Help trouble shooting a faulty solder station

Discussion in 'General Electronics Chat' started by bance, Nov 22, 2012.

  1. bance

    Thread Starter Member

    Aug 11, 2012
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    Hi guys,

    I'm looking for some help in trouble shooting my solder station. It's a Duratool D00673 also sold as a Zhongdi ZD 916. The problem is that, when switched on the tip heats up but instead of stabilising at the set temp, it continues until it is glowing RED hot.

    At first I thought it was probably the sensor, but I swapped that out for a new one, and this did not fix the issue. The LCD works and reports the tip temperature but for some reason the control system fails to work.

    I opened up the box and inside are three circuit boards:-

    1. Power supply board
    2. Main board
    3. Push button board
    The push button board can be discounted as it merely contains the switches and no other components.

    The main board consists of some power supply circuitry, amplification circuitry (for the sensor ), an LCD driver and the micro-controller. There is a 2 wire connection from the handpiece, this gives a measurable and rising voltage as the tip heats up.

    There is a two wire connection from the main board to the power supply board, this gives a steady voltage until the temperature reaches the set point, when it drops away to zero.

    The power supply board contains a rectifier, a MOSFET and an opto-coupler for isolation.Whilst testing the two wire connection from the main board, I noticed that the Mosfet was providing power to the handpiece regardless of whether the control wires from the main board were connected or not. I think the problem lies here.

    Perhaps someone could explain how the power supply circuit works, or point me in the right direction towards resolving this situation.

    I have included some photographs, data-sheets and a schematic that I traced from the power supply circuit (this my first time at reverse engineering, so please forgive any errors).

    TIA Steve.
     
    Last edited: Nov 22, 2012
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Display the sch file on your PC and save it as a png file and post that. Even if you need to use pront screen.
     
  3. bance

    Thread Starter Member

    Aug 11, 2012
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    I think I did that....
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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    The voltage at the gate of Q1 should drop sharply when the tool reaches it's setpoint. If it doesn't, the optocoupler output is probably shorted. If it does, the MOSFET is probably bad.
     
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  5. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    There was an sch file when I looked at it. You must have changes it will I was reading the message.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    optocoupler is drawn reversed emitter/collector, what signals are on the opto led side?
     
  7. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    I would begin at the beginning.

    The schematic is incomplete. The mcu is reading the sensor and then the mcu responds by controlling the opto coupler.

    There is an mcu involved that is not being shown,

    I think you first need to see if the mcu is turning the opto coupler off and on at the appropriate times.
     
  8. bance

    Thread Starter Member

    Aug 11, 2012
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    @Dodgydave,
    You're quite correct the optocoupler symbol should be flipped, the pin numbers are also incorrect, but I used a generic symbol the closest I could get.
    The signal provided by the microcontroller is ~1.1V squarewave---- I've included a picture.

    @Spinnaker,
    Does this not imply that the MCU is functioning?
    I clearly show in the schematic that a microcontroller supplies the signal to the optocoupler.
    Are you a sailor by the way?
     
  9. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Just because the mcu "appears" to function does not mean that it is. You could have a input or output pin bad. Unlikely but it happens.

    That is why I said begin at the beginning. When the iron reaches it's intended voltage you should see the pin change state. What is the photo you posted. Is that the output pin of the mcu after the iron is hot?

    If so (and I am not expert) but it says to me the the mcu keeps trying to turn the iron back on even though it is hot.



    Yes I am a sailor though sold my boat a couple of years ago and don't get out as much. Plus my trips to the Chesapeake have dropped off too. My big boat handling skills are really getting rusty.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    lift the diode D1 out and see if the iron still gets hot, if it does then the mosfet is duff, if not then its possibly the sensor,which maybe a thermocouple in the iron body.
     
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  11. bance

    Thread Starter Member

    Aug 11, 2012
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    Ok,

    I removed the diode and the MOSFET continued to conduct, which must mean that it has failed short.

    I think I understand the circuit better now, the positive rail, all the way to the tip and back to the MOSFET holds potential, but has no return path and so cannot conduct.

    When the opto-coupler receives a signal from the microcontroller the LED energises and makes the circuit through the photo-transistor which in turn makes the circuit via the gate of the MOSFET (a domino effect) the potential now has a return path and the tip heats up.

