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"Crack" powering off 80s vintage practice amp

Discussion in 'Amps and Cabs [BG]' started by JimChjones, Nov 22, 2017.


  1. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    I dug my little 1980s Ohm practice amp out of the loft, thinking it ought to be rehomed because I play through the DAW at home now. Powers on and sounds OK, but I noticed that there's a much sharper crack when turning off than I remember. Is this likely to be a symptom of sick capacitors? If its early to mid 80s, is it getting to that sort of age?
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Some solid state amps will do that, it depends on the design. If it always made the noise, perhaps you simply don’t remember it being so sharp.

    An impulse send to your speakers isn’t good for them. If it helps, turn the volume down all the way before powering down the amp.

    In terms of power supply capacitor health, it is always good to periodically power up your amp when it is in storage. The allows the electrolytic capacitors to reform the internal insulating layer on the alumium foil. Do this at least once a year for an hour or so.

    If your amp does not have a louder power line related hum, chances are your caps are fine.
     
    Last edited: Nov 22, 2017
    bobyoung53, JimChjones and mobdirt like this.
  3. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    Thanks: I'm getting the crack even with volume turned down... Its a much harsher noise than I remember - it really made me wince for the speaker. I'll give it a couple of hours sitting turned on and see what we get.
     
    beans-on-toast likes this.
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Nope, not likely to be a capacitor issue, regardless of what all the internet mythology might suggest. (internet guru mythologists might want to break out their voodoo dolls at this point)

    Your symptom is common, especially on small inexpensive amps. Almost always, it's due to the lack of a back EMF snubber on the power switch, which is installed to quench the tendency for a small arc to form when opening the power switch. Why would this happen? Because interrupting the current to an inductive load like a transformer causes a large voltage rise across the open circuit and will momentarily arc while the magnetic field collapses. It won't happen to the same degree every time because it depends where in the AC current (not voltage) cycle the interruption occurs. If you were to open it at the zero CURRENT crossing, it would be silent, but obviously that's a crap shoot. The voltage spike and arc emit a high radiated and conducted noise field which gets coupled into the audio. Normally there's a small ark quenching cap across the switch contacts, but on the cheap amps it's often left out. If the amp is small enough and the switch of adequate capacity, it won't hurt anything.

    I do not recommend modifying anything because the cap is special type for use in the AC primary circuit, and messing with the primary circuit should be left to somebody who recognizes the inherent dangers and safety requirements associated with this area.
     
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  5. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    Just took the amp unit out for a quick eyeball inspection and made an interesting discovery. The spade connectors from mains in to the back of the switch were really loose, and on squeezing them with a pair of pliers to make a better connection they actually broke up. They are/were extremely brittle. The connections were loose enough that I wouldn't want to guarantee there wasn't an arc there. Oddly the spade connectors from switch to transformer aren't exhibiting the same looseness, but I haven't pulled them off the terminals to check. Better go out and buy some connectors. I've never heard of a connector going brittle like that - I just squeezed them gently and there was an audible crack as they failed. I'd better replace all 4 I guess, but is this a thing one might expect?

    And, unsuprisingly, Mr Horse is correct, no cap across the switch.
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Cheap FastOn connectors can fail like this, it's all about paying for quality (or not)
     
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  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Inexpensive amps are designed to last not much longer than the warranty. They cut corners and use low end parts. The switch arcs when you use it, eventually this takes a toll internally and on the terminals. Heating cycles cause the brittleness.

    If you can do it safely, as a one time test you can try bypassing the switch.
     
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  8. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    OK thanks folks. I'll replace all the faston connectors, then give it a couple of hours sitting in the corner of the room turned on and see how we go. I was especially surprised to find the earth connector brittle, I sincerely hope that one hadn't been suffering a lot of heating cycles or action from the switch:).

    I'll have to check out every connection on the mains side thoroughly, and if anything beyond the fastons comes up dubious then I guess I own a 20W extension cabinet...

    [Later] Interesting. On stripping the whole cabinet down and checking everything there's some evidence the faston connectors have been brittle for a very long time, maybe even from new. Which in a way is kinda reassuring that its just a bad batch of cheap components, not that the whole thing has had some kind of maltreatment and other stuff could have aged badly.
     
    Last edited: Nov 22, 2017
    beans-on-toast likes this.
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Be sure to use the proper crimping tool, not some off-shore POS.
     
  10. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    Any reason why I shouldn't solder them? I have a distrust of anything self crimped, doubtless due to lack of high quality crimping tool...
     
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  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Fastons ease service and assembly time. Too much heat can be a problem with some switches, just be careful. Some switches have solder terminals, they are designed to be soldered.
     
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  12. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    I meant solder the fastons, not crimp them... I wasn't proposing to solder directly.
     
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Crimp with pliers and soldering works.

    To quote one supplier....Sealed Crimp & Solder Connectors are crimped for mechanical reliability, soldered for superior strength and conductivity, and sealed for maximum durability. When there is no margin for error, choose Sealed Crimp & Solder Connectors...the ultimate connector.

    Sealed Crimp and Solder Connectors
     
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Soldering is not as reliable as crimped, and soldering of some materials can damage their spring properties (like some fast-on type connectors)

    NEVER use pliers or dikes to crimp any connector, it's a sure way to failure down the road. A proper crimping tool adds a deformation that locks the wire into place with a gas tite bite.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Last edited: Nov 23, 2017
  16. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    Well the local electronics store had some fastons sold as suitable for being soldered, so I've used those. The amp is definitely safer now. Unfortunately it doesn't seem to be much quieter turning off. Oh well.

    Anyway, thanks for your contributions gentlemen.
     
    Last edited: Nov 23, 2017
    beans-on-toast likes this.
  17. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Almost certainly due to the missing part ;)
     

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