Power Amp IC Pin Identification? (With Pictures)

Discussion in 'Wireless & RF Design' started by Electronautilus, Mar 4, 2015.

  1. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    Hello! I'm currently working on a project that required me to utilize something called the Non-inverting Input of my integrated circuit. I'm pretty sure that it's power amplifier chip, despite it not having a heat sink, since it never seems to have overheating problems. Unfortunately, the particular piece of electronics that I'm working on is so old that there are no data sheets online available for me to use! I can't go connecting pins to things at random to try to figure it out, and I'm so new to this DIY thing that I have yet to figure out how to troubleshoot with a ground wire. I'm hoping that maybe someone more experienced might be able to help. I might have the wrong IC altogether, but the only other one is connected to the cassette deck portion of the system, as far as I can tell. This chip is connected to the speaker leads, so I'm pretty sure this is the chip I need

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    If you need more specific photos, just let me know! Thanks a lot!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    That does not look like an audio amp at all; more likely that is part of a receiver IF/Detector/Stereo Decoder?

    All I got from Googling "T900-BI" was a bunch of those ^%$##%^& web sites that put the word Datasheet on their web pages without having one....
     
    Last edited: Mar 4, 2015
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  3. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    That's very much possible! I can see that it's also connected to the radio antenna via some attenuation pots... Also, there's only one speaker for this, as it's not a stereo speaker circuit. I'm not quite sure how to identify a non-inverting input without a data sheet, however, I do have soldering and multimeter usage experience. Maybe you know what I should be looking for?
     
  4. kdcouture200

    Member

    May 22, 2010
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    Unfortunately just using a multimeter wont help you identify much more than power and ground connections. You could power the circuit and probe activity on the chips pins using an oscilliscope to figure out the pin config. With out a datasheet anything you do is an educated guess at best though sorry to say.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  6. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    Hey, that looks promising, Dodgydave! I'll be sure to check out to see if the specs match up from your data sheet to my chip- if all seems to line up, then perhaps I will cut the necessary traces for my project. I don't know the likelihood of things magically lining up between differently labeled components (my guess is about zero), but it's worth the time and effort! The worst case scenario is that I'll simply fry my plastic cheapy mp3 player with whatever I might accidentally connect to on the chip, then I can just repair the traces.

    (EDIT 5MAR2015) Also, I might make note that I don't possess an oscilloscope, though I am interested in procuring one or even building one out of an old CRT I have- I've been doing more and more extensive reading on the subject and hope to maybe repair and modify one that I picked up from Goodwill last year.
     
  7. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    Perhaps you can postulate more now that there's a bit more input on the subject, Mr. Mike?
     
  8. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    [UPDATE 08MAR2015]

    Thanks a lot to DodgyDave! I'm pretty sure I've identified the audio input pin for the chip. I've run into a couple problems though, causing me to doubt if I've correctly identified the input pin. The one that I'm pretty certain I've identified as being the input pin is the bottom rightmost pin in the first picture.

    I've been able to disconnect the native audio input from the chip, in order to avoid any impedance mismatching between my mp3 player and the original radio/cassette player. After that, I soldered a 0.1μ ceramic cap in series with my mp3 player audio signal, in hopes that I'd be able to get an audio input of some kind. Unfortunately, all I got was a lot of intermittent crackling and popping from my 8 ohm speaker. the speaker luckily hasn't blown out, to my knowledge. I also twisted in series a 10K resistor to my audio line, in hopes of somehow solving my speaker's problem, but it didn't do anything. I'm currently at a loss! Any suggestions?
     
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  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If its the same chip as i posted, then pin 9 is the audio input, and you need to input your audio from the volume control, it will have 3 terminals, use the end terminals for your audio input via a 10uF cap.
     
  10. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    If you are referring to a single gang pot for volume control, then that unfortunately is incorrect. The volume control for this unit is one of those vintage 1980s wall plate sliders, and it has something like four or five through-hole contacts to the board.

    Either way, I don't quite get your meaning. I can't use the native volume control due to the original radio/cassette signal running through it. The impedance mismatch might damage any devices I hook up to it, or so I've read. My intention is to attach my own pot to control the volume.

    Thanks for the 10 uf cap suggestion! I'll be sure to go get some in town today.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Ok then input the signal via a 10uF cap to pin 9.
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    low level audio signals will not damage anything with an impedance mismatch. at worst, they will either make the audio bassy or high pitched.
     
  13. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
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    Is it possible that an old clock radio would have a larger amount of impedance, though? The page I'm using as reference is here.

    Do you have any information as to why impedance mismatching would distort the frequencies like that?
     
  14. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
    12
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    ALRIGHT! So, I've been able to get a slight sound signal through my chip and out of my speaker!

    I attached half of my stereo audio signal to what is thought to be the mono input pin based off DodgyDave's data sheet. I had some problems with crackling and popping- I hadn't attached my GROUND connection! I looked at the data sheet again, attached the audio ground, and now the popping/buzzing is limited to when the portion of the audio jack is touched directly by my finger, or when plugging in the jack to a device. I'm assuming, in this case, I'm shorting the input to myself or the device, as it sounds just like when someone touches the input of a guitar amp.

    This is really good news! Or at least I thought it was, until I actually tried playing something. I got about a half second of scratchy, distorted audio output from my speaker, until the sound went completely silent. I don't think I burned anything, since I can still get my buzzing when I touch the contact, but for some reason the speaker now refuses to respond to any other audio input whatsoever. Any thoughts?
     
  15. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
    12
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    I've been looking around for 10 uF ceramic caps, and I'm not coming up with much, really. I'm pretty sure dielectric caps won't be able to do the job in this case, correct, given that audio signals are AC, correct? What am I to do?
     
  16. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
    12
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    I found a Radio Shack that had some electronics components left for me to peruse, and picked up a 10 uf tantalum capacitor. Not ceramic, so I don't know if it will work or not, but I hope it does! I really want to get done with this project, haha.

    I wish the Radio Shacks everywhere weren't shutting down so frequently!
     
  17. Electronautilus

    Thread Starter New Member

    Mar 4, 2015
    12
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    The tantalum cap works like a charm, so I mounted it with my resistor in a water bottle cap, and I'm getting ready to screw my assembly back together. Wow, I didn't think my project would take this long!
     
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