issue with AKAI AA-1030 Receiver

Discussion in 'DIY' started by rotco, May 17, 2018.

  1. elnaldo

    elnaldo Addicted Member

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    The way to prevent a short-circuit is to use a "Dim Bulb Tester", an incandescent bulb in series with the AC line. If the unit is shorted, the bulb will light. If the unit is not shorted, the bulb will stay dim, or off, depending of the wattage of the bulb and the current draw of the amplifier at idle.

    I'd start another thread on the Rotel, posting a schematic and the exact transistors you found shorted, we can suggest other parts that could be damaged, so you replace all at once.
    DBT-short.gif DBT.jpg
     

     

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  2. rotco

    rotco Active Member

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    while using the "Dim Bulb Tester", i need to chose the wattage of the bulb carefully?
    so that the maximum current will be less then the ampere rate of the fuse?

    my country is 220v
    assuming the fuse is 2A
    applying
    p=v*i
    thus
    p=220*2
    so that the maximum bulb i should use is 440W?
    and the fewer is better?
    or maybe i missing something...

    thanks
     
  3. sregor

    sregor AK Subscriber Subscriber

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    You are on the right track, but you want the bulb to allow just enough current to go through so if everything is ok, it will operate, but if something is wrong, bulb will limit the current so damage is minimized. If no load, that usually is a lot lower than the rated fuse.
    Usually 60 to 100 watt bulb will keep it safe but allow it to work. Bulb must be incandescent. My practice is to start with 60, and then larger if not enough current gets through to get things running.
     
  4. rotco

    rotco Active Member

    Messages:
    131
    Great,
    Actually, i need this "Dim Bulb Tester" lets say just for the first 30 second to see if its reacting fine.. then i can go back with the receiver directly to the wall power.

    am i right?


    @elnaldo
    you're really helping me!
    i'm quite of new in this hobby, and your tips helps a lot!
     
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  5. sregor

    sregor AK Subscriber Subscriber

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    Pretty much. I usually do a little testing before taking it off the bulb. It needs to be off the bulb to run full power. The bulb is just acting as a current limiter for protection.
     
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  6. elnaldo

    elnaldo Addicted Member

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    +1, In small amps I use something in the 60-75Watt range, and leave it connected for a while until the amp produces sound. Or, if I don't trust the amp 100% but need more power, I fit a 100-150W bulb. It allows almost full power for a small amp, but will bright if there is a short.

    A bit of hum can be normal with a small DBT connected.

    Actually I have a switch to bypass the bulb, but I don't recommend this to a newbie. Many chances to make a mistake.
     

     

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  7. rotco

    rotco Active Member

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    Hi guys

    Im assuming that we use the bulb just to get pure resistance.
    I used an incandescent bulb in, On it its printed 60w.

    I used the mathematical formula
    R=v^2/p
    220^2/60=807ohm

    I measured the bulb with dmm
    I found its 62 ohm

    What is going on here?
    The bulb is working just fine while i test it.
     
  8. sregor

    sregor AK Subscriber Subscriber

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    Bulbs are not simple resistors. They are stabilized by the resistance going up as the filament gets hotter, so cold resistance is low, but as soon as current starts running through it, the resistance goes up. That is one reason why the work well as a protection circuit (current limiting). It's also why most incandescent bulbs fail (go open) when you first turn them on....It's also why incandescent bulbs are specified for dim bulb circuits.
     
  9. rotco

    rotco Active Member

    Messages:
    131
    Thanks a lot!
    But is is make sense ots goes up by factor of more then 10? (Actually x12)
     
  10. rotco

    rotco Active Member

    Messages:
    131
    Now,
    I think is something related to AC.
    I didthe calculations as if it was DC.
    I think its something of factpr of sqare root of 3.

    Never mind.
    I used the "Dim Bulb Tester" with this 60w bulb.
    And measured the voltage at the transformator outputs, ive got a reading of about 1VAC.
    (Instead of 22VAC without the Dim Bulb Tester)

    Now i need to raise the power of the bulb?
    I think you guys probably using a VR in series to the bulb instead of changing bulbs arent you?
     
  11. elnaldo

    elnaldo Addicted Member

    Messages:
    7,244
    Location:
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    is the bulb dim, or full bright?

    Even with a 60W bulb, I'd expect more than 1/2 of the rated voltage, I'd expect around 15V at a 22V secondary.

    If the bulb is not bright, if it stays dim after one second, the unit is not shorted and you can try full voltage and check that secondary again, or try a larger bulb. But if the bulb stays dim, the 22V winding reading 1V is not OK.

    PLEASE CHECK YOU ARE MEASURING AT THE AC SIDE, NOT THE DC SIDE OF THE RECTIFIER.

    I change bulbs when I need more current. I have a 53W, 100W and 150W.
     

     

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  12. rotco

    rotco Active Member

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    Hi @elnaldo

    As you recomend me, i opened a new thread for the rotel, in order not to intefer this thread.
    Please,i would really like if you could seewhat o wrote here and respons.

    Regarding yiur reply here, i just say i measured the 1VAC at the point before the fuse. Not on the rectifier.

    You can see on DIY the thread i open one hour ago named "Rotel rx-830 immeditly blow fuses"
    Thanks
     
  13. elnaldo

    elnaldo Addicted Member

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    Location:
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    Is the DBT test on the Rotel, or on the AKAI?
     
