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Hit ceiling light with bass - power goes out - GK 800rb not working

Discussion in 'Amps and Cabs [BG]' started by skeevy, Jan 31, 2013.

  1. skeevy


    Mar 2, 2012
    At rehearsal last night, I accidentally hit the headstock of my bass on the ceiling light (this group practices in a basement with low a pretty low ceiling). This resulted in the power going out to the entire basement. Now after we sorted out the blown fuse and got the power back on, everything turned on just fine, and all of the gear seemed to be unaffected.

    However, my rig wasn't producing sound. I determined that neither my bass or instrument cable are the problem, as I was able to tune up just fine. The problem must be some part of the 800rb. It will power up, and the lights are on but nobody's home. I can still hear a slight pop followed by a low level hum (just like before the incident) coming out of the cab when I switch the head on, so I know there is signal going from the head to the cab. If I adjust the master volume (or any of the knobs), the volume level of the hum coming out of the cab does not change.

    Any ideas what might have gone wrong, and if there is any chance I could fix this myself? Amp work seems to be very pricy around here, and getting an amp fixed often means the local store shipping it out of town to the nearest qualified tech, which takes forever.

    Thanks! And no, I was not plugged in through a surge protector. And yes, I have learned my lesson on that one. Any recommendations on making sure that a given surge protector will do the job, or should they all work fine?

    Probably better my 800rb than our guitarist's Orange, but still not fun :(
  2. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    Phillipsburg, NJ
    I'd have to guess the circuit protection inside the amp(Fuse or breaker) did its' job but fried itself in the process.
  3. Aren't lights and sockets generally meant to be on seperate breakers?

    Might be worthwhile finding out if there are any internal fuses on the amp that can be replaced.
  4. 800 RB is not class D and no protect circuit. I would try an instrument cable from effects send to effects return first. If no good still, the slight pop and hum may mean the power amp is okay. You can check this by plugging your bass directly into the effects return. If the amp plays (with reduced volume and no tine shaping) the power amp is okay. I would suspect from that point that a voltage surge from the light through your bass fried the first opamp (little black part with eight legs). The opamps are available everywhere, even at Radio Shack but they are soldered on the preamp board so some skill is needed to replace. Last time I looked at Radio Shack they were $0.99 but people like Mouser.com or digikey.com are way less.
  5. This, probably. But it's a bit of a mystery since one of your tuners probably hit power, which sent it to ground, through your strings/bridge ground.
  6. Induced 60Hz from the mains to the signal would certainly be possible? I don't remember the break down voltage on the TL072 exactly but it is pretty low compared to even half of the mains voltage. (Mains voltage is AC, bass's signal is AC, so no way to protect against what you are wanting to amplify.)
  7. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    I've guess the bass acted as a short for the elements in the bulb rather than as a ground path itself.
  8. MRuss


    Jan 23, 2008
    Los Angeles
    How can your amp not have a fuse?

    Edit: I looked it online, it does. Maybe check that.
  9. tbsodsmuck


    Aug 4, 2012
    .....laughter ensues....ahhh to be young again :D
  10. That is the reason I first mentioned the effects return jack bypass. Sounds as though the power amp is okay? The accident could be coincidence.
  11. If the amp had a fuse at the socket which had gone, it wouldn't be lighting up at all.

    Any internal fuse may have gone.

    Though, if it was a massive spike into the input of the amp, I don't think there'll have been anything sacraficial in the way.
  12. Bassmec


    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    The juice from the light went to earth mate, I don't have more than one earth handy, this is the only one we got:
  13. I'd pull the head case apart and start to look for blown up resistors. It does indeed sound like the grounding of the strings (and consequently tuners) contacted the light socket and it created the short through the instrument and amp. If this did happen it would one hell of a light show while it all went down.

    I had something similar happen:

  14. skeevy


    Mar 2, 2012
    Thanks for the replies, I will try plugging into the effects return, then ultimately opening her up when I get home.

    You guys are saying it was a spike from the bass end of things and not the wall outlet end of things then?
  15. Yes the 800RB is pretty stout and if it were from the wall, more than the 800RB would have had trouble.
  16. skeevy


    Mar 2, 2012
    It does work (at a reduced volume) when plugged into the effects loop, so it appears the power amp is in fact working fine. Going to pop her open in a couple hours here and see if anything visually looks blown.
  17. Don't forget to loop the effects send and effects return with an instrument cable and try the input again. Just in case it was just the effects return internal switch acting up.

    Good news about the power amps working! :D
  18. chadds


    Mar 18, 2000
    At least you're still here to discuss the whole thing.
  19. Bassmec


    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    Also kind of handy that the bridge to jack earth lead in the bass held up as they ain't too thick or sometimes can be quite high ohms insulated by paint varnish and body wax under the bridge.
    Still it was only that half arsed electricity they let you ex colonial geezers have, not like the full 240 big ones us Brits get to electrocute people with.:bassist:
  20. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    While you are looking, there are two zener diode (a small black tubular component with a line at one end) pairs, one at the input (D1 and D2) and one at the effects return jack (D3 and D4). They might be marked 1N759. Inspect them and see how they look.