Surface mount resistor compatibility

Discussion in 'General Electronics Chat' started by RB2004, Mar 31, 2014.

  1. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Hi,

    I have a circuit board I need to repair, it has a burned surface mount resistor.

    On the resistor it has the number 1000 which I gather to mean 100 ohms with 0 multiplier with <=1% tolerance.

    is this item suitable?

    http://uk.rs-online.com/web/p/products/2232120/

    its 100ohms with <1% tolerance and same physical dimensions.

    only thing I cant understand is, its supposed to be 100 ohms if I am correct... but the existing other resistors with the same 1000 written on them im measuring 33.3 ohms on the multimeter which is a lower resistance.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,777
    1,103
    Measuring resistors in circuit is always problematic because other components can be in parallel with them and affect the measurement.
     
  3. AJMetal87

    New Member

    Mar 15, 2013
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    0
    I have found that usually when I come across a burnt or fried resistor that there are usually other IC's within the circuit that are affected as well too, so make sure to verify the integrity of the IC's in the seemingly affected circuit.
     
  4. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0

    Hi,

    The resistor in question was placed between the 2 legs of a capacitor, I have a feeling the capacitor blew, and that was possibly the cause of the resistor failing, im replacing the 2 capacitors on the board, so hopefully the rest is ok.

    on this particular board, I will post a photo later its actually quite basic, its a heater control board from a TVR Tuscan, mostly just resistors.
     
  5. AJMetal87

    New Member

    Mar 15, 2013
    21
    0
    Keep us in the loop.
     
  6. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Hi,

    here is a photo of the circuit board, as you can see mostly transistors and resistors.

    Only IC are mostly just the servo motor driver and the PIC chip with the programming on.

    DSCN4186.jpg

    Its a heater control ecu from a TVR Tuscan 2003

    Problem with it, was no voltage being output for the compressor. rest of the functions that im aware of worked fine... heater motor control, and temperature blending etc.

    Im no expert electronics engineer, but upon opening it, what immediately struck me was the slightly burned resistor between the 2 legs of the capacitor, and the fact the strip of ceramic that should of been underneath it was missing... the resistor had actually separated from the board, and when I nudged it, it fell off. So a clear problem. Rest of the board looks fine.

    So I thought id replace the 2 capacitors (on other side of the board) with equal value 1000uf 63v components, but of a slightly higher temperature rating 105 degrees Celsius instead of original 85 degrees Celsius... hope the ones I've used are ok, they are a lot smaller in physical size than the originals, but the specs are the same as above, ripple current slight difference but not much.

    then replace the burnt resistor, ordered a load of 1% 100ohm ones from RS Components...

    https://uk.rs-online.com/web/p/surf...22677633D4E4F4E45267573743D3734302D3930373926

    and also ordered a replacement tyco relay which is on the reverse of the board.... worth a try, I wanted to change the mofset but cant find anybody selling them, its apparently an obsolete part.

    But hopefully, 2x new capacitors, new relay and resistor will fix it.
     
  7. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Well, got the replacement resistor soldered on now, and 2 capacitors.

    Just got to try and remove the old relay, and install new one.. then hopefully all good :)

    Any tips anybody for removing the relay? ive tried copper braid to absorb the solder, but its still not free :(
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    1. There has been really obvious heat damage from a massive over-current fault. You could have a fault elsewhere like shorted wiring external to the module.

    2. This is automotive? The forum rules ban automotive modifications, although this might be considered a repair not a modification. You need to sort that out with the moderators. :)
     
  9. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Hi, module fitted to car is a 2nd hand unit which has been changed before. Has yellow breakers hard writing on it.

    Part was probably changed by a previous owner to repair the fault, but ironically the unit fitted was faulty (it has faulty not going written faintly on it)

    So I'm sure this particular unit was damaged on another vehicle. Breakers sold it in that state.

    Also to confirm this is not an automotive modification or upgrade, it is a like for like repair only and not to any safety critical system. All the module does is control heater controls.
     
  10. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Quick update, got the new relay installed :)

    So hopefully, 2x new capacitors, 1 new relay, and a 100 Ohm resistor should fix the problem... will try it as soon as I can and post back on the results.

    Really hoping all it was, is the burnt resistor / capacitor and that its all fixed now.

    If that resistor and capacitor is related to the air conditioning compressor after all, and was the cause of the problem.

    Then it could of blown on the vehicle it was removed from if the air conditioning compressor clutch coil went faulty.

    Bit of a bad design really, most vehicles have an independent fuse for the compressor clutch coil rated at 10amp... also, usually a separate relay sometimes in the fuse box, or an external box somewhere.

    TVR in their infinite wisdom chose to not do that, no external separate relay, and no separate clutch coil fuse.

    Instead they have 1 20 amp fuse which is for the entire heater control module, and if the clutch coil shorts out, chances are it will blow the module before it blows the fuse.

    Also, lack of an external relay, I think that the relay is the one I've changed as I cannot think of anything else to do with heating that would require a relay, as the only other thing the module does is control the heater flaps and vary the fan motor speed.. no on off functionality for anything other than the compressor... relay I removed had a higher resistance than the new one as well.
     
    Last edited: Apr 2, 2014
  11. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    I see so many posts from people trying to fix electronic things by simply
    "replacing the burnt components"
    The success rate must be frustratingly low for this activity.

    Yet few attempt to dissuade people from trying it?

    I find it rare that bad parts actually look bad, especially semiconductors and IC's.

