on testing my station. I haven't completed all the steps yet, but this is how far I got before needing to ask another question.

    I can see why you would have thought my latest photo showing the wiring was the bottom of the case, but actually it's the top! The large hole is where I removed the temp control pot.

    The element seems okay. It measured 1.52K (Live - Neutral). I also get OL readings from live and neutral to earth - so far so good.

    With the connections moved around, I did the quick test and..... well I'm still here! The iron heats up fine and is melting solder again nicely.

    I have got all the bits and pieces I need to proceed, including an old fridge 15W lamp and holder (I have a habit of keeping things I think might come in handy one day!). I just wanted to check with you regarding the replacement resistor. I have a selection of 1/2W carbon film types. Will they be okay, or do I need metal film? (Not that I know the difference). I think the 1/2W I have will just about fit, but probably a little bit tight. If I need to order metal film, perhaps I should get 1/4W?

    Thanks once again
  7. 73's de Edd

    73's de Edd

    Aug 21, 2015
    I think the 1/2W I have will just about fit, but probably a little bit tight.

    Don't even worry about passing its leads thru the holes,as this is just being temporary . . . for evaluative purposes.
    I , myself, would just get its leads well tinned and then solder tack it to the Pee Cee Bee's foil side, where those pads are already tinned.
    Yes . . . . .Carboniferus Filmus . . .speciae . . .will be satisfactory .

    I have a habit of keeping things I think might come in handy one day!)
    ! . . . . . . you thir (lisp-th). . . are my kinda guy ! (Electronics components stock inventory at 1,000,000 plus+ )
  8. Pejayuk


    Dec 6, 2020
    Okay, so I think things look promising. I tried the test lamp and not only does it come on, it also dims. The dimming range is somewhere between half brightness when first switched on, to full brightness when temperature dial would be on max. It doesn't dim right down to just a glow like in a standard dimmer, but I guess that might be because of how the temp dial is calibrated rather than another fault?

    Look forward to your expert opinion and what I should do next.
  9. Pejayuk


    Dec 6, 2020
    Hi Edd,

    Hope you are okay. I just wondered if you saw my last post. And, if you had drawn any conclusions regarding my testing with the temporary resistor and 15W lamp.

    The only further update is, when testing it, I've noticed the LED power indicator still isn't working.

    Thanks for your help

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