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Amp problem: Easy fix?

Discussion in 'Amps and Cabs [BG]' started by mattsk42, Aug 4, 2005.


  1. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    I have the same problem as this guy I think. It's not my cable coming from my bass, I already checked and used a few different ones. http://www.talkbass.com/forum/showthread.php?t=194588
    It is an older amp, can I just replace the head somehow? I know absolutely nothing about this, so bear with me. :meh: Thanks!
     
  2. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    Oh, and it's an older "Kustom" amp.
     
  3. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    Matt, you're gonna have to provide more information if you want a fix...what kind of Kustom amp? Is it a combo or a head/cab set up. What are the specifics of YOUR problem. Depending upon the amp and set up, there could be a few things happening. You might also post over at the vintage Kustom Message board, where someone might have info about your amp specifically.
     
  4. Is this one of the really old Kustom tuck-and-roll?

    There is a thermal protection switch on the very old models (at least on the K250). This will cause all power to shut off until it cools off.

    So if you lose all power and then it comes back on later, this is one thing to check. These amps run very very hot, they sit in a completely sealed metal box surrounded by wood, foam, and vinyl insulation.

    But if the lights on the switches are still on when the amp loses all its power, then the problem lies elsewhere.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A cutting out problem is usually caused by overheating of the output transistors or a defective thermal breaker on same. Overheating is usually caused by too low a speaker impedance. If the thermal breaker is blown it must be replaced.
     
  6. Actually on my K250 the thermal breaker is on the incoming AC side, not on the power amp section. And IIRC it resets....

    Again, if there is one design flaw of the old Kustoms, IMHO it would be the lack of heat sinking. The output transistors are fixed to the bottom of the chassis on the inside, directly over the wood box, with absolutely no airflow.....The inside of this sealed chassis gets ridiculously hot. But hey, in 1971 this was one of the best amps on the marke supposedly.

    There is anonther possibility: the power switches on the old Kustoms were prone to breaking, they're a pretty complicated plastic pushbutton affair. I'm not sure what's going on with his amp, since the description of the problem is sorta vague, but if the issue is the amp cutting out, perhaps the mechanical pushbutton switch is malfunctioning.

    Naturally, this switch is obsolete and there are no replacements available.
     
  7. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    The model is a Kustom II Bass. It's a combo thing on wheels. It doesn't ever totally shut off, it just quiets down and crackles. Not really constant noise, just sounds like the volume got turned down and tons of distortion was added. You can still clearly hear individual notes. I used to be able to play for a minute or two before it did that, now it does the crackling sound right away. Oh, and I tested the speaker on a different amp and it works fine.
     
  8. That sounds like an output transistor going, though there's other possibilities too.
     
  9. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    So would that be easy or not?
     
  10. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    That's one of those things you'll want to take to a qualified tech. It could be a number of things, and they'll have to be tested. Doesn't sound like a home-fix job, unless you're more than handy with doing electronics work.
     
  11. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    Hmm... well we don't have any here. I can do some electronics stuff, and it'll get thrown away if it's not fixed because I got it free. So as long as I don't get electrocuted I'd experiment some. Is there any way to find out what it is or just replace the head cheaply? (less than 100 dollars)
     
  12. Warwick player

    Warwick player

    Dec 31, 2002
    Bucks, UK
    Just be careful if you're taking the lid off, theres possibley 230v AC (uk) 110 V Ac (States) in there even after turning the power off, make sure you wait for the capactiors to drain, leave it for about ten minutes and check with a meter if you go in there, try and use the one hand approach to, although it is sometimes it is ackward it is better to be safe than sorry.
     
  13. Output transistors *may* be within your capabilities. If these are BJT you can buy them yourself and replace them usually without any soldering.

    The Kustom II is a later design than the early Kustoms, so I don't know what's there. But look on the back of the amp, or inside the amp. For BJT's they will look like this:

    http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=2n3055

    The part number on the transistor (if you're really lucky) might actually be a 2n3055 but maybe not. If you see them, (make SURE the amp is unplugged) you can unscrew the screws, pull them out, and replace them. Note carefully all the insulation pieces. Also pay attention to the way the wires plugged into the transistor.

    I'd buy a whole set and replace them all, there's lots of cheap places like Parts Express or www.mouser.com

    If the transistors look like this:

    http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=IRFZ44N

    they are MOSFETs which can be replaced, but this would involve some solder work, so it's a little more work.

    Hope this helps.

    P.S. As you can see from the Parts Express page, usually these aren't terribly expensive. That's why it'd be worth just buying a whole set (if they're BJT's) and replacing the whole set, if you don't feel the amp is worth taking to a tech.

    If you can post a pic of the inside and back of the amp, that may help.
     
  14. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    Here's an update. As I said before, it works great, but only at low volumes. I cannot turn it up very loud at all. When I do (or when I play at low volumes and hit a note above open D string), it cuts almost all power and adds distortion.

    HOWeVEr....

    If I strongly slap the SIDE of the whole combo, the sound comes back right away. Not the front, top, or back, but the SIDE of it.

    It happens no matter what cab I hook up to the head, so it's not the speaker's fault.

    Here's photos: http://s11.photobucket.com/albums/a163/mattsk42/Kustom II Bass Combo/
     
  15. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    Well I went back in and tightened absolutely everything in there. None appeared to be very loose, but it now works fine. :confused:

    So, something must have been loose. Thanks for the help!
     
  16. mattsk42

    mattsk42 $100 off new Directv subsp.PM me BEFORE signing up Supporting Member

    Wrong, problem is back. Still is "fixed" by slapping the side of the amp. Nothing can be tightened though. I may take it to a tech but would rather do it myself.
     
  17. Take it to a tech. If U got the amp free.... even a small cost fix- you're still on top.

    Even an Output tranny replacement aint gonna cost a lot IIRC. As always I could be wrong....
     
  18. wave rider

    wave rider

    Jan 5, 2005
    Have you cleaned the input jacks with DeOxit?

    That it responds to "therapeutic massage" has me wondering about a bad solder joint.

    =wr=
     
  19. Hi.

    That's because You didn't fix anything in the first place, but rather had a lucky shot by fooling around the insides of the amp.

    There are number of things that can be wrong with the amp, the cold or cracked solder joint(s) being the most logical one. Cleaning all the connectors and switches isn't such a bad idea either.

    Regards
    Sam
     
  20. mbart203

    mbart203

    Dec 29, 2009
    I agree, it sounds like cold solder joints. Hit all solder joints with an iron and clean all contacts.