    When a resistor burns, it's usually because there is a fault in another component, it's not the resistors fault, replacing it does not cure the original problem.

    The "shotgun" approach of replacing all the parts in a given function block works sometimes, but it's expensive and inefficient, there is still a high probability that it will remain broken after a lot of hard work and $$$$ spent.

    There is no substitute for really knowing how a circuit works and troubleshooting it accordingly.

    Thoughts?
     
  12. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Hi, yes you are correct and I agree with you 100%.

    I'm not an experienced electronics engineer, which is my defence.

    However, as the resistor sits between the 2 legs of the capacitor, it is likely the capacitor was the cause of failure.

    It doesn't sit between any other component, if it was infront of an ic then I would be inclined to believe that the other component was faulty and damaged it, but it doesn't, it just bridges the 2 legs of a capacitor which is why I thought maybe that was the cause.

    Also, there could of been an external fault on the car it was originally removed from which doesn't exist on this car.
     
  13. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Another quick update.

    As above, changed the 2 capacitors, a 100ohm resistor, and the relay.

    Connected the module up, and everything which worked before still works fine... so no ill effects from my repair.

    Relay also now clicks on and off when you press the Aircon button, which it did not do before... so another bonus.... and my repair must of fixed that. Improvement!

    Still didn't get power though on the air conditioning compressor pin though.

    HOWEVER, after another look at the circuit board, here's what I discovered,

    1) I know there is power to the relay, as it clicks on and off.

    2) the relay is powered from the same 12v supply source as the compressor.. it splits off, 1 way goes through the relay and is switched to power compressor, other way goes off through a resistor and powers the relay coil.... so if the relay is clicking on and off there is power to the relay and therefore nothing wrong up until the relay.


    So I tested between the output pin of the relay, and the connector pin for compressor feed... nothing... then worked my way back, tested between relay output terminal, and where the connector pin board joins the main pcb... bingo a connection.

    now, on that particular pin, the solder pad is on the reverse side of the board to the circuit track... so while the pin was connected to the solder pad, it wasn't connected to the reverse side of the board through the hole.

    Reflowed the solder, and added a bit more solder... now there is a connection between the pin and the output of the relay, and by a process of elimination, if there is power to actuate the relay, then there is power going into the relay as well... and now the pin is reconnected to the relay power should flow all the way now.

    Going to try it on the vehicle again tomorrow, but im confident that the electronics side is fixed now.

    No idea though why it should of become disconnected in that way though? maybe it never had enough solder in the joint to begin with and over time separated creating a dry joint.
     
  14. RichardO

    Well-Known Member

    May 4, 2013
    1,231
    382
    I once heard of a technician using the "quadrant method" of troubleshooting and repair. He would replace all of the IC's in 1/4 of the board and if that did not fix the problem, he would do the same for the next quadrant!
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I would have sacked him. And anyone else who went to the "Eenie meenie miney MO!" school of appliance repair. ;)
     
    Last edited: Apr 5, 2014
  16. BReeves

    Member

    Nov 24, 2012
    412
    64
    Like in this case, sometimes you really do not have a choice but to shotgun, replace the obvious/burnt or just guess. The manufacture will not supply a schematic or any service info at all. The module is not normally serviceable but with a little ingenuity like the OP showed sometimes you can trade time for the cost of a new module and get lucky.

    Good job on getting your issue fixed.
     
  17. RB2004

    Thread Starter New Member

    Dec 28, 2011
    11
    0
    Hi,

    Thanks to everyone who replied :)

    I resoldered the connection on the pin where it joins the PCB... and as above had a 0 resistance connection :)

    Plugged it back into the vehicle today, and all working now! really pleased with that.

    Obviously the resistor was burnt, and as people have said usually there is a cause, but couldn't be sure in this instance if there was a fault with the board, or if it was damaged on the previous vehicle it came from... as whoever removed it from the previous car had written on the module "Faulty - Not Going" yet it still somehow got sold on and fitted to a car.

    But because I gather the capacitors are a known problem with the units, and this resistor sits right in between the 2 legs of the capacitor... it seemed logical to replace the capacitors, plus the obviously damaged resistor.

    Relay may or may not of needed changing, but it is the air conditioning compressor relay and what I had a problem with... so I changed that for good measure, worth noting that the relay which I removed had a higher coil resistance than the new replacement relay... so there may of been a problem with that also.

    The components, Capacitors cost a grand total of £2.20, a strip of 50 resistors cost about £4, and a relay under £5 also... so it cost me £10-12 roughly to try and fix, vs £60 to send it off and get repaired with a 5 day turnaround excluding transit time in the post.

    And my gamble paid off, because I was really pleased when I connected it up that the relay was now clicking away which it was not doing before.... but then I became really frustrated that I still wasn't getting the 12v feed... while I had made a step in the right direction and fixed 1 problem there was still another :(

    So did some circuit tracing with multimeter... and discovered that the pin for the compressor clutch, its soldered from the reverse side of the track and had no connection, resoldered it and added a little bit extra solder... and after that a nice 0ohm reading.

    Plugged it back into the car today....and....IT NOW WORKS!!! so im really ecstatic about that.

    Module is not available new any more, and if it cant be repaired, its £225 for a reconditioned unit... so well worth the gamble.

    It is a reasonably simple circuit anyway, not as complicated as some PCBs.

    But I do agree that trial and error can be expensive and changing components because they look faulty doesn't always guarantee a fix.
     
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