Advice regarding transformer for powered speakers (Studophile AV40)

Discussion in 'General Electronics Chat' started by dackjaniels, Jul 9, 2011.

  1. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    Hi,

    Im brand new here and hoping for some advice.

    As you might guess i'm not educated in electronics and only understand the very basics.

    My son has a pair of M-Audio Studiophile AV40 speakers which recently stopped working.
    Firstly the left speaker was knocked over and stopped producing sound but the right speaker continued to work. A couple of months later and the right speaker has stopped working. When plugged in and switched on the power light fails to come on and the speaker shows no signs of life.

    The two-way speakers are powered by an amp inside the left speaker that plugs into the wall socket (240V UK), an output from the left speaker then feeds the amplified audio to the right speaker. Each speaker also houses its own crossover circuit.

    My troubleshooting steps so far are (using a DMM):

    1. Checked fuse in plug (for mains wall socket) and it is fine.

    2. Checked fuse in left speaker (between mains input and transformer) and it is fine.

    3. Checked voltage at transformer input [primary] (fine reading around 244V).

    4. Checked voltage at transformer output [secondary] and it shows no voltage. This to me shows the transformer is faulty.

    5. Partially stripped the transformer and removed the thermal cutoff component and checked for resistance/continuity. The continuity test failed to produce a beep. The resistance test indicated an open circuit (DMM display read 1 before test and stayed on 1 when contacts were touched). It seems to me the transformer got too hot and blew the thermal cutoff.


    Now for some questions...

    a) Does my reasoning sound right so far?

    b) The thermal cutoff is an A4-F as listed here...
    http://www.aupo.com.hk/egproduct/af.htm
    As im based in the UK, where can I get a replacement (same or equivalent) for this component.

    c) Would it be easier to replace the whole transformer as im not confident I could solder the new cutoff with causing it to blow from the soldering heat.

    d) The label on the current transformer has this text...

    MODEL: BDT-57A178n(AV40) LF
    I/P: ~115V/60Hz WHITE/GREEN-YELLOW/BLUE
    ~230V/50Hz WHITE-BLUE

    O/P: ~14Vx2/1.4A RED-BLACK-RED

    SHENZHEN XINGYAODA ELECTRONICS CO. LTD

    The distance between the mounting holes on the PCB appears to be about 65mm center to center.

    I have tried searching for a replacement transformer but just get confused by the ratings, especially when it comes to serial and parallel figures I have no idea what to choose. It seems I need one that has two secondary outputs of 14V though im not sure if it would be 1.4A current in each one or if it means combined (0.7A each).
    Any help in this area would be appreciated, especially a link to a suitable replacement part, and perhaps comments on transformer make/quality if it may affect the quality of the sound output by the speakers.

    e) Finally, when a transformer overheats such as this appears to have done what might be the cause? Is it likely to have caused other components to fail (no scorching on PCB that I can see or blown/mis-shaped capacitors). Could the fact that only one speaker was working for some time have caused these overheating problems.

    If you got this far, thanks for reading.
    Eagerly awaiting your responses and thanks.

    Steve
     
  2. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    One more thing...

    I asume the transformer outputs 14V (x2) AC. Would the entire circuit run on AC or is there likely a rectifier on the PCB somewhere that converts it to DC? If so, what might it look like?

    Thanks
     
  3. RFactor

    Active Member

    May 1, 2009
    33
    3
    First let me say that your troubleshooting techniques seem to be good. If the left speaker was knocked over and stopped working, something may have come loose inside and is possibly shorting the output of the left channel. This could cause the eventual failure of the built in fuse or transformer. Are you able to test the transformer output on the hot side of the fuse?
     
    dackjaniels likes this.
  4. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    Hi RFactor,

    Thanks for the reply. Firstly, I noticed an error in my post above, it was the right speaker (the one without the amp internals) that got knocked over and stopped working first so i'll edit that now. I opened it up to check for anything obviously loose but could see nothing wrong, I also checked the speaker itself using an audio signal into it's inputs and it produced sound. I also checked continuity between the outputs on the left speaker (that feed the right speaker) and the inside where the speaker ports connect to the PCB in case the wire was yanked and damaged the speaker connectors but it seemed fine.

    When you talk about the hot side of the fuse I assume you mean the thermal fuse in the transformer? If so, by the hot side do you mean the checking the voltage on the secondary before it reached the fuse, therefore removing the fuse from the equation?

    Thanks.

    P.S. Can't see an edit button for my first post so it remains unchanged for now
     
    Last edited: Jul 10, 2011
  5. RFactor

    Active Member

    May 1, 2009
    33
    3
    Hey Dack,

    Yes, I meant the thermal fuse. What I was getting at is do you really have a bad transformer that may be expensive and hard to find or is it just the fuse or thermal fuse. When I said the hot side I meant (assuming the fuse is on the secondary) is the transformer bad? I haven't seen a transformer with a thermal fuse embedded but I have seen a few with regular fuses. In the states, thermal fuses are usually shaped like a bullet and are usually used in devices that produce heat such as coffee pots.
     
    dackjaniels likes this.
  6. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    Thanks for that RFactor. I will have another look later on today and see if I can test the transformer as you have suggested. Will report back...
     
  7. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    Update:

    I had a small problem where 3 of the wires going into the transformer primary broke off from the thin copper strands (coming from the coil(s)) to which they were attached. Only about 6mm of these strands was showing but I managed to solder and extend them so got them re-attached.

    Next I bridged the gap where the thermal fuse was located (now removed) with a piece of copper wire and carefully plugged the speaker back in and tested. When I flicked the switch a pleasant hum came from the transformer, the speakers power light lit up and when I attached my mp3 player music came out :) I then attached the second speaker and to my surprise it also bellowed music just as it should. Not sure why it had stopped working prior to this but it seems fine now.

    So the upshot is I need to replace the thermal cutoff fuse in the transformer with a new one. Would I be ok to just find a similar fuse with the same or very close specs or should I replace with the exact same make/model that was there originally (as linked in first post)?

    Also, any suggestions where I should buy this from (im in the UK) to avoid hefty postage and allow small order quantities?

    Thanks.
     
  8. RFactor

    Active Member

    May 1, 2009
    33
    3
    IMHO a fuse built into a transformer is not the best option, for many years fuses were separate from the transformer, one on the primary and one on the secondary. If for some reason the designer thought it necessary to put a thermal fuse integrated into the transformer then something is wrong with the design. For safety sake, you should probably replace with the same fuse if you can find it. Glad you got it going and good luck finding a thermal fuse. Here is a link to thermal fuse supplier in the UK
    http://www.maplin.co.uk/components/fuses/thermal-fuses
     
  9. dackjaniels

    Thread Starter New Member

    Jul 9, 2011
    6
    0
    Thanks RFactor, I'll take a look at Maplin's website. And thanks again for your guidance during my little foray into electronics troubleshooting :)

    Regards,
    Steve
     
  10. RFactor

    Active Member

    May 1, 2009
    33
    3
    You seem to have a natural talent for electronics, you should keep at it, it's a very interesting, challenging hobby.
     
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