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    St70low hum at idle

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    dynacoman

    Posts : 13
    Join date : 2011-09-03

    St70low hum at idle

    Post by dynacoman on Fri Sep 28, 2012 4:06 am

    Ok guys, I have had my stereo 70 a long time 12+ yrs. Over the last 3 years I finally went thru the entire thing and updated it. Even though I keep the stock circuit w/ 7199's in it on the front end I'm not totally unhappy with it. Anyway I have notice one thing I can't stand and this is the low hum at idle. Is there anything that can be done to quite this amp down? All my friends that builds amps say that this the way they were. really? Any thoughts on how to make it more quite?Anyone chime on in.
    Thanks! Smile

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: St70low hum at idle

    Post by Bob Latino on Fri Sep 28, 2012 6:21 am

    Hi,

    You said you updated the amp. I am assuming that you replaced the quad cap. The most common cause of a low level hum that does not change with volume AND is the same volume on BOTH CHANNELS is a leaky quad cap. Even if you replaced the quad cap 3 years ago, it is now possible that one section is bad.

    To really test the quad cap you need a capacitor meter but you can do a pretty good job with a resistance meter. Before you work on the quad cap discharge all four sections to the chassis with an insulated screwdriver. Check the resistance from each of the four sections of the quad cap one at a time. (red probe on the section and black probe on the chassis) Normally the resistance will rise as the cap section charges from the battery in your meter. Each section should (eventually) show over 1 meg ohm by the time the meter stops. If any section shows a LOW resistance (maybe 10,000 ohms or less?) it usually means that this section is leaking voltage to the chassis. Sometimes two or three sections may show a low resistance because all the quad cap terminals are interconnected by the choke and resistors. The only way to test to see which one (or multiple) sections are bad is to remove all the wiring between the connectors and then test each section separately. If just one section is bad you can bypass this section with a separate capacitor of equal value but IMHO - I would replace the entire quad cap.

    Bob

    dynacoman

    Posts : 13
    Join date : 2011-09-03

    Re: St70low hum at idle

    Post by dynacoman on Fri Sep 28, 2012 8:03 am

    Thanks bob! I do have a capacitance meter and will inspect the quad cap! I think it may be time to look at finding an upgrade and delete the weak link. I'll test it later today and let you know what I've found.

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