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Discussion in 'Electronic Repair' started by mark g, Mar 18, 2013.

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  1. mark g

    mark g Guest

    I am in the process of repairing an audio amplifier that employs a TDA7295 as
    the woofer driver.
    The repair of the original fault, which was causing the TDA7295 and a TDA7265
    to be held
    permenently muted, went fine, but having got the mute control level working
    again, I find that I
    have no output from the TDA7295. It seems to be shot - for reasons that are
    however unclear to me
    - but I would appreciate a second (or more) opinion. The levels around the
    chip are as follows:
    Supplies: +/- 28V,
    Vin+: input signal of +/- 0.7V,
    Vin-: just a dc level of -9.6V,
    Vout: just a dc level of -24.8V,
    Bootstrap input: just a dc level of -25.6V.
    These levels remain the same regardless of the state of the MUTE input.
    STANDBY is always inactive, i.e. >+3.5V. Any suggestions on how this state
    could have arisen?

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  2. Phil Allison

    Phil Allison Guest

    "mark g"

    ** Those voltages suggests the + supply is missing on the TDA.

    Look for cracked pins, bad soldering, cracked PCB tracks etc,

    Or the IC was damaged during previous trouble or service work.

    If all still looks OK, stick a new one in.

    .... Phil
  3. N_Cook

    N_Cook Guest

    What is the simple, no powering, DVM ohms measurement between , separately,
    output and each supply rail ?
  4. The word 'around' might be significant. You need to measure the levels
    actually on the pins of the chip (make sure you have a probe that will
    do it without risk of shorting) because it looks as though your +ve
    supply isn't getting through.
  5. Phil Allison

    Phil Allison Guest

    "Adrian Tuddenham"

    ** Absolutely.

    Many scope probes have a simple accessory that shrouds the point so it will
    not possibly short adjacent pins on a DIL package.


    When troubleshooting, it pays to use a scope set to DC coupling first -
    rather than a DVM.

    Cos the scope screen shows what is really there.

    And I mean a REAL scope - ie an analogue one.

    .... Phil
  6. mark g

    mark g Guest

    responding to , mark g
    Thanks for all the above replies. I'm very grateful.

    The measurements I reported were made using the pointed tip of an oscilloscope
    and I thought I had been on pins of the chip, but I agree, the negative
    voltages do suggest
    the positive supply is missing.

    What I forgot to mention was that the circuit actually worked for a few
    Prior to, and after that brief period, the symptoms were the same, i.e. as
    described earlier.
    It does rather suggest a bad joint, doesn't it, but I couldn't sort it out.

    Anyway, I'll investigate again and let you know.
  7. Jamie

    Jamie Guest

    maybe you have latch up.?

    Some chips have problems if you over drive the input past the rails,
    they latch up and remain that way until you reset them or some how over
    come it.

    It appears that it has MOS technology in it, so that could be the

    A good test would be to just power it up but don't inject anything in
    it. See how long it'll sit idle.

    If it seems that is the case, you can put diodes from the rails to the
    input that is being driven to prevent the input from exceeding the rails.

  8. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** The yellow glue problem is at least 30 years old.

    Roland " Cube " series amps had problems with it and they were made in

    VCRs had problems it in spades as do many SMPS too.

    Class D amps used in high end line array boxes made by dB Technologies have
    it splashed about.

    The stuff does NOT melt and move around, it simply goes dark with heat and
    time and becomes conductive and corrosive to copper and tin.

    Long as it has no contact with metal, it hardly matters.

    Now, the colossally stupid Chinese spread it about on PCBs like jam on

    They have the exact same philosophy on these matters as Alfred E. Neuman.

    .... Phil
  9. Guest

    Been there, done that, didn't get the tee shirt. The picture on a 23"
    LCD TV was badly distorted. All voltage out of the power supply were
    spot on, as were the outputs of the LDO regulators on the main board
    EXCEPT for one in the area of the digital video output. The 3.3 volt
    output read 3.1. Checking it with a scope showed a clean input and
    nearly half a volt of noise on the output. In this case the fault was
    a pair of 100µF electrolytics on the output.

  10. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** In early Roland Cube amps, the reverb drive IC was a SIL power amp -
    with yellow glue all over the pins to stop them from breaking under

    One Alesis powered monitor speaker has the stuff all over a toroidal choke
    in the SMPS, to hold it firmly to the PCB. That choke gets hot and the glue
    eats right through the enamel - so you have to rewind the damn thing

    ** The same glue is pale yellow when fresh, turns light brown with time and
    heat and finally black like burnt toast.

    This looks like the stuff.

    Comes in a convenient sauce bottle.

    .... Phil
  11. Guest

    Plus and minus supplies OK right ?

    Negative voltage on the minus input right ?
    Not negative voltage on the plus input right ?
    Negative voltage on the output right ?

    If it was an OP AMP it would be bad.

    It IS an OP AMP.

    Also, since manufacturers are not in the habit of using split supplies without DC coupling, the woofer is either open or the fuses would blow or the IC would probably blow a hole out the case.
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