TBA820m amp problems

Discussion in 'The Projects Forum' started by Stephen Flynn, Sep 29, 2015.

  1. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    [​IMG]
    [​IMG]

    Bought a couple of these little amp kits. I soldered one and it works good with a electret microphone. I 'm now trying to breadboard the second kit to make it more suitable for piezo disc pickup. My plan is to get the stock kit amp up and running on a breadboard, then experiment with various buffer circuits. Sounded easy, but for the life of me I can't get it up and running. Obviously fundamentally wrong with my interpretation of the schematic???
    included a couple of pics..hopefully someone more experienced can spot what I am doing wrong. Disconnected the battery terminals to take pic (in case anyone spots that :)??? I'm stumped.

    heres the data sheet http://www.st.com/web/en/resource/technical/document/datasheet/CD00000118.pdf. kit looks pretty close to figure 2.
    I swapped out the chip to rule that out, also done a continuity test on breadboard..it seems OK. Anyone got a suggestion.If someone could explain how I could test the amp with my dmm, and do continuity through the circuit. that would be appreciated too. For now I'll treat it as a good learning exercise.
    cheers
    Steve
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Steve
    What are the symptoms? Do you hear anything from the speaker??
     
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Check pin #6, I do not see a +V power link, also pin #2 appears incorrect.
    Its hard to tell from your rainbow choice of wire colours.;)
    E
     
  4. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    No real output from speakers. not even a crackle unless I play around with caps. Pin 2 in 33 ohm resitor and 100uf electrolytic going to ground. hidden behind the red lead.
     
  5. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    ericGibbs....YES I had hooked power to pin 7..my eyes are shot: now when I hook up I get wild screaming feedback from my piezo pickup and speakers attached. Not controllable with the volume pot???
     
  6. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    Update: piezo pickup doesn't have to attached...just acting like a tone generator..high pitched frequency continuously howling.
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi,
    I still do not see a wire in pin #6 on the amp, unless its hidden behind something else.??

    EDIT:
    You posted at the same time , so ignore this!
     
  8. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    limited to just a couple of actual breadboard leads..the wild colours are actually paperclips. I don't have access to any electronics stores.
    I measured the resistance on the paperclips and they were .9ohm...oddly enough i had some 0 ohm resistors and they have slightly higher resist than the clips..so I figured they would be ok to use here..a bit tight but they fit ok.
     
  9. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    yep you right..there was no power on pin 6..I have attached and now getting a howling high pitched tone
     
  10. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    It is possible by connecting +V to pin #7 has blown the TBA, try another IC.

    EDIT:
    I am still not convinced that pin #2 R1 and C2 are connected correctly.??
    upload_2015-9-29_9-35-2.png
     
    Last edited: Sep 29, 2015
    absf likes this.
  11. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    Heres a better pic. I have jumpered pin 1 to 5 instead of straddling over the chip with C3..

    Swapped out a new chip. Back to being dead? Hmmm...the wild oscillationing noise..would that have come from a damaged chip?
     
  12. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Hi.
    That R1 is short circuited by the board sockets.
    upload_2015-9-29_10-18-16.png
     
  13. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    what do you mean ericgibbs?? Pin 2 shows a R1 followed by C2.. Do you mean i have laid it out wrong, or is their something wrong with the BB?.
     
  14. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    R1 is not touching the pin 2. Still dead..rejiggled every component. This is my second attempt. Tried on a cheap breadboard, same result..thought the breadboard was faulty. Kinda giving up hope. Might be beyond my capabilities? Got no more chips ( just the 3) to try. Is there a way to check its functionality using my dmm. What info on the spec sheet should i look for?
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    As Eric pointed out. Both legs of R1 are in row 6 of the board. All the sockets in a given row are connected together internally, so R1 is shorted. Likewise the small cap in row 5 is shorted.
     
  16. dannyf

    Well-Known Member

    Sep 13, 2015
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    Double check your wiring - it is wrong.
     
  17. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    OK the penny has dropped, finally figured out what you meant by "short-circuited by the board sockets" ...duh :). That fixed it!!!! Amazing what a good nights sleep will do...Also the cap on row 20 was a short. Wow I really learnt something about the fundamentals of breadboarding... Thanks very much to all who replied, especially ericgibbs. No doubt I will have some more questions when I tackle the second part of my project..making the amp more suitable for capturing more bass frequencies of my ukulele via the piezo pickup. Its quite a tinny high-end sound and if I lay into the strings, quite a bit of distortion. Will now play with some buffers. If anyone has some suggestions on that subject..much appreciated.
    cheers
     
  18. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    Ok I have tacked this onto the breadboard, not getting great results...crazy distortions when turning the volume pot on the amp, there is a signal coming through on lower volume but slightly fuzzy. Can someone suggest a way to connect the output of the Tl072 R4 (220ohm)>10uf - - into the 1uf of the Tba 820m amp???
     
  19. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    I would try a 'volume/gain' pot from the TL072 to the TBA amp, say a 25K pot.
    One end of the pot to the 10uF and the other end to 0V, with the pot wiper to the TBA input cap.
    E
     
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  20. Stephen Flynn

    Thread Starter New Member

    Sep 29, 2015
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    Ok that sounds good. I don't have any 25k pots on had. Would a 10k pot work??? What would happen if a connected 2 x 10k pots in series?? Would that that get it close enough....would it even work..which one would you adjust??? Thanks very much.. When I get this working, will record and post a sample.
    cheers
     
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