  14. rotco

    rotco Active Member

    Messages:
    131
    Yeah, i know i start to confuse you guys (and even myself)
    Thats why i opened a new thread for the Rotel:
    http://audiokarma.org/forums/index....tantly-burned-the-fuses.825048/#post-11664705

    The Akai here, the issue is in normal volume start to makes explosions noises, we agree i should replace the elc. Caps on the p.s board. I will update as soon as i will done with it.

    The Rotel has a short somewhere which immedietly blows the fuses.
    I checked all output transistors, they found OK.
    Next componnent to check will be recitifier as the guy on the Rotel thread recommend me.
     
  15. rotco

    rotco Active Member

    Messages:
    131
    Hi again
    Got back to the AKAI.

    I will remind you, that the issue is with explosion noises only while high volume in both channels at the same time and tempo/strentgth.

    I was already performed the idle current and bias adjustments.

    I measured at the speakers output terminals a low value of mvdc (about 5) in low volume, but when i raised the volume up, the reading jumped fast and changed alot up to 300mvdc in the same time and tempo i got the explosion noises.

    Is it help some how?

    P.s i still havent replace the caps.. Will do it soon. Maybe this new information of the DC intermittent existance at the speakers terminals bring us to better solution or other suspects? (Right now the suspects are the elect. Caps in the power supply board)
    Thanks.

    @mbz any chance you help me here, im stuck!

    Thanksss!
     
  16. sregor

    sregor AK Subscriber Subscriber

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    Did you try this? Also chop stick test - sometimes tapping on components with non conductive object will get you to the bad component.
     

     

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  17. mbz

    mbz AK Subscriber Subscriber

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    Like the guys have already said, tracking down intermittent faults without an
    oscilloscope is as hard as it gets. The "good" point is that you can reproduce
    the fault by playing bass audio at moderate/high volume.

    Yes, both channels affected does point to the power supply, but struggling to
    understand how this translates into "booms"/surges in output. For completeness
    you should check out the psu (see below). I would also make some tests on the
    power amp.

    Do you get "booms" if you select mono?

    For the following tests, need to play bass audio at low/mid volumes to see how
    the test point behaves normally, then increase the volume and see if any change
    in behaviour. Use AUX input.

    Monitor dc Voltage (black probe connected to GND/chassis) on power supply board
    pins 6, 11, 12, 16. Any significant change when "booming" occurs?

    Repeat measurement on pin 11 then 19 (STV-4H diode) any change when booming?

    As a guess, I'd consider replacing C4 and C1 on the power amp board, ie, maybe a
    gain or coupling issue but very doubtful. Almost like the loudness was switched on?
     
  18. rotco

    rotco Active Member

    Messages:
    131
    Unfurtunayley, not yet.
    I will do it very soon, thanks!

    @mbz yeah! You gave me lots to make a progress! Thanks
    I will do it and let you all know what is going on.


    Now, please, two things:
    1. I found this 3A fuse in parallel with other "normal" 3A fuse.
    Is it matter? The ahape of it and the thick conductive spiral in it.
    Is it "regular" or any special?i thought of replacing it to normal one.
    See pic:
    20180710_095446.jpg

    2. I thought of making a "level up" and upgrade myself with an oscilloscope.
    As i see here in that case, it would be easier if i had it.
    Plus, i have also AKAI AA-1040 which works "fine". But i notice a big difference of sound quality/strength between L/R channels.
    I thought it would be handy to help me figure out whats going on there.

    I found the top order numbers on AliExpress:
    MDSO ISDS205A digital usb scope for 61usd.

    http://s.aliexpress.com/bMRnmqyQ?fromSns=WhatsApp

    Any recommendations or disrecommendations?
    Other product?
    What do you say..

    Thanks
     
  19. mbz

    mbz AK Subscriber Subscriber

    Messages:
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    Location:
    Melbourne, Victoria
    Are you sure the fuses are in parallel, that is, they divide the current. Don't know "anything" about fuses but that
    would suggest to me that you effectively have a 6A fuse if they are in parallel. Your probably looking at the Left
    and Right channel speaker protect fuses, they are mounted side by side but are not in parallel electrically.
    For such fuses I'd be going for fast burn, the fuse in the pix is probably fast burn, find another.

    Regarding the oscilloscope, I haven't used that type, best to wait on others with first hand experiance or better
    still would be do an AK search or create a new thread, Best budget oscilloscope. Certainly they are adding more and
    more features and cutting price. You would want signal generation (sine, square), the spectrum analyzer helps and a
    crude distortion measurement would be nice. Some really nice ones for $400- but there should be a good choice in the
    budget range.
     
  20. elnaldo

    elnaldo Addicted Member

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    Never used one of those PC based scopes, but the problem I see is that you need a computer connected to it. If your work bench is very organized and you have a dedicated shelf for a computer, and everything wired there, you could use it, but I've found very useful to have a standalone digital oscilloscope like the "Hantek DSO5102P Digital Storage Oscilloscope 100MHz" (NO AFFILIATION), which is on sale in ebay for $208 now. Rigol is other very well regarded brand, Rigol and Hantek have both good reputation, similar models at similar price. If you plan to continue doing this, consider to get one of this. If this is a one time project, I'd try to solve it with a PC software scope as "VISUAL ANALYZER", (no affiliation), a PC software to turn your computer into a very powerful scope, signal generator, spectrum and thd analyzer and for sure much more functions that I never used. Only caution is to protect your sound card from high voltages using a 10X attenuation and DC blocking capacitors when needed.
     
    Last edited: Jul 10, 2018
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