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Bose CD-3000 tablle radio/CD player sending dc to speaker.

Discussion in 'Electronic Repair' started by David Farber, Apr 9, 2013.

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  1. David Farber

    David Farber Guest

    This Bose CD-3000 tabletop player was brought to my attention because
    intermittently there were popping/static noise was coming out of the
    speakers. It was quite loud. When I powered it up via the 12 volts input,
    the problem didn't seem so noticeable. I took it to the shop, opened it up,
    and noticed there were two car stereo-type speaker amplifier chips powering
    the speakers. Using these two datasheets, and I
    was able to figure out that the TDA7375 powers the left and right speaker
    and the TDA7396 powers the subwoofer. I checked the prices of replacing
    these parts and each one was less than $10. Since the pc board where they
    resided was difficult to access and make tests while powered on, I decided
    to replace the stereo amp chip. It didn't help. I figured then it had to be
    the other chip. I replaced the other chip with no improvement. Then I
    figured out a way to remove all the pc boards and cables and power it up
    outside the unit where I could make some meaningful tests. First of all,
    there was some small dc offset going to the subwoofer. I soldered a 330 ohm
    0.5 watt resistor across the subwoofer output terminal so it would have some
    load. It was about 340mv. Not great but certainly passable. Then after about
    15 seconds, the dc shot up to 12 volts and my bench meter showed a spike in
    current. This voltage swing delay occurs every time I power it on. If I just
    disconnect the subwoofer, the left and right speaker outputs are fine. A
    schematic would be of immense help here or perhaps someone has had some
    experience with this unit. Any and all suggestions are welcome.

    Thanks for your reply.
  2. N_Cook

    N_Cook Guest
    7375.html I
    Either that or a failing minor supply to an op-amp farther back, eg a small
    cap on one rail, is my guess. Either way requires tracing back from the
    input/s to the output device
  3. David Farber

    David Farber Guest

    I was leaning toward a conductive glue problem because there are a few wires
    that are fed from the amp board to the main board that are protected by a
    flimsy sleeve that is covered with what looks like a foam material. It was
    sticky to the touch. I cleaned it off from the parts of the pc board that it
    was touching but it made no difference.

    Then I replaced the two input caps going to pins 1 and 2 of the TDA. The
    originals were .47uF film type. I replaced them with .1uF ceramic type just
    to see if it made any difference. It did. Noise and popping stopped. Voltage
    swings stopped. And I even tested it for ten whole minutes. I'll give it a
    full burn in tomorrow but usually it wouldn't hold steady even for one
    minute, especially powered by AC. I think the AC was more of a headache
    because it put a higher supply voltage (15VDC) than my bench supply which I
    only powered up to 12VDC. Maybe not. Anyway, I'll replace it with the
    original valued capacitors if this problem stays away for good. By the way,
    the reference numbers of the caps are C62 and C64.

    Thanks for all your suggestions.
  4. David Farber

    David Farber Guest

    I powered it back on this afternoon. It lasted for about 30 seconds before
    it acted up again. Back to the drawing board for me. I'm going to see if I
    can figure out a way to just power up the TDA chip by bypassing the cpu
    power up signal with everything else disconnected and then see what the amp
    output does. It would be nice to know if the problem is on the amp board or
    coming from somewhere else. Would I need to put some dc on the two signal
    input caps to bias this correctly?
  5. Hi David
    I looked at the "standard test and application circuit" in the PDF
    link you sent and although there is no schematic of the inside of the
    chip the test circuit shows a 2200mfd electrolytic in series with each
    speaker. If your radio indeed has those then in theory there should be
    no way to read DC on the speaker unless the cap was leaking.
    (I'm assuming it's only one speaker you're having this problem on). In
    any case until you do resolve this I would temporarily put put a
    quarter or half amp 3AG type fuse in series with the speaker. 12VDC
    should be capable of wiping out a voice coil. I lost a nice 12" Jensen
    once when a 60 watt amp I was working on decided to unload 60VDC onto
    it. Now my bench speakers are fused. Some lessons come hard. Lenny
  6. I lost a nice 12" Jensen when a 60W amp unloaded
    As was the Jensen. <ahem>
  7. Guest

    According to that datasheet the only thing that can put DC on the output isthe input. If the input is capacitively coupled it would seem to have to be a defective IC but you have changed it which pretty much rules that out. So now what ?

    I say when shit like this happens look at the PC board. If there are no setting resistors for the DC level on the input, any leakage path will do it.

    I called Jethro bodine to cypher this out and we got 26 dB gain. well 20 dBis about 100, plus 6 makes it about 400. (approximately, fuckya about precision). The input impedance is 60 K, and since it a semiconductor and we ain't talking Mhz that means the input resistance is 60 K.

    Let's get conservative on the gain. It is say, 200. That means for a six volt output it would take what ? You have 14 volts on the board, a leakage resistance of six megohms could result in 1/whatever at the input. Multiply it back up by the gain and you got volts, not millivolts.

    Such leakage paths have many causes. Sometimes customers spill something init like soda, and after years it starts becoming conductive. Another big one is electrolyte leaking from the caps. This is another insidious one because sometimes you cannot see it and it (or the soda, or bigscreen CRT coolant) gets more conductive as heat and voltage is applied, and sometimes it reverses when turned and cooled off.

    Tell ya what, take a 10 K resaistor between pins 1 & 2 of that chip and seewhat happens.

    Either that, or take the thing apart and clean the PC board with acetome and alcohol. (acetone first, then HOT water, and then alcohol)

    If you replaced the coupling caps, it has to be some kind of leakage. thereis no bootstrap or any of that shit here. This is simply two OPAMPs with common feedback, there ain't no more.
  8. David Farber

    David Farber Guest

    Hi Lenny,

    I believe you're looking the link to the quad power amp datasheet (TDA7375)
    which turned out not to be the source of the dc output. That chip powers the
    left and right speakers. The problem output was coming from the TDA7396
    which powers the small subwoofer. I didn't test this with the speaker
    connected. I used a load resistor to monitor the dc output.

    Just to follow up on the progress since my last post, I isolated the
    amplifier board and hooked up a 14 volt source to power the amp and a 5 volt
    source to pin 8 to turn the amp on. I didn't hook up any input to the amp. I
    just sat around and watched the dc level out. It was stable without any

    Then, I followed the input wires back to the main pc board which is home to
    the AM/FM tuner, preamp, and everything else except the display board. I was
    led to pin 1 of a TL0748CN. The data sheet is here.

    I resoldered the connections in the vicinity, hooked everything back up
    again, and so far the popping and dc shifts are gone. It was playing for an
    hour today. The problem is, I don't want to just let it sit and play if I
    can't be within listening range for the reasons you stated earlier. I don't
    want dc going into the speaker or to damage any other components.

    Thanks for your reply.
  9. David Farber

    David Farber Guest

    The owner does live about a mile from the Pacific Ocean so I considered that
    there could be damage from the salt air but there was no evidence of any
    corrosion. The telescoping antenna does have some slight signs of pitting

    I explained in a response to Lenny (aka klem kedidelhopper) how I resoldered
    some connections in the preamp which feeds the main amp. So far things are
    looking good. If things go haywire again, I'll try that 10k resistor

    Thanks for your reply.
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