Help identifying component

Discussion in 'The Projects Forum' started by themow, Dec 28, 2012.

  1. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    I have a bash 300 watt subwoofer plate amplifier.

    http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-750


    I had not used it for a year it was sitting in a closet. Last night I went to power it on and it blew its 3A fuse. I put another fuse in and it blew too. I pulled it apart and found what looks to be a burnt ceramic capacitor on the power supply. There is nothing written on it. I called parts-express to see if they had a schematic for it but they said they did not have anything and that the company never offered it. It was made by a canadian company called Indigo. They have since been bought out by a chinese company and still do not offer a diagram. I have searched high and low but have turned up nothing. I am turning here for help. I realize that its not an easy request but I'm hoping someone can point me in the right direction. Please let me know if you need any more info

    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,132
    3,052
    That might be a varistor that has absorbed a surge and failed to a short.
     
  3. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    Interesting I had not thought of that. Is there any type of test I can do to help narrow this down
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    What is the silkscreen label of the part? (Silkscreen is the white text with component labels, etc). It looks like "R" something, but I can't see under it.

    Do you have a DMM? Put meter in resistance mode, and one lead on each terminal of that physically damaged component. If resistance is low or zero, then that is part of the issue, but I wouldn't stop searching there.

    De-solder it and continue to check board for shorts.
    A closeup photo of the other side of board may be of some help, though fully tracing it will be difficult with the heatsink in place and generally bulky components "hiding" much of the board/connections.

    If the terminals J400 and J401 (silkscreen numbers) are where the power entered, that would be useful information as well.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    If it is a varistor. It will be part of the Input Transient Protection. And the device will be in parallel with the mains. You may take it out and your power supply may come to life(only for fault finding of course). However it could also be a NTC that is part of the inrush current limiting. Such a device will not be in parallel with the mains input. A failed NTC may also indicate bad caps (those black caps in the upper right corner in pic 3)
     
  6. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    The silkscreen under the part says R400
    Attached is a pic of the bottom of the board. The pencil is between the two leads.
    The part is reading 22.7 ohms however the longer i keep the leads attached, the lower it drops. Slowly. The caps do not look like they are bulging.

    J400 and j401 are where power entered.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    If the supply is 120V, the 33Ω would cause a ~3.5A or more current draw.

    The meter is changing due to the capacitors in the circuit. You'll need to take it out of the circuit to test the actual value. There is no doubt it is bad, and it sounds like a Varistor which is in place to absorb input voltage spikes. Unfortunately, they are similar to fuses, in that when doing the protecting, they destroy themselves.

    You'll still want to check the circuit with that component removed to see if the low resistance path still exists, or if you were measuring the capacitors charging.


    What is the intact similar looking disc (but yellow/cream colored instead of green) labeled? Rxxx as well?
     
  8. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    The round orange object is the VDR (symbol under it is for VDR). Round grey damaged object is a PTC device (burnt out) Is in series with one side of the AC feed in.
     
  9. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    Thanks for all the help so far guys. Much appreciated.

    I removed the burnt part. I can now see the some writing on it.
    Some of what was written is gone and can not be seen But this is what i can make out
    SCK
    04
    There was writing to the left of those numbers but its gone. They are the end of what was written. Removed, it is measuring 13.5 Ohms.

    The cream colored disc is has
    14n471k
    The some strange symbol then below that is
    8071 or 8074.
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    You have not give any answer to this (and that is kind of dumb). Is the device in series or parallel with the mains(power) input that is important. IF the device is not in parallel it is most likely NTC or PTC like those in this datasheet
    http://www.jinzon.com.tw/pdf/SCK_Series.pdf
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Well doing some work with Google. I found out that 14N471K is a Spiratronics 300V 80J Metal Oxide Varistor. Then your device has to be the inrush current limiter
     
    Last edited: Dec 30, 2012
  12. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    I dont know how to tell if it is in series or parallel with the main power input. Any more info I could provide to help?
     
  13. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    Heres what i thing the circuit is. And the PTC is most certainly damaged & it in series with the AC mains. Since the MOV isnt damaged then its probably OK as they usualy split when damaged. I would be thinking that the Bridge rectifier is shorted by what you have described. Or there is a short in the SMPS.
     
  14. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    This is a picture of the top of what you have labeled as the bridge rect. It tests at 515 or 1200 ohms on any combination of the leads except when testing on each end. That is zero ohms.
    It is labeled kbu 604 AC.
     
  15. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  16. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    If by each end you mean across the 2 outside pins, those are the +/- DC pins.
    If so there is a short in the amp or filter caps. That's sometimes shorted outputs if it has discrete output devices.
     
  17. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    Yes I mean the two outside pins. It reads zero ohms. What should I look for next (besides a new amp)
     
  18. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You might want to test with the rectifier removed from the pcb. If you still have a short across the pcb outside pins then you maybe want to think of a new amp.

    Just my opinion - Now day's it's frequently cheaper to just replace the amp unless you have experience in repairing electronic equipment.
     
    Last edited: Dec 31, 2012
  19. Relayer

    New Member

    Jan 1, 2013
    18
    1
    You can replace the PTC with a 4.7Ω 5 Watt resistor to do your testing with, but ONLY after you've found the short/s.
    You need to look at the chopper transistor (MOSFETs are mainly used now though), which is usually located against the heatsink, not too far from the chopper transformer.
    You may also need to look at any fusible resistors, as I'm certain one or more have gone open circuit. Fusibles usually have a gray body with the usual color coding bands and are around 1 to 2 watts in size.
    You also need to look at any diodes around the chopper transistor, especially zeners.
    Please let us know your findings.
    Regards,
    Relayer :D
     
    Last edited: Jan 1, 2013
  20. themow

    Thread Starter New Member

    Dec 28, 2012
    17
    0
    Are you confident that there is a short in the unit and that it was not hit by a voltage spike and the only damage is to the burnt unit? I only ask because I have limited experience in searching for shorts. And I hate to be the total newb on the forum asking tons of elementary questions. I see the chopper transistor but dont know how to test it. 2 of the 3 caps on the board have zero resistance.

    Thanks
     
Loading...