SA 9500 Mk I intermittent crackling on LH channel - advice?

Discussion in 'Pioneer Audio' started by oz-2, Apr 20, 2017 at 10:43 AM.

  1. oz-2

    oz-2 AK Subscriber Subscriber

    Messages:
    31
    Location:
    Cranleigh, Surrey, UK
    Hello AK'ers
    I have a small problem with my SA 9500 Mk 1

    After about 10 - 15 mins of use I get a crackling sound on the LH channel. Observation of the L/S shows that this is accompanied by a large, gradually varying, dc voltage which moves the cone in and out over about 2 seconds. The sound and the movement of the cone then disappear for a while. This is only noticeable in no signal conditions. The problem reoccurs every ten to 15 mins or so but it isn't consistent.

    Observations so far:
    1. If I separate the control preamp and the power amp - no crackling.
    2. The sound appears to be independent of volume control position i.e. it is always about the same loudness
    3. It is always in the LH channel. If I turn the switch to Rev, or L etc it still comes out of the LH channel.
    4. It appears to happen on any input selection i.e. aux 1 and 2 or tuner etc.
    5. Tone controls are switched off.
    6. LF and HF cut are Off

    As the problem does NOT occur when the pre and per amps are separated I conclude that the per amp section is fine.
    As the volume control does not seem to vary the crackling I assume that the problem is after the vol control. My suspicions are on q3 and q5 on the Switch Ct ass AWS-082.

    Alternatively I think that q1, q3, q5 on awg-032 could be suspect.

    Does anyone else have a constructive opinion that might help please? Is my reasoning ok?
     
  2. QSilver

    QSilver Well-Known Member

    Messages:
    977
    If disconnecting the control amp (preamp) from the power amp stops the crackling, you probably have an issue in the pre amp section after the volume control. If its the left channel only, your probably looking at Q3 and Q5 on the schematic as both of these are known to fail like this.

    Q3 is a 2SA725 - replaceable with a KSA992FBU but note that the orientation of the new transistor MAY NOT BE THE SAME as the original.

    Q5 is a KSC1312 - replaceable with a KSC1845FTA IIRC. Again, the orientation of the new transistor is likely not the same as the original.
     
  3. oz-2

    oz-2 AK Subscriber Subscriber

    Messages:
    31
    Location:
    Cranleigh, Surrey, UK
    Thank Qsilver. I will get her on the "bench" (dining table!) and perform an extraction and a filling.
     

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