Connect with us

PCB for Dishwasher repair

Discussion in 'Troubleshooting and Repair' started by cjcharles, Sep 10, 2016.

  1. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    Hello all,

    I'm a newcomer here but this looks like an awesome forum so look forward to exploring more in the next few days!

    I was an electronics engineer (a long old while ago), but still do bits and pieces, so keen to have a go at repairing my dishwasher given it will save me so much money... Long story short, my Bosch dishwasher stopped and showed no signs of life at all, after I opened it mid-cycle. I got a very faint smell of burning electronics and no amount of fuse replacements showed any life! (it is annoyingly about 12m out of warranty but was annoyingly expensive).

    I have dismantled it down to the key PCB and have found a few components that look to have died:
    • 1 power resistor - seems to be 1GOhn, which sounds like a crazy high value to me and Id have no idea how you could blow this.... (would need a 32kV spike....).
    • A few small SMD components near a power regulator (SMPS?) device have also burnt out. These ones are harder to understand since I cant tell what to replace them with....
    So that leaves me with a few questions...
    Do you think replacing these components has a good chance of fixing the board (it was giving nothing at all before so some signs of life would be useful...)?
    What do you think could have blown the power resistor and do you think I should worry about it? Or put it down to age and bad luck after something else failed?
    Is there any way to identify what the small SMD components are, so I could have a go at replacing them?I can see them driving the big inductor just above it, so can imagine they will be important to fix...

    Any advice very very much appreciated!
    Thank you!
    [​IMG]
    [​IMG]
     
  2. Minder

    Minder

    2,806
    583
    Apr 24, 2015
    Looks like it might have a switching power supply, if so these are prone to fail in the power semi area.
    It is very seldom the very small SMT devices fail, apart from the devices mentioned, power resistors and relays are high on the list.
    SMPS power supplies can be tricky to fix, I have even seen short in the power inductors.
    M.
     
  3. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    Thanks @Minder
    So you would suggest buying a new board rather than trying to repair? Or just that I might need to replace the SMPS chip in the middle? (Ive no idea how to tell if that chip is alive or dead!) The SMD chips around it definitely seem to be blackened but I have no idea what they should act as (res/cap).
    As you can tell my enthusiasm is possibly higher than my experience, though hopefully I do have a fair bit of knowledge there somewhere...!
    Thanks again
     
  4. KTW

    KTW

    273
    14
    Feb 22, 2015
    Does the door switch function?
     
  5. debe

    debe

    228
    62
    Oct 15, 2011
    Sugest you search TNY264 data, theres several typical application circuits for that SMPS chip. There isn't a lot to them, & I would suspect the chip has failed to have destroyed the other components. Its probably easier for you to just replace the board.
     
  6. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    Thanks both, I cant tell if the door switch functions as it seems to go into another chip, but looking at it and listening to it carefully it certainly seems to click as I would expect. I will have a look at the circuit diagrams and see if they match the layout seen, though hard to see what the traces do underneath the chip itself so might be a fair bit of continuity testing i guess.

    If I were to replace the board, is there anything I could easily test on the other components that might have caused it? E.g. a motor/heater burnt out and that surge killed the things on main board?

    Thanks again for the advice!
     
  7. Minder

    Minder

    2,806
    583
    Apr 24, 2015
    I would also check any valves or motors (pumps etc).
    They used to be simple AC motors but things have gone a little exotic in recent years so the motor technology might not lend itself to easy testing.
    M.
     
  8. 73's de Edd

    73's de Edd

    2,579
    1,056
    Aug 21, 2015
    Sir cjcharles . . . . . . .

    To see if you might possibly be able . . .TODOITTOIT.

    Take METERING . . . . . place on ohms mode . . . . . . short probes together to see its manner of display . . . . . then.
    Move probes to the "Tiny . . .switch mode in a box " I.C. and meter between pin 5 and 2 /or/ 3 and confirm if that internal power FET . . . . IT GO BOOM !
    Dat dere mentioned grey metal current limiting resistor is a low resistance value.
    If you find "Tiny" with shorted internal FET. . . .that suggest why it is now open . . . . possibly with a FWB rectifier also associated with that failure

    Let's see what 'cha got now . . . . .


    ILLUSTRATION OF A ? able AREA:

    dishwasher PCB.png

    73's de Edd
     
    Last edited by a moderator: Sep 12, 2016
  9. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    @73's de Edd
    Thank you for that. I can detect no diode voltage between pins 2/3 and 5, but I do detect about 36ohms of resistance. Is this to be expected, it seems a bit strange for sure! Also it does strangely seem like the small SMD components are still working, though it could be external capacitance's which are causing the chips to appear not as open circuit.

    Having checked the inductor above it, it seems to show low resistance in both separate coils, implying they are not open circuit and broken.
    I also tried checking the power resistor which seems to show a large resistance (and not open circuit), but it is very hard to test - especially given my multimeter doesnt go near 1GOhm measurement.
    Is there perhaps a resettable fuse somewhere that may also need pressing?

