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what's this circuit thingy

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  1. #1
    Community Moderator phild01's Avatar
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    Default what's this circuit thingy

    I have a cheap amp that went bang. It still works on 12v but the 240v part is the issue. The yellow circled area of the board has what looks like a disintegrated gold inlay. It has direct connection to neutral and the AC input of the DB107 bridge rectifier via that squiggly inlay, The DB107 is rated to 700v and I don't know if that is the problem. I was going to replace it but want to understand the function of that squiggly inlay suspecting it might be either a resistor or a fuse.

    amp-br-rect-bott.jpgamp-br-rect-top.jpg

  2. #2
    Community Moderator phild01's Avatar
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    I just found what looks like a very similar circuit to what I have except I don't have the switchmode type coils.The squiggly thing in mine seems to be a crude type of fuse. Perhaps what I have is an unstable version of what the circuit should be.
    This is a circuit I found online:
    image_2021-11-22_175829.jpg
    and a pic of mine:
    amp-ps-pic.jpg

  3. #3
    Seasoned DIY droog's Avatar
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    Could be wrong but I believe the missing track does not connect to the bridge. From what I see the bridge is the connectors in blue.
    amp-br-rect-bott_1.jpg


    It looks like a feed from the blue neutral (Main supply ?) to the black neutral on the other plug, so might be something else on the red black connector is faulty ? what does it connect to ?
    There is also unused pads on the circuit either side of the zig zag track, my suspicion is that the pads are for a protection or filtering component, thermistor or choke and if fitted the zig zag track is removed.

    I do not believe the zig zag track will be anything other than just copper like the rest of the tracks.

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    Community Moderator phild01's Avatar
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    Thanks droog. Unfortunately what wasn't obvious are the AC in then to the power switch. The neutral is definitely connected to the bridge rectifier via the zigzag. You might have missed my follow up that shows an identical circuit and a fuse.

    From what I see the bridge is the connectors in blue.
    Correct.

    Perhaps I should scrap what is there as I think it is an abridged version of what it should be (being a cheap Chinese stereo amp). The amp works great so I might get another 240-12v module to replace what is there.

  5. #5
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    It does look like a printed fuse, there to stop the house burning down when any one of a number of devices in the chopper circuit fails. Without the mains input filter using this power supply is going to turn any house in an EMI generator; it certainly won't comply with electrical regs here.
    Before you speak, ask yourself: Is it necessary, it is true, does it improve on the silence? - Baba

  6. #6
    Community Moderator phild01's Avatar
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    Quote Originally Posted by John2b View Post
    It does look like a printed fuse, there to stop the house burning down when any one of a number of devices in the chopper circuit fails. Without the mains input filter using this power supply is going to turn any house in an EMI generator; it certainly won't comply with electrical regs here.
    Fair enough, already suspect the design is a dud and will bypass with something better.

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    Seasoned DIY droog's Avatar
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    What is the red / black connector?

    The circuit diagram also shows a choke filter on line and neutral that isn’t on the board so maybe the fuse isn’t either.
    The pads at either end of the zig zag coils also be for a fuse if fitted.
    While the zig zag will act as a fuse and vaporise if overloaded I still do not believe it is anything other than a copper track.

    Edit, removed comment as it was incorrect.

  8. #8
    Community Moderator phild01's Avatar
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    Quote Originally Posted by droog View Post
    What is the red / black connector?

    The circuit diagram also shows a choke filter on line and neutral that isn’t on the board so maybe the fuse isn’t either.
    The pads at either end of the zig zag coils also be for a fuse if fitted.
    While the zig zag will act as a fuse and vaporise if overloaded I still do not believe it is anything other than a copper track.
    Those RB wires simply to the power on/off switch. The leftover fragments certainly have gold rather than copper colour but yes just a copper track that might have been gold flashed!
    The circuit is one I found that looks identical, I just think in typical Chinese fashion mine was cheapened with deleted parts.

  9. #9
    Seasoned DIY droog's Avatar
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    Quote Originally Posted by phild01 View Post
    Those RB wires simply to the power on/off switch. The leftover fragments certainly have gold rather than copper colour but yes just a copper track that might have been gold flashed!
    Makes a bit more sense now but if you are relying on the circuit diagram to determine the components there is no indications of the switch, and there is no choke on either circuit board, don’t believe there is a fuse either.

  10. #10
    Community Moderator phild01's Avatar
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    Quote Originally Posted by droog View Post
    Makes a bit more sense now but if you are relying on the circuit diagram to determine the components there is no indications of the switch, and there is no choke on either circuit board, don’t believe there is a fuse either.
    Again that circuit is not mine, it is one I found to be very similar and with identical components except I reckon mine has critical components absent. There is no fuse on the board and I believe the zigzag is now established to be the fuse...nasty.

  11. #11
    Community Moderator phild01's Avatar
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    The amplifier is tiny and only 130mm deep. I would prefer to replace it with something smaller that is also an sd slot mp3 player, same as I have now but just can't find much to do the job.
    Doesn't seem too much around like that or I don't know where to look!.

  12. #12
    Je pense, donc METRIX's Avatar
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    Quote Originally Posted by phild01 View Post
    The amplifier is tiny and only 130mm deep. I would prefer to replace it with something smaller that is also an sd slot mp3 player, same as I have now but just can't find much to do the job.
    Doesn't seem too much around like that or I don't know where to look!.
    The sawtooth trace appears is a crude fuse, saved the manufacturer 2c by not having to put in a physical fuse / holder.
    There are calculators you can use to determine trace requirement to "act" as a fuse.

    https://www.qorvo.com/design-hub/des...ing-calculator

    I would be tending to ditch the device, it's a fire waiting to happen, you can attempt to fix it, but probably not worth the risk.
    If it's still working via the 12V socket just use that.

