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    Faint Sizzling Noise (both channels)

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    Avitar

    Posts : 2
    Join date : 2010-07-03

    Faint Sizzling Noise (both channels)

    Post by Avitar on Sat Jul 03, 2010 4:20 pm

    Hello,
    I recently purchased a used ST-70 in mint condition. I've listened to the ST-70 quite a bit and I'm hearing a faint sizzling noise coming through the speakers. I purchased new tubes, tried different speakers, adjusted the bias voltage, also grounded the chassis and the sizzling is still present. I checked the quad capacitor with a fluke multi-meter, each of the four quadrants to ground read approximately 150K to 220K ohms. All measurements were taken with all of the tubes removed from the unit and of course no power. Any help would be greatly appreciated!

    Thanks


    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Faint Sizzling Noise (both channels)

    Post by Bob Latino on Sat Jul 03, 2010 6:41 pm

    Hi,

    In order to test the amp for noise I am assuming that you either shorted the inputs or plugged interconnects into the two input jacks and terminated them on the other end by plugging them into a preamp (keep the preamp OFF). If the input jacks are left "open" they will pick up noise from the amp's circuitry and the amp will have "sizzling" or a "buzzing" noise.

    Even though an old ST-70 amp may LOOK mint the parts inside are still 35 to 51 years old. If the quad cap gives you a relatively high resistance then it is probably still OK. What I would do is replace the driver board and parts set. The stock driver boards were cheaply made with open solder traces. The fact that the sizzling is on both channels leads me to believe that something common to both channels is at fault. If the amp still "sizzles" with the input jacks terminated then I would suspect that the high voltage power supply (which is common to both channels) has some issue.

    Bob

    Avitar

    Posts : 2
    Join date : 2010-07-03

    Re: Faint Sizzling Noise (both channels)

    Post by Avitar on Sun Jul 04, 2010 10:13 am

    Hey Bob,
    I'm assuming that my next step is to check the output of each of the transformers. Do you happen to know what voltages and from what wire or connection will produce a certain value? Also, the wires that are used for the speaker output, 4,8 & 16 ohm, one of them is not being used (brown wire). It's taped up but hanging loose, I'm wondering if it's acting as a antenna and picking up noise? What are the impedance values for the Quad Cap? Thanks for your help!

    Bryan

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Faint Sizzling Noise (both channels)

    Post by Bob Latino on Sun Jul 04, 2010 10:51 am

    Hi Bryan,

    If you hear the noise from BOTH channels then it is probably not an output transformer that is bad. Something in the main power supply is probably not right.

    The brown wire is the 4 ohm secondary tap wire. Taped up it like that it won't cause any noise.

    Every quad cap is different and just measuring the resistance to chassis ground is not a real reliable way to see if a quad cap is good or not. If the resistance is below 10K or so (IMHO) - then the quad cap should be replaced. To really get a good idea of the whether a quad cap is OK or not you really need a capacitor meter and then check each section to chassis ground with the amp OFF.

    Bob

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