    Thanks again
     
  10. 73's de Edd

    73's de Edd

    2,579
    1,056
    Aug 21, 2015
    Sir cjcharles . . . . . . .

    Did you give back info on the RED circled area . . . as I view it, it possibly just might be some ferrite beads on a wire.
    I CAN make out a 200 and 100 ohm surface mount resistor and a centered transistor.
    Notice that they make no effort to assign designations to any components on the board.
    Since this unit is typically being mounted in an enclosed " sarcophagus " for shock and humidity / water protection, one can only SEE the board when it is exposed . . . such as you now have it.

    SURELY that TNY unit is bad . .at 36 ohms . . . D to S and the open supply resistor that feeds the power supply for it.
    If you want to confirm the proper value of that open metal film resistor, try this.
    I am reading it as brown black brown and a gold or silver tolerance band. . . . with an insignificant very end band.
    That would make it 100 ohms . . .totally realistic.

    To test, take a single edge razor blade and VERY carefully scrape away a bit of the paint cover in the very center of the resistor.
    Take ohmmeter in a suspected 100 ohm range scale and touch one meter lead to the wire lead at either end of the resistor and then see if a reading is obtained with the other leads probe as it is touched onto the very center, of the just scraped area.
    If no reading, then move from one resistor end lead to the other , in reading to that central area.
    With the next paint scraping . . or another . . . if required.
    A reading should eventually show up from one of those ends to the center. Double that value and you have the value of the blown resistor.

    Here is how I am " reading " other areas:

    Main AC power input at A connector and then as activated, power relay B or B' feed power down to A' as either a motor or water heating element supply.

    AC line from A also gets line filtering with C series of X and or Y cap types. AC then passes thru dual AC inductive line filter for EMI / RFI at D.
    AC line from D makes its way down to J ( now open resistor ) passes thru and then needs to find its way to a usual used Full Wave Rectifier Bridge * to get DC voltage for the main B plus storage capacitor H.
    I can't see it * in the picture.
    Also you need to test K and L to see if they are two power diodes or ferrite shielded feed thru lines . . .which would test out as shorts.

    Back up at E is your shorted TNY which is relying on its B+ supply from cap H . . .I can read 100 ufd . . .is its voltage rating on up around 400 VDC ?
    If E was working, it would drive low voltage power transformer F which then supplies its voltage to be rectified and feed into G storage capacitor. That meets the low voltage DC supplies for the circuit board .
    Check the part numbers and give feedback on items M and N, as M looks like it could be a power dual Schottky rectifier, or and looks like N could either be a 3 terminal regulator or power FET.

    Need more picture coverage of P, as some SERIOUS " bidness " is going on over there, possibly power and adjustable speed control of main motor.

    BTW . . . . don't you realize that YOUR problem is deviating from the norm . . . . . with the USUAL failure mode, being one of the high current handling terminals on B , B' or the other like big relay nearby having developed into the condition as is being shown on inset A below..

    PCB MARKUP :


    Bosch Dishwasher PCB II.jpg



    73's de Edd
     
    Last edited by a moderator: Sep 17, 2016
  11. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016
    Here this might help.

    The burnt component seems to be a capacitor. I could remove this capacitor and check it's value if that helps.

    If by "power resistor" you mean the large grey resistor near the relay, i can tell you it's not a 1Gohm resistor, off the top of my head i believe it's a 100 Ohm resistor, but yours looks toast.
     

    Attached Files:

    Last edited: Oct 22, 2016
  12. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016
    Hey Edd,

    I hate to be contrary, as you're obviously far more knowledgeable on electronics than me, but this contradict my experience with PCB failure in household appliances.

    Rarely have I seen "dry joints" be the actual cause of a PCB issue. Most frequently it's SMPS problems, smoked transistors, or smoked relays (stuck shut more noteably). Once have i encountered dry joints on a transformer, but that was just the one time.

    I always check for dry joints on relays and power plugs, because i know it's a common failure, but i'm always disappointed ;-)
     
  13. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    Hello both. @73's de Edd @Maxwell Thank you so much for your help it has been really useful to do some investigation of my own. I have some updates to give you:

    I guessed that the power resistor was 100Ohms and bought some new ones, but when swapping them out they blew up instantly. I then tried guessing a few of the small components based off the example diagrams online but could not get any life back into it.

    I guess the next step could go one of two ways, one being repeating the resistor replacement and slowly plugging in cables until the resistor blows? Alternatively I need to do more detailed checking of components on the PCB to see if anything else is wrong with it.

    To answer some of your questions Edd: Both K and L are diodes (L with about 0.7V drop and K with about 1.1V). I cant see any dodgy connections on any of the soldering sadly, as yes that would be a nice fix!
     
  14. 73's de Edd

    73's de Edd

    2,579
    1,056
    Aug 21, 2015
    .