    What sort of amp is it, how powerful, I assume by the circuitry shown the specs for it wouldn't be too good, with high THD low SNR and poor crosstalk.
    If it's just one of those small amps, there are thousands of these on ebay,

    I would tend to go for a 12V one because this will allow you to bypass the fire risk of the dodgy 240v circuits used in these cheap devices by using a good quality SAA certified plug pack.

    https://www.ebay.com.au/itm/18505281...Cclp%3A2047675
    Mieux vaut prévenir que guérir

  13. #13
    Community Moderator phild01's Avatar
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    Quote Originally Posted by METRIX View Post
    The sawtooth trace appears is a crude fuse, saved the manufacturer 2c by not having to put in a physical fuse / holder.
    Without a doubt now...

    No, I have decided not to pursue a repair simply because the design is incomplete and just rubbish.

    On the other hand I am impressed with the power and how clean it sounds so will somehow devise an inconspicuous 12v supply, unless I find a better mini/micro mp3 amp.


    Unfortunately no sd slot for mp3.

  14. #14
    Je pense, donc METRIX's Avatar
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    Quote Originally Posted by phild01 View Post
    Without a doubt now...

    unless I find a better mini/micro mp3 amp.



    Unfortunately no sd slot for mp3.

    https://www.ebay.com.au/itm/22429713...cAAOSwH9tf6toH

    Lepy actually have good ratings, they are one of the better cheapy brands.

    Here you go, this one has everything except the kitchen sink.

    https://www.ebay.com.au/itm/22429713...cAAOSwH9tf6toH

    Or you can get the Chinese copy of a Lepy copy being Lepai

    https://www.ebay.com.au/itm/20206145...0AAOSwGh5Zw7Ka
    Mieux vaut prévenir que guérir

  15. #15
    Community Moderator phild01's Avatar
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    Quote Originally Posted by METRIX View Post
    https://www.ebay.com.au/itm/22429713...cAAOSwH9tf6toH

    Lepy actually have good ratings, they are one of the better cheapy brands.

    Here you go, this one has everything except the kitchen sink.

    https://www.ebay.com.au/itm/22429713...cAAOSwH9tf6toH

    Or you can get the Chinese copy of a Lepy copy being Lepai

    https://www.ebay.com.au/itm/20206145...0AAOSwGh5Zw7Ka
    I saw those but was dismissing everything that didn't have 240v, however zip, so yes, those linked ones are great but the shelf will also have an ugly 240 supply on show. The power socket is recessed and a plugpack/wart won't fit.
    Think likely I will buy the Lepai or Lepy then.

  16. #16
    Community Moderator phild01's Avatar
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    Hey Metrix, just bought the Lepy on AliExpress. Much cheaper with a choice to get a 3A wart. I suspect I can use a travel adaptor to extend the reach to the recessed socket, will see. $A35 free del.

    https://www.aliexpress.com/item/3277...685f3db2f72a15

  17. #17
    Je pense, donc METRIX's Avatar
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    Be careful of those no name imported plug packs they don't have any standards and they can be lethal (literally).

    https://www.sbs.com.au/news/woman-di...-electrocution

    If I get things that are 12V from ebay etc, I always get a local SAA approved adapter and never take one supplied from the overseas supplier.
    Local Approved ones don't cost that much, last one I got was $21 for a 3A.

    I was glad I got the approved ones, as I accidently overloaded one a while ago, I thought it was 5A, but grabbed the wrong one, it was 3A.
    Put some led strips on it that added up to 4.2A, after about 3 hours the led started to flicker, I thought there was a connection problem so turned it off.

    Back on the next day it was ok, but after a while started to flicker again, then everything went off.
    Went to the plug pack and it was quite warm, warm enough to have warped the plastic casing slightly.

    It was dead after the second go, I looked at the label and saw it was 3A, so it was running around 70% over spec.

    I would have been really concerned if it was a non complied one, it would have probably caught fire, and the plastic would have most likely been a non fire retardant type, so who knows what might have happened.
    Mieux vaut prévenir que guérir

  18. #18
    Community Moderator phild01's Avatar
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    Quote Originally Posted by METRIX View Post
    Be careful of those no name imported plug packs they don't have any standards and they can be lethal (literally).
    Yes, I stupidly optioned it without thinking...too late when I realised what I did a few hours later.

  19. #19
    Community Moderator phild01's Avatar
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    Can't believe I am replacing one dud power supply with another dud power supply

  20. #20
    Je pense, donc METRIX's Avatar
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    Quote Originally Posted by phild01 View Post
    Can't believe I am replacing one dud power supply with another dud power supply
    There is council e-waste drop off point not too far from you
    Mieux vaut prévenir que guérir

  21. #21
    Community Moderator phild01's Avatar
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    Quote Originally Posted by METRIX View Post
    There is council e-waste drop off point not too far from you
    Yes.

  22. #22
    Community Moderator phild01's Avatar
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    Darn it Metrix, you foiled my plan. I muddled things when I had a plan to interface the plug pack to the outlet socket to a travelling adaptor because the socket is recessed. I felt I could only do this with an overseas type plugpack. There is no room in the recessed socket and need a standoff for it.

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