    The micro-controller monitors the voltage from the thermocouple and in combination with some histerysis switches the opto-coupler on and off to maintain the tip temperature.

    Question 1 : What role does C2 play in this circuit?

    Question 2 : Will this be a suitable substitute for Q1

    Thanks all for your help.

    @Spinnaker
    I'm a sailor too, I was lucky enough to do a trans-Atlantic this year, only got as far as Cuba, didn't quite make it to the good ol' USA.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The first thing I would have checked was to disconnect the load (heater) and test the FET pins for shorted...

    Other common faults are bad sensor and/or wires (sounds like you eliminated that).

    You can test the output stage (ie FET) still in circuit by disconnecting it's control wire and manually 0v or 10v to it's gate and seeing if the heater is controlled.

    (edit) whoops missed that you already found the dead FET. The orignal P60N06 is a NFET 60A and 60v rated so any similar NFET should work. If the voltage from the R2:R1 divider is < 10v you might need a logic level rated NFET.

    It's also good practice to put a 13v 400mW zener across R1 (FET G-S) as this stops the FET being killed by any over voltage noise spikes etc on its gate.
     
    Last edited: Nov 22, 2012
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  13. oliglaser

    New Member

    Feb 2, 2010
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    Hi folks - I know this an old thread, but hopefully someone can help. I have a couple of these (faulty) ZD-916's just landed on my workbench.

    After a quick test, I found the output of the 78L05 regulator was at around 2.5V. The input was at around 16V.
    Looking at the power inputs to the control board, there is a 19VAC and a 7.5VAC line. Now what puzzles me is why the 19VAC would be rectified and used for the input to the 5V regulator. I have a suspicion they have got them the wrong way round.

    The TO-92 package has a Tj-a of 200C/W, so at this drop it's only going to manage 40mA at best before it shuts down, and since it appears to supply the microcontroller and backlight I 'm assuming it's pulling at least this (although the case is not hot to touch. So it's probably faulty (I have some on order)

    Anyway, if someone has a schematic, service manual or has one of these (working) stations and could take a quick look to see if the supply leads are the other way round I would be very grateful.
     
  14. oliglaser

    New Member

    Feb 2, 2010
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    Following up to the previous post:

    Being impatient, I just swapped one of the suspected faulty 78L05s with a TO-220 7805 and it seems to have fixed one of the stations. The other I managed to damage the output pad (looked like it had been hacked at before, or just terrible soldering in the first place) and the connection to the other side of the PCB is broken (which is unfortunately under the LCD so I can't get at it)
    The leads on the TO-220 package are thicker so I can't easily push them right through, so I'll wait till the TO92 versions arrive and use one of those for this one (hopefully I'll be able to wick enough solder through to get a connection with a thinner lead)

    It also seems the backlight is supplied from the negative rail coming from the 7.5VAC input, so just the micro and LM358 plus a TLV431 are on the 5V rail.

    So it looks like the supply wiring is probably correct, and it's just the 78L05s that are dodgy, although I'd still be interested in the original info requested if anyone has it (I can't quite understand the need for the 19VAC input - the iron is powered from another winding in case anyone was wondering)
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    They probably just used a cheap common commercially available transformer, and picked one with windings that were "good enough". The two low power windings are probably meant for 12v DC and 5-6V DC supplies.

    I don't like 78L05 much, if you have instances of them blowing I would replace with TO-220 7805 parts, provided they fit in the space provided.
     
  16. romanko

    New Member

    Jun 9, 2010
    3
    0
    Hi guys. I got replacement iron for this soldering station after it's gone for exactly the same and now after few minutes station permanently stopping heating the element until you don't push any of three micro switches on the front panel. When you push the buttons temperature is increasing and element is starting to heat, display showing. Temperature is still stops on 200°C. Do anyone come across with this issue?

    Best regards Roman
     
  17. elmachino

    New Member

    Apr 11, 2016
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    0
    Huge necro bump.
    I had the same problem on a ZD-912 (the solder/reflow station). My iron was going red hot.
    My problem came from a broken trace between the diode and the capacitor on the soldering iron power board.
    I fixed it by hard wiring the diode to the capacitor on the power board.

    Regards
     
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