    Sires . . . . . . .



    CJ . . .Can the ORIGINAL film resistor . . .guesstimated to be a 100 ohm unit . . . .be tested by the half / half method I described.

    As for the BLACKENED . . " Is this smoked " . . . . . .YES! . . . . . It is being a 0 .1 ufd monolithic block capacitor, thanks be for Maxwells pic.
    It is used for shot noise filtering of the zener diode to the right.

    I read it possibly as a 6D but that seems too high and C-D9 seems too low . . . of a zener value.

    Maxwell . . . can you test your . . . hopefully good . . . .diode to see what its zener threshold is ?
    Hit me for procedure if required.

    CJ . . .can you check the Vf junctions of the units CW . . . (2N3906 family). . . . PNP transistor ?
    Hit me for procedure if required.
    As well as that just mentioned zener diode on your board. . . .mainly to just see if it is dead shorted . . . by virtue of that adjunct block ceramic capacitors condition.

    CJ just changing any part and plugging back in to test is going to result in the WHOLE board being like that
    " Is this smoked "
    It is IMPERATIVE to series insert a 60W incandescent lamp in place of the "100" 0hm resistor.
    THEN the lamp just glows BRIGHT, and limits power input, in case of a BAD power supply fault downstream.
    Otherwise, the power supply could start working . . .but . . . . with a reduced voltage output.

    This small cluster of discrete parts, is related to sending back info to the TNY of the voltage and current
    conditions of the power supplys output for tracking corrections.
    Plus I think that we already determine that you have a shorted power FET that is within the TNY . . .has that TNY
    been changed ?

    Has a model number of this Bosch unit been given ?


    " I hate to be contrary, as you're obviously far more knowledgeable on electronics than me, but this contradict my experience with PCB failure in household appliances. "

    Maxwell . . . .I was referring to HIGH current carrying areas on systems solder joints.
    That illustration was showing a ring joint in the very back that had cavitational ring erosion around it, then a closer lightly RED circled one with a more progressed condition.
    And THEN the final condition of high current having to flow through an ever smaller conductive connection and the arc out and blackening of the whole connection of the last illustration.
    This one is being on a relay to a high current pull motor or a heater element supply.



    73's de Edd

    .
     
    Last edited: Oct 25, 2016
  15. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016
    You need to fix the short first or it will keep blowing.

    No, the short will most likely be in the PCB itself. If the short is external then a more likely fault on the PCB is blown transistors that act as switches, like the more beefy SOT223 ones, or even TO220 (motortriac).

    The blown 100 Ohm indicates a fault in the SMPS, as Edd suggested, you need to check the TNY.

    I think the best thing right now for you to do is to get the PCB to power up, once it's powered up you can put it back into the dishwasher and start up a cycle, you can then check if it works and what works, if anything. The machine will produce error codes when trying to activate specific components in the machine.

    When activating a cycle the machine will first drain, the washing motor will be checked (measure water resistance by motor current draw to see if it's empty), then it will open the solonoid valve to let in water and the flow meter (reed switch sending pulses to the PCB to measure flow) will start to a measurement of the water, once full the machine will enter wash cycle and after a few minutes (on 30m program) the tablet dispenser will open and the heater relay will be pulled on. If it has passed this stage, it is relatively safe to assume the fix is complete, if not, read out error codes and/or determine at which point the cycle failed to determine the fault.

    The module is confirmed working, so it will be good, BUT, check picture below, should be more useful.

    Also, how did you figure out the capacitance value of the smd cap?

    Great info, thanks, i wondered about that myself.

    This PCB is used in many Bosch and Siemens models, only the software is different.


    Thanks Edd, makes sense, the one time i had to resolder a joint was on a high current PCB transformer.
     

    Attached Files:

  16. cjcharles

    cjcharles

    6
    0
    Sep 10, 2016
    You guys are brilliant, thank you so much for your help on this. I swapped out the cluster of blackened surface mount stuff and TNY264, and it no longer blew the resistor. It must have failed fairly catastrophically in order to blow that resistor as must have been a decent voltage spike...!

    Sadly I still dont have a working machine just yet, though I need to check my connections as my SMD soldering is never great. Hopefully that will be enough, for later this week when I next have time. Too tired now to keep going.

    Thanks again,
    Chris
     
  17. 73's de Edd

    73's de Edd

    2,579
    1,056
    Aug 21, 2015
    .

    Sir cjcharles . . . . . . .


    Can you pass to me . . .your units brand . . . .Bosch ? . . . . . and its model number ?
    Also can you now confirm the two output voltages out of the secondary of the switch mode transformer.?
    Also your zener diode , just to the right of the blackened out monolithic black ceramic capacitor, must have still been good.
    For the future benefit of any others can you pass us the DC voltage being present across it.
    Currently . . . . .rough estimating at 18VDC.
    You say . . . . .



    That's. . . . . due to the cratered output FET within the TNY

    73's de Edd

    .
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-