Loud hum in amp

Discussion in 'Amp Tech Center' started by greasamizer, Jul 18, 2019.

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  1. greasamizer

    greasamizer TDPRI Member

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    I own an old Spectra 210 amp. I hadn't used it in a while, about twenty years. The other day, I tried it, just for the hell of it, and as soon as I turned it on (dry, no instruments, effects, etc), It had this very loud low-pitched hum; sounds sort of like a motor with a locked rotor. When I pulled it apart, I had noticed one of the 10-watt resistors was corroded at the terminals. I isolated it, and checked it for resistance; no deflection. I am looking at three possibilities: there are two big electrolytics , four big transistors, and two power regulators I see as possibilities. I checked the transformer for shorts and found none. To me, this is a run-of-the-mill solid-state amp, nothing special about it's circuitry that jumps out at me; but this has me puzzled. Any takers?
     
  2. schmee

    schmee Poster Extraordinaire

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    I'd replace the elytics and especially the filter caps. It sounds like a typical filter failure to me.
     
    Paul G. and BigDaddy23 like this.
  3. VintageSG

    VintageSG Friend of Leo's

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    The smoothing caps in solid state amps need some loving too.
     
  4. LightningPhil

    LightningPhil TDPRI Member

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    The caps are probably not happy. Over time, the dielectric can degrade. If kept in use, the potential applied every day keeps the dielectric working. After all, an electrolytic's dielectric layer is formed by applying a voltage and building it up in the first place.

    When turning on old equipment, I use a Variac, and slowly increase the applied mains voltage over an hour or so. Usually works. But on occasion when it doesn't it's time to replace the caps. After 20 years, your amp may appreciate it anyhow - even if they weren't totally dead (as they get more and more electrically leaky over time).

    While you're at it, if it's easy, increasing the capacity of the bus capacitors may give you a slightly lower noise floor than you initially had. But overdoing it too much can blow the rectifying diodes. If ever I take an amp apart, it ends up with a larger tank capacitor than standard.

    Of course, it might not be the caps - just a good place to start before attempting anything else.
     
  5. greasamizer

    greasamizer TDPRI Member

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    Thanks, Guys, for the input. I had, as mentioned, found a failed resistor, and a little casual exploration had, indeed, found a cap that was ill-dispositioned. As a precaution, I replaced all four of the electrolytics, the ceramic (concrete...) resistors, and as just a 'for the hell of it' the two big power diodes(?) that, with a furious battle, I finally removed and replaced. Have some 'duties' to attend to and then will re-check, re-assemble and re-test (sort of like 'Rah-Rah-Rah' with adenoids). with any luck, this amp will work OK. I had used it for a ton of gigs way back when, made a tone of cash, so really, it doesn't owe me diddly. OK, thanks again for the feedback.
     
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