Blackstar Club HT40 humming fault

Danny1969Danny1969 Frets: 3555

My mates got an HT Club 40 and it's developed a strange fault. After it's been on about an hour is starts to make a noise but if you tilt the amp back the noise stops. I've had a quick look and resoldered the valve bases and cleans the contacts on the output valve sockets .... it still does it. All joints on the PCB look good ...   the trouble is because it takes an hour of use before the fault occurrs it's a time consuming thing to blindly prod around  .... any ideas @ICBM @ecc83  or anyone else
www.2020studios.co.uk 
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Comments

  • ecc83ecc83 Frets: 807
    edited July 9

    Does it hum with all controls at zero? If so it could be a biasing fault. I would have to dig out a schematic to tell you the sense resistor but it will be a 1W 1R with a diode across it.

    If it is biasing and that is not a duff EL34 then it could get deep and you will need an informed tech.

    D36, R 217, TPs 10 & 18. Do not measure to chassis, should be 25mV

    Dave.

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  • ICBMICBM Frets: 31118
    If tilting the amp stops it, it must be a physical bad connection - it could be a solder joint that’s too finely cracked to easily see. It could also be internal in one of the valve bases (ie in the valve itself) even if the sockets are fine.

    Set the amp up with the cabinet face down and the chassis the right way up on the back of it, and wait for the fault to develop - you should then be able to find it by carefully moving the components. You may need to tilt it to the angle where it comes and goes if it’s very sensitive.

    If I had to guess I would put a tiny bet on a filter cap joint.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • ecc83ecc83 Frets: 807
    ICBM said:
    If tilting the amp stops it, it must be a physical bad connection - it could be a solder joint that’s too finely cracked to easily see. It could also be internal in one of the valve bases (ie in the valve itself) even if the sockets are fine.

    Set the amp up with the cabinet face down and the chassis the right way up on the back of it, and wait for the fault to develop - you should then be able to find it by carefully moving the components. You may need to tilt it to the angle where it comes and goes if it’s very sensitive.

    If I had to guess I would put a tiny bet on a filter cap joint.


    Good plan. You can also remove all the valves except the output pair, if the fault still occurs you know it is in the OP stage or possibly the MOSFET PI though quite how the latter would cause hum defeats me at the mo'!

    Dave.

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  • PsygoatPsygoat Frets: 0
    I'mm with ECC83 on this one - check heavy components like the filter caps for dry joints, it's very common and will cause hum, usually very loud !!!
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  • PsygoatPsygoat Frets: 0
    edited July 9
    if it's a power supply fault you will hear it even if the controls are turned down.
    A gentle thump or probing with an insulated object will help you find the offender.
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