What's happening with this circuit?

Discussion in 'The Projects Forum' started by hooger, Jul 25, 2012.

  1. hooger

    Thread Starter New Member

    Jul 25, 2012
    4
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    [​IMG]
    Hi could you please tell me what is going on here?...the red voltages are what I have measured.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    Apparently output pin 2 of IC303 is at a logic low (0V) so the base of Q707 is also at 0V and it is turned off. This means there is collector current through Q702 and thus no base current through Q701. Q701 is therefore also off so its collector voltage is zero (you don't show the rest of the circuit so I have to assume that Q701's collector load goes to common). IC701 is a 10V linear regulator which provides 10V (9.9V measured) from the 29V supply.

    Does that answer your question?
     
  3. hooger

    Thread Starter New Member

    Jul 25, 2012
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    0
    Q701 goes to a box fan...the protection circuit is switching the sub woofer speaker off(via a relay)as soon as the power is switched on.
    I have checked all the voltages and speaker,wiring,fuse,solder,connectors etc. and this was the only bit that was different from what I expected.
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Are the printed numbers with the arrowheads supposed to be the expected voltages on those nodes (under some specific set of conditions)? If so, the I don't understand why 15.0 is shown on the output pin (pin 3) of the IC701. It's suspicious that it is 10V below the assumed 25V on pin 1.
     
  5. hooger

    Thread Starter New Member

    Jul 25, 2012
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    Yes as far as I know those are the test voltages at power up,I tested all the other arrowed voltages and they were within 10% of the arrowed value.
     
  6. Skeebopstop

    Active Member

    Jan 9, 2009
    358
    3
    There really should be a pullup between base and emitter of pnp as leakage current of q701 might get amplified.

    Ic301 could in theory as such be overloaded but unlikely. Seems the 3 terminal reg might be damaged?
     
  7. hooger

    Thread Starter New Member

    Jul 25, 2012
    4
    0
    Yes you are correct!...IC701 had damage,replaced and everything works fine.
    Thanks!
     
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