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Ampeg SVT-4Pro problems/questions

Discussion in 'Amps and Cabs [BG]' started by Randy Warren, Oct 17, 2009.


  1. Randy Warren

    Randy Warren

    Oct 26, 2008
    Hi, any techs out there? Got a SVT-4 pro on the bench that has distorted/hissy sound, tracing the signal to the tone stackI've got great signal to C13, but on C17, signal craps out. Any ideas here? the owner said it worked fine, but one day someone else connected an ipod up to the amp and it has played badly ever since. Any help appreciated!
     
  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Not sure if you have signal "at" C13 but not C17, or if you have signal "thru" C13 and it is gone after C17.

    Also not sure what if any signal is getting through/to C17. lows only? highs only? nothing?

    if gone at/after C17, V2 pin 7 grid might have issues.

    Otherwise, the tone stack isn't too complicated, it must disappear in there somewhere, and the problem should be findable.

    Doubtful the IPOD made a difference, but.....
     
  3. Randy Warren

    Randy Warren

    Oct 26, 2008
    Thanks, Jerrold, I was thinking that pin 7 may be at issue, perhaps the plate or cathode resistors are off and pulling the signal down, I'll check it again tomorrow and see what is up...
     
  4. Randy Warren

    Randy Warren

    Oct 26, 2008
    well, got signal all the way to TP6 on the preamp board, but cannot pick the signal up again, where does it go? Also, on the pwr amp print, TP6 & 7 read 0V DC, although I am getting the + & - 15vdc on my various IC's, any ideas? thanks
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Pin 4 of J18 page 3. graphic EQ. Also Q9 on page 2, co-ordinates B3.

    Not sure on the +-15V. Does the voltage appear on the other side of the 4.7k resistors?
     
  6. Randy Warren

    Randy Warren

    Oct 26, 2008
    Thanks, Jerrold, nope, nothing on either side, but I think I may have an answer to the problem. The 32v circuit breaker is damaged, replacement on the way, I noticed that when I turn on the amp, I get a buzzing kind of sound for less than a second, it seems to be coming from that breaker...
     
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Say WHAT?

    Either I have completely missed what you meant, or something is kinda strange here...... WHAT "32V breaker"?

    The only breaker is the line voltage one, which is not "32V"....... and problems with it would not cause just teh low voltage to be funky in one area.........

    Do you mean a relay? Do you mean something else?

    The relay may buzz if the +-15V is weak, which might also have some relation to your other problems.
     
  8. rbonner

    rbonner

    Sep 25, 2008
    This is usually when the BUNZ-O-MATIC torch comes out with the roll of acid core solder.

    BOB
     
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Bob, you are not being nice..................

    I'm waiting a while before making any such judgment.
     
  10. rbonner

    rbonner

    Sep 25, 2008
    I'm sorry, I've just seen that happen too many times... The 4PRO is a pretty tight chassis. Yeah I take that back.
     
  11. Randy Warren

    Randy Warren

    Oct 26, 2008
    HAR! good ones! Darth Bob, I feel the force, and it is good! FWIW, it is a 32v 15 amp CB, part #W28-XQ1A-15, looks like a CB or relay of some sort, I'll be putting it in tonite and then fire the ampeg up...
     
  12. rbonner

    rbonner

    Sep 25, 2008
    If it in fact is a CB, far out... That will probably cure 20% of the failed 4PO's not to be confused with 3PO's out there. I need to know where that bugger is on the chassis, Randy... I didn't know there was another one in there at all?

    BOB
     
  13. Randy Warren

    Randy Warren

    Oct 26, 2008
    actually, it's on the chassis right above the IEC plug in series with the AC and the switch, odd that it is marked 32V 15A, but there it is. While it is physically broken (the front part of the reset lever), as Jerrold has noted, if it is not working, there would be no power to the amp at all, which is not the case. I'll put the new one in tomorrow and see what happens..
     
  14. basspro

    basspro Supporting Member

    Mar 10, 2009
    Las Vegas,NV
    I remember reading the pro's had some bad relay issues.I've not had a problem with
    my 5pro yet, knock on wood.But that dosen't seem to be your problem here as you
    have power.
    Have you checked the preamp tubes BTW?
     
  15. rbonner

    rbonner

    Sep 25, 2008
    Basspro, what you are meaning is dirty SPEAKER RELAYS. The relays are discoed until everything comes up to speed and then connects the speakers. What I DO NOT recommend is play thump-thump-thump on your E string until the relays energize and SMACK an instant 1200 watts to the relays. That causes them to pit. It's not a toob amp where the toobs warm up and the music comes on slowly.

    BOB
     
  16. rbonner

    rbonner

    Sep 25, 2008
    SIGH, that is the main power CB... 32V 15A figures. I seriously don't have the foggiest Idea how that got in there.

    The amp would NEVER have gotten a UL approval with a DC rated Circuit Breaker in there.

    I would suggest that maybe the breaker got replaced by DAVE the Repairman?

    If it came from the factory I'm totally discusted. This might be worth me actually popping the top off one of my amps here and look...

    BOB
     
  17. While it would also disgust me, if it was a LOUD Tech amp it would not surprize?
     
  18. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The breaker is, or at least was, a P&B W28 thermal breaker rated for line voltage....... now probably the chinese equivalent......

    A 32V ONLY breaker, if present, SHOULD BE REPLACED IMMEDIATELY. it may not open when it should.

    But that will NOT explain your problem in any way whatsoever...... The breaker is either open, or closed, or has a bad connection....

    Whatever it does, happens to ALL the voltages in the amp, if open , they all drop to zero. if closed, they come up together. if nasty connection, maybe they are all low until the breaker actually opens.

    Point being that you said SOME voltages are good, and some others are not. Bad ones are from a fault in the specific circuit, and you, as the tech, have to find it. That's why you get the big bucks......... ;)

    EDIT......

    I looked up the W28 part, and see that for DC it is rated 32V, for Ac it is rated 250VAC. I never have used one for the DC rating, so I had forgotten..... No doubt that is where the confusion comes in........ But if it has the 250VAC UL rating marked it's fine, if it ONLY has the 32V, replace it.
     
  19. rbonner

    rbonner

    Sep 25, 2008
    OK good, 250/32 standard ratings....
     
  20. Randy Warren

    Randy Warren

    Oct 26, 2008
    agreed, Jerrold, that main CB would not allow any part of the amp to come on, and I do have voltages on the preamp board (and all tubes tested good), god, getting to the rail board(s) and outputs around the back is gonna be a bitch, really crowded in there...
     

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