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Blown NAP 110 boards - what components to repair

Discussion in 'd.i.y.' started by grahamdocman, Mar 21, 2021.

  1. grahamdocman

    grahamdocman pfm Member

    Several years ago, I made the 'classic' mistake of either disconnecting or reconnecting the +ve and -ve leads from the PSU (Minicap6) (I can't recall which) BEFORE checking the PSU was fully discharged !!

    CONSEQUENCE damaged board and DC off set measured about 150mV ! (possibly higher as I'm working from memory).

    Now I've a little time on my hands I thought I'd try to repair them as they are currently useless, so I've nothing to lose.

    Any suggestions of most likely blown components or where to begin ?

    I dimly recall a previous thread on the subject, but despite various searches I can't find it.
  2. Gervais Cote

    Gervais Cote Predator

    I would replace all the tantalum and electrolytic capacitors at first and then verify every little resistors for traces of heat or incorrect value.
    Transistors might be damaged as well but they are more difficult to evaluate.
  3. MJS

    MJS Technical Tinkerer

    You probably removed the -ve supply before the positive and you've reversed biased the 47uF feedback tant. You probably only need to replace that.
    Mike Hanson and misterc6 like this.
  4. grahamdocman

    grahamdocman pfm Member

  5. misterc6

    misterc6 Wasted and wounded, it ain’t what the moon did

    Yes, that’s the feedback capacitor.
  6. MJS

    MJS Technical Tinkerer

    It also has the wrong polarity in that diagram.
  7. geoturbo

    geoturbo pfm Member

    Very newbie one.. What would be the best way to test transistors? And could it be done with transistors in the circuit?
  8. martinab2

    martinab2 pfm Member

    You can test the base emitter and base collector junctions with a multimeter. They should behave like diodes (for an NPN transistor base is anode and emitter and collector are cathodes - reverse for PNP). If a junction conducts both ways or not at all then there's a problem. Also check collector to emitter as you can sometimes get a short between the two (usually in power transistors). A good transistor won't conduct collector to emitter unless there's current flowing in the base emitter junction.
    In circuit testing depends very much on what else is connected around the transistor but you would normally see 0.7v between base and emitter for a transistor that's biased on.
    Mistrale and geoturbo like this.
  9. MJS

    MJS Technical Tinkerer

    If the amp just has a high DC offset in the hundreds of mV range then it's probably working fine. Any major issue will almost certainly cause it to go fully DC almost to the rail voltages. Like I said, low offset like this is usually due to leaky capacitors especially given the suspected reason for the fault.
    Mistrale and 337alant like this.

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