Changing filter caps on '78 Acoustic 220?

Discussion in 'Amps and Cabs [BG]' started by donalddemon, Jun 14, 2020.

  1. donalddemon


    Feb 26, 2010
    Asbury, NJ
    I have an old 220 amp that I love the sound of but it's developing some static like the caps are going bad. I don't think they have ever been replaced so it's long overdue.

    Anyone done this before and are there any tips to ordering the right parts? I'm handy with the soldering iron but never have done a cap job, this will be a good chance to learn. I have done some work before inside amps so I know about the harmful voltages and such.

    I found a schematic here:

    It looks like there are several (8) electrolytic filter caps, not sure which ones to replace or where to source the parts. Any help would be greatly appreciated!
  2. What if it is a jack, or a pot that is in need of cleaning or something else out of spec?

    Caps do not have an expiration date and only NEED to be changed IF they are bad.
    Find the true cause of the noise before swapping parts that may not do anything.
    Omega Monkey and abarson like this.
  3. abarson


    Nov 6, 2003
    Santa Cruz
    +1 for doing actual troubleshooting. Otherwise you can spend a lot of time and money and not solve the problem, as well as running the risk of introducing more issues.
  4. BassmanPaul

    BassmanPaul Inactive

    Indeed find out what's wrong with the amp first before spending money on parts that may be perfectly fine. I'll bet it's got nothing to do with the caps!
  5. Definitely this. I'd like to go one better. Take it to a tech. It'll be the best money you ever spent. They have the tools and the expertise to tune it up good as new.
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Quit blaming the poor caps for something that's quite likely something else.

    I see more damage from folks swapping random parts than just about anything else (with the exception of spraying "magic cure-all miracle spray" (tm) in pots and other areas where there should be nothing contaminating.
    Geri O likes this.
  7. mc900ftj


    Jan 21, 2014
    Don't replace anything without proper troubleshooting. If you don't know how to troubleshoot an old amp, take it to someone who is experienced in amp troubleshooting and repair. You may save your amp, some $ and possibly your life if you aren't knowledgeable about high voltage safety.

    Good luck.
  8. Geri O

    Geri O Endorsing Artist, Mike Lull Guitars and Basses Gold Supporting Member

    Sep 6, 2013
    Florence, MS
    One question further than the good advice you've already received....

    What about the problem leads you to believe that the caps need replacing?
  9. lug

    lug Supporting Member

    Feb 11, 2005
    League City, Tx
    Plus new caps contain 87% less mojo. You will however likely get 26% more spank out of them.

    Pro tip: If in the unlikely event the caps are really going, you will sometimes notice an odor like dead fish coming from them.
  10. I wonder when the Vintage Capacitors will be a big seller like Vintage Tubes? :greedy:

    This is funny... Like how people think a tube amp means you always need to replace tubes, yet tubes that are over 30 years old are very sought after. :cautious:
    Omega Monkey and agedhorse like this.
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    A crazy world it is, replace all capacitors we must ;)

    [/marketing grunge]
    turcmic, Geri O and S-Bigbottom like this.
  12. turcmic


    Jul 30, 2006
    Montreal, QC
    Is changing caps the new mandatory thing to do, right after diy bass cab with an 18-inch speaker for more bass? :D

    OP, a good tech is a good investment.
    agedhorse and S-Bigbottom like this.
  13. seamonkey


    Aug 6, 2004
    I'd like to hear what that bad cap static sounds like. Can you post a clip?

    Does it have an effects loop? Do you get the crackle when you plug into the "Return" jack?
    agedhorse likes this.
  14. donalddemon


    Feb 26, 2010
    Asbury, NJ
    When powering down it takes almost a minute to fully shut off, a symptom I've seen on amps that need new caps. It's also the same static sound my Peavey made before replacing the caps. Beyond that, I don't know. Finding a tech has been difficult right now because of the virus and in in the middle of recording so I need it done ASAP.
    Geri O likes this.
  15. donalddemon


    Feb 26, 2010
    Asbury, NJ
    I don't think it has one but I'll double check and see if that works.
  16. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Those are not symptoms suggesting that it needs new caps (no matter what the internet might say)
    BassmanPaul likes this.
  17. donalddemon


    Feb 26, 2010
    Asbury, NJ
    Ok, care to share any further thoughts on what it might be?
  18. JohnMCA72


    Feb 4, 2009
    I bought a brand new Acoustic 450 back in the day, & out of the box, from the very first time I powered it down (after the first time I'd powered it up), it did that.
  19. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Any of a hundred possible causes.

    With proper troubleshooting of the circuit, you will be able to rule thing out (like filter caps) to narrow down the likely causes step by step.

    Repairing isn't about replacing random parts that somebody on the internet who hasn't tested the amp says. It's about correctly identifying the cause of the symptoms so that the correct part(s) are replaced (IF it is even due to a bad part)
    S-Bigbottom and BassmanPaul like this.
  20. beans-on-toast


    Aug 7, 2008
    What you are describing is a higher internal resistance inside the electrolytic capacitor that can be seen with age. This can cause the cap to operate hotter and eventually bulge and fail. I would carefully inspect the caps, looking for signs of bulging. Some caps have a small rubber covered port at one end that can bulge. Look for that. Any bulging, change the cap.

    As these power supply caps age, the amp can loose headroom and have an earlier onset of distortion. Static as a cap discharges can be heard of you pull the plug and hit a low note. This is normal. The amp doing this while playing could be a sign of this early onset of distortion due to a bad cap.

    As others have said, these symptoms could be due to issues unrelated to the caps. There are many possibilities. It could be as simple as a bad resistor for example. With the aid of test equipment, the problem can be identified and fixed.

    It is important to note that a cap is not a cap. They are not all alike, specs are different. Low ESR is usually good. Working voltage is important. Long life is good. A high temperature rating is good. Even size is important. A higher performing cap may change the character of the amp. For example, some amps have a certain characteristic distortion. Upgrading the power supply to perform better can change that. Sometimes in a positive way, making it cleaner. Sometimes not though, leaving people thinking that the magic is gone.
    donalddemon likes this.