Identifying Blown Components

Discussion in 'General Electronics Chat' started by mr_merlin990, Jan 16, 2014.

  1. mr_merlin990

    Thread Starter New Member

    Jan 16, 2014
    5
    0
    Hi All

    Someone accidently connected a 240v AC supply to my SESTOS Counter C2S-R-220 unit's 12v DC out terminals. Apparently there was a flash and the house circuit protection device tripped (cut the power).

    On opening the unit for inspection, I can clearly see damaged transistors, resistors, and some desoldered joints caused by the heat generated by the fault. All other components "appear" unaffected. I include a picture below (visible damage to surface mount components on bottom right hand side):

    [​IMG][​IMG][​IMG][​IMG]

    I cannot clearly see the markings for components Q5 (I can just make out 2T1), Q6, R19, and R20 (472 possibly).

    I have a new unit on order already and wondered if it was worth having a go at replacing these relatively cheap SMB components to fix this unit. A little project I guess...

    When the 240v supply is attached to the correct input terminals the unit starts up OK and I can access the menus etc. I suspect it is functionality like start, pause etc that is impacted by the fault because they need the 12v DC as supply. The unit makes a humming noise when I try to start counting as a result.

    Anyone think it's worth trying to replace the damaged components? if so, how can I tell which resistor and transistor to select as replacements for the ones I can't identify? I can tell that the resistors are 0603 size and the transistor is SOT-23 following measuring.

    It was me who blew it by the way :( a moment of complacency....
     
  2. mr_merlin990

    Thread Starter New Member

    Jan 16, 2014
    5
    0
    BTW, I know I can wait for the new unit to arrive and investigate myself but delivery is at least 4-5 weeks away and no where near as much fun :) .....
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Sometimes a nice big magnifying glass and bright lighting - from various angles - can allow you to read far more off a part than you thought was there.
     
  4. mr_merlin990

    Thread Starter New Member

    Jan 16, 2014
    5
    0
    Thanks Wayneh, tried that. The tops are completely blow away, zero chance of ever getting the info that way I'm afraid. Can't find any service type docs either #stuffed
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    The first thing would be to try to locate a circuit schematic.

    You say the counter powers up ok.
    What works and what does not?
    Do the IN, GATE and RESET inputs work?
    Does the relay function work?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Can you post a clear photograph of the bottom side of the board?
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Get a schematic, then replace Q5, Q6, R19, R20.

    Fortunately they look to be separate from the rest of the circuit and the fault current path was through them, into the main ground plane. So hopefully that means you only need to replace those 4 parts and run two wires where those two PCB tracks were blown away (you might have missed the blown track on the GND terminal, or maybe it's ok).
     
  8. mr_merlin990

    Thread Starter New Member

    Jan 16, 2014
    5
    0
    Thank you THE RB and MrChips. I had thought that the fault path had been contained to those components too. I might be lucky. I have written to SESTOS requesting a schematic, no reply as yet so still have no idea what Q5, Q6, R19, R20 are specifically. I had planned on using a small jumper wire to bridge the damaged tracks on the PCB and provide connectivity to the terminal on the back of the unit again. Ground seems fine even though it looks damaged. It passed a continuity test as expected. Lets see what SESTOS come back with, if they ignore me, I might have a stab at reverse engineering and drafting my own schematic - might be fun! Thanks Guys.
     
  9. mr_merlin990

    Thread Starter New Member

    Jan 16, 2014
    5
    0
    MrChips - The unit powers up, I can configure all the settings via the menu but I can't get it to start counting down. It just sits at the configured start time. I can't test the gate, reset etc because of this but if I set the unit to start counting down from "0" then I hear the relay change state with the typical click noise. Thanks for your help.
     
  10. Richie121

    New Member

    Jan 12, 2014
    23
    4
    I see a couple of vias between Q5 & R19 and I presume GNL is the ground plane, so you can check GNL is the ground plane on the other side of the board.
    It looks like the components affected are just to handle a low voltage supply from 12v and step down to the working voltage of the main chips on the board.
    Have a google for standard circuits that do the same job with two transistors (or 3 pin components) and try to match the layout of the components you have to the circuit examples you find.

    I find this a useful way to identify what should be there based on what needs to be there in order for it to work as designed. Although there are many ways to achieve the desired effect, it is usually ony a choice of less than 5 ways of doing it - many of which appear on data sheets of major components.
     
    Last edited: Jan 18, 2014
Loading...