Doh. My P.A. is dead.

Discussion in 'Amps and Cabs [BG]' started by Gabu, Oct 5, 2001.

  1. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    I just got my new PA I bought from Ebay.

    No volume.

    I have tried both speaker jacks, I have tried all mic inputs (1/4" type... i have none of the other type of cable). I have tried many combinations of knob/eq settings.

    One other weird problem.

    It has low and high eq knobs. If I turn the low eq knob up on more than one channel at a time, the speakers start pulsing. ... Like every second they go pop, pop, pop... But if I turn up just one of the low eq... it does not do that. All of the other knobs produce no result at all.

    The speaker cab works. If I plug my mic into my Carvin amp, and into that speaker cab... it works fine.

    Any tips or whatnot would be greatly appreciated. I told the seller of this already and he said he would work with me either on getting it repaired, or returning it... whatever.

    I am pretty disapointed. :(
  2. Blackbird

    Blackbird Moderator Staff Member Supporting Member

    Mar 18, 2000
    Oh, you mean you P.A. I thought you meant something else. (perish the thought)

    Lemme fix that for you...

    Anyway, Bummer, dude. I just won the bid on a 5 string jazz-like bass on eBay. I hope I'm luckier than you were. Actually, I've had pretty good experiences.
  3. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    I think you might get a better response in the Amps forum. After all, what you are talking about is an amp with a mixer stuck on the front.

    The technical experts on amps hang about in the Amps forum and very rarely visit anywhere else - but there is some serious technical knowledge there.

    I suggest that a moderator would be doing you a big favour if they moved this thread over there.
  4. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    I know that ther are lots of things this could be and that PAs are pretty complicated. Like, I have set up our band's PA several times and no sound has come out - things like wrong cables. some of the speakers require power and others don't, master volume and individual outs volumes etc. etc.

    What sort of cables are you using - you are using proper speaker cables aren't you and not guitar leads!!
  5. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    First let me say I am really, really sorry to anyone who thought that I was saying that my father died. I didn't mean to cause any confusion. My Dad is alive and well and he still kicks my ass at basketball anytime he wants to.

    Bruce, I am using a guitar cable. I the only amp cables I own are on my Carvin rig. I didn't think to try them, but I sure will.

    Some more info:

    My cab is currently setup as a 2 ohm load. This PA on the back of it says that it supports down to 2 ohms. So I am using just one jack (it has to speaker jacks), but my speaker load is 4 8 ohm speakers all in parallel. I have tested the speaker cab in my bass rig. I can get output, running my mic through my bass amp into the cab in question.

    The exact model is a Crate PA B4150. It has 4 inputs and 2 speaker jacks. It has both types of inputs, 1/4 and XLR (is that the right word?). It has a 5 band eq. It has the following knobs: gain, reverb, monitor, low eq, high eq. It also has some jacks in the front for monitor, main, reverb footswitch, um... I think 2 others. And it has monitor, main and master volume knobs also (sepperate from the individual channel ones).

    The cab is plugged into the speaker jack on the back with a guitar cable.

    The mic is getting plugged into any one of the 1/4: inputs.

    All the knobs are set to 1/2 way up except low eq.

    low eq are all left off because when more than one of them are turned up at once the speakers start pulsing.

    Thanks Bruce, and everyone else who reads this! :)
  6. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    Well the people in "Amps" are having just this sort of technical discussion as we speak - I've just been looking; but they very rarely venture over to Misc. I am a rank amateur in this area and need to have the gear in from of me, but people like Joris and bgavin etc. etc will instantly understand what your technical details mean and whether or not it should work! ;)
  7. Bruce Lindfield

    Bruce Lindfield Unprofessional TalkBass Contributor Gold Supporting Member

    Have you tried taking a cable from the front output labelled "main" into the speaker cab - this would be my logical choice?

    I think you need to be using speaker cable though, as you will probably just "fry" any guitar leads that you use for this purpose!
  8. Blackbird

    Blackbird Moderator Staff Member Supporting Member

    Mar 18, 2000
    Well, Tally-ho, to Amps we go...
  9. 1) Check fuses with a VOM
    2) Always replace fuses anyway
    3) If brave, look inside the amp for something loose. If in doubt, don't touch.
    4) Use a higher than 2 ohm load. The amp may be instantly shutting down under 2 ohms. This is awfully low, and the amp must specifically state it will handle 2 ohm loads.
  10. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    Howdy Bgavin,

    Thanks for your input!

    I will check the fuses. I can check the other components to some degree (as far as looseness or obvious breakage). I used to be an Electronic Technician in the Navy, so I do have some knowlege of working with circuitry... however it's been more than 10 years so I will go slow and quit if I feel above my head.

    The amp does say that it will handle 2 ohm loads. It has 2 4 ohm speaker jacks and claims a 2 ohm min. I can certainly change my speaker config to a 8 ohm load though.

    Do you have any thoughts on the pulsing sound?


