Help with dehumidifier PCB - NO POWER

Discussion in 'Technical Repair' started by Mike Baker, Jan 10, 2018.

  1. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Hi All,

    Im guessing there is a problem on the power circuit on this PCB as there is no power.

    The fuse is ok.

    The chip on the top P-IC1 is a ViPer 22A - How do i test this with a multi meter?

    How do i test the relay with a multi meter?

    Any other things to test?

    Thanks
    IMG_4042.JPG IMG_4044.JPG
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    5,896
    1,381
    The fault with this type of circuit is very commonly that large grey rectangular capacitor at the top right of your photo of the top side of the board.
     
  3. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Cheers Albert, Unfortunately that is a new replacement as i already tried that fix :)
     
  4. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    the board is quite discolored around the Viper 22A chip which makes me think its got very hot....
     
  5. Lyonspride

    Member

    Jan 6, 2014
    126
    19
    ^^ Viper chips as a general thing, do run hot, but they do on occasion fail unexpectedly for no apparent reason.

    Check all the external inputs, one will be an interlock of some sort, the others may be a temperature switch, a thermal fuse or similar devices that need to be "ok" for that to operate.

    The 4 diodes on the back the PCB look like a crude a bridge rectifier, it's worth checking (very carefully) that your getting your mains x 1.7 (ish) out of that.
     
  6. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2

    Thanks for the reply,

    I changed over the chip for a new one today and still its completely dead.

    I am a bit nervous about testing 2 much with the multi meter when its got mains voltage in it..... lol
     
  7. Lyonspride

    Member

    Jan 6, 2014
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    Understandable, I do it every day so I guess i'm just used to it, it'd be nice to know whether it's getting power or not.

    Rather than trying to reverse engineer the PCB I would test components, diodes are easy with a multimeter in diode test mode, 0.2-0.6 volts one way, zero the other, short circuit is more likely than open. Everything else on that PCB i'd expect to be visibly faulty, except the chip and the relay. Take the relay out to test it (so as not torisk damage anything by applying voltage to the PCB), you might get lucky and find high contact resistance, but relay contacts are tricky without a low resistance high current meter, personally I would replace that relay simply because it's a brand (Zong Chuan) that I have experience of being unreliable..... I actually just realised I have a pile of them on my bench in front of me as I type this.

    If you want to test the PCB though, you need to emulate the inputs it would normally get, which means figuring out what is connected to it and what it should be doing.
    When I did Delonghi DEM10 early last year I pulled the whole thing apart and then reconnected everything with all the covers and the PCB enclosure missing, the annoying part was having to find a way to keep the collection draw interlock switch closed (duct tape) I can't remember what was actually wrong with it though, I think in the end it was something stupid like a loose fuse holder or corroded connector.
     
  8. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Thanks for your reply, so i have tested the diodes with my multi meter and have found some interesting results!

    D2( DC M7) and D4 (DC ER1J) have readings in both directions, D1(DC M7) and D3(DC ER1J) do not.

    So those diodes are shot..... Now where do i get some without spending a fortune on carridge or waiting weeks? IMG_4071.JPG
     
  9. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
  10. Lyonspride

    Member

    Jan 6, 2014
    126
    19
    Might not be shot, there could be something connected across them, if the diode tester reads 0.00v in both directions then they're shot for certain, but otherwise lift a pin and measure again.

    Also If you don't want to start prodding a live PCB with your meter, you could solder some wires to the PCB before powering up and then measure via croc clips, less chance of shaking, slipping, yelping and swearing.
     
  11. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    The 4 diodes on the back the PCB look like a crude a bridge rectifier, it's worth checking (very carefully) that your getting your mains x 1.7 (ish) out of that.[/QUOTE]

    Sorry to sound like a complete noob but how would I go about this with a multi meter?

    And you were correct that once lifted it was ok
     
  12. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    7,214
    1,190
    To test diodes, set your dvm to Diode, and measure across it both ways, (the Cathode has the Black lead to it) it should read 0.7V and open circuit the other way round. Ideally lift one end of the diode from the pcb to get better results.
     
  13. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Can you recommend a replacement relay for me please?
     
  14. LesJones

    Well-Known Member

    Jan 8, 2017
    1,295
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    Have you tested to see if there is any mains input to the board ? Are there any interlock micro switches such as water collection container presesnt or full ?

    Les.
     
  15. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Hi, yes there is mains power to the board and the water collection micro switch is working correctly.

    Thanks
     
  16. Lyonspride

    Member

    Jan 6, 2014
    126
    19
    Datasheet here:
    https://www.relayspec.com/specs/017775/835.pdf

    Relays can always be a bit tricky to source, as there's no real standard for pin configurations.

    Start with what you know, 8A 250VAC, 12V DC coil, and read datasheets. You could simply replace with the same relay, as they do appear to be easily available, but I would probably go looking to see what Omron or TE Schrack have to offer..... It's up to you :)

    Perhaps http://uk.farnell.com/oeg-te-connec...000/relay-spst-no-250vac-30vdc-8a/dp/1891654#
     
    Last edited: Jan 14, 2018
  17. Mike Baker

    Thread Starter Member

    Aug 31, 2017
    144
    2
    Thanks for this lionspride, Unfortunately the relay seems to be fine.

    The problem seems to be with the low power side of the circuit as we have good mains coming in.

    Any ideas where and what to start testing? Thanks
     
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