  11. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Gabu, Gabu, Gabu. A guitar cord is designed to carry very low current ... you know, a few milliamps at about one volt maximum. A speaker output can operate at as high as a hundred volts. Using Ohm's law, E=IR, 100 volts across a 2-ohm load results in 50 AMPS of current! 50 AMPS, for the love of God! The resulting effect on a guitar cord carrying 50 amps would be something that would look really cool in a cartoon. (OK, gearheads, I know it would have to be a 5,000 watt amp for this to happen, but my point is the same.)
  12. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    50 amps! I just need two or three.... ;)

    You are right, I should really invest in a few now that I have two amps and a PA and one true speaker cable. :)

    But anyways, I have decided to let the UPS take this one back. They will give me $100 insurance for my troubles. I had one heck of a time trying to figure out how to get inside of the thing... There are 15 screws in the back. I took off the outter six... but the cover only wobbled. I was worried about the last 9 because they seemed to hold some componants on the inside. In short, I chickened out.

    But I don't think it was a fuse issue, because I do get power to the unit... just no output to the speakers. (and the weird low eq problem)

    If UPS pays me and leaves me with the unit too then I will get more adventurous... Otherwise... it's back to ebay for me. (or Carvin perhaps)
  13. Gabu


    Jan 2, 2001
    Woodland Hills, CA

    This will be my pa replacement. It seems like a great deal. I have a buncha cash in my paypal account so it won't be much out of my pocket. :D

    The guy who sold me the crate pa was very cool and refunded my order.

    Thanks for everyones help. If you need a smaller PA check out this speakerhole one... it seems cool!
  14. Hope things work out dude. Even if a PA head of that type (a box mixer) states it will handle 2 ohms, I would be extremely dubious. If it is a stereo head, ie, has two power amps inside it, then that's OK, but a mono one should never go to 2 ohms, even if it says it can on the back panel. 4 8ohm cabs on a stereo head are only loading it with 4 ohms per side. 2 ohm loads put a huge strain on any amp, generating mucho heat and therefore mucho unreliability.
  15. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    I finally decided to just push my luck and undid the screws again, then pulled... pulled... pried and finally Ka-Boink! The front panel came off.

    Shortly thereafter, the back panel came off too. I eventually found the fuse, and examined it. It was fine. I looked for any more and was not able to find any more fuses.

    Both the front and the back circuitboards are screwed on to the front and back panels upside-in... So when I took off the panel, it was kind of hard to see all the circuitry... But I did get a fairly good gander and nothing was obviously loose or burnt.

    Between the two boards, screwed to the bottom of the case (with rubber shock absorbers even) was a metal box. It had two cables connected to it, one saying out and the other saying in. I unscrewed that and looked under it. The cables connected to what looked like a small transformer and between the two transformers were a couple of springs. These springs had a bunch of support wires attaching them that seemed to keep them from moving around too much. Weird! Well, I didn't see anything loose or burnt so I put it back together.

    Next, the great experiment:

    Okay, the power light does come on. Also, when I turn it on, the speakers give a normal sounding pop. So I thought, maybe something that I can't see is just loose. So I lift the pa about an inch off of the cab and drop it. It makes a really funky boooiiinnnggg sound through the speakers. That much be those springs I saw imbetween the circuitboards. Now that would indicate to me that the back circuitboard is probably working. Since it is what handles the power, and also is what the speakers connect to.

    What's next?

    Frankly, I have no idea. What is it that might be wrong with the front circuitboard that is killing the signal before it gets to the back circuiboard? The preamp maybe? Hmmm...
  16. dsmith


    Mar 29, 2001
    Mt. Vernon, KY
    First, buy yourself a speaker cable, you need that no matter what. Make sure it not a guitar cable. After you get that, we can go from there.
  17. Gabu


    Jan 2, 2001
    Woodland Hills, CA
    Done! I have replaced the instrument cable with speaker cables I bought yesterday. No change yet.
  18. dsmith


    Mar 29, 2001
    Mt. Vernon, KY
    Ok, hooking up your powered head is very simple. Just remember that you have 2 sets of outputs, and one should be for the left channel, and one for the right. Look for a switch that says stereo/ bridgeed on the back of the head. Make sure its in stereo, we don't want to bridge the amp, unless we know it works in stereo first. Run your speaker cable from one 1/4" jack on the back of the speaker, to the L output of the power amp. (Make sure amp is off).

    Hook up your mic, and turn all eq's on that channel at 12 O'clock, and turn the gain, and the volume knob all the way down.

    Power up the amp, then turn the gain up to about 9 oclock, then slowly turn up the volume while checking the mic. You should be able to hear yourself now, and if not repeat the same procedure on the R output of the amp. (Never run L and R outputs with 2 speaker cables to 1 speaker that has parallel jacks or you will not have much of an amp left.)
    The springy sound you heard was probably the reverb, just like a peavey guitar amp, you kick it, you get thunder.

    If you still cant get a signal, you might want to take it to a local music store and see if they can help you. I don't mess around with powered heads that often, but they are fairly simple. You might also consider calling Crate, they should have a customer service department that can get you in touch with someone who will have a maunual, and get you going.