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Thread: Ampeg SVT 4 Pro schematic needed

  1. #1
    Supporting Member Jazz P Bass's Avatar
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    Ampeg SVT 4 Pro schematic needed

    Any kind soul care to share a schematic for the SVT 4 Pro?
    I have a preamp out signal. No output signal.
    Went in power amp in with a result of no signal also.

  2. #2
    Old Timer Tom Phillips's Avatar
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    I can email it to you if you PM me your email address that will accept attachments. Can't send attachments thru the Ampage email system
    Tom

  3. #3
    Senior Member Enzo's Avatar
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    Oh, I can fit it here in sections.
    Attached Files Attached Files
    Education is what you're left with after you have forgotten what you have learned.

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Cool Ampeg SVT 4

    Thanks Tom.
    Thanks Enzo.
    So far the Output MosFets are not shorted.
    The relays are going into protect for sure.

  5. #5
    Supporting Member Jazz P Bass's Avatar
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    Fixed

    Got the SVT 4 fixed.
    1 hour to take it apart.
    1 hour to resolder the main board.(found a couple of really bad solder joints)
    1 hour to put it back together.
    1 hour to final test & do paper work.
    Nice amp!
    Why do people think this stuff is easy?
    Last edited by Jazz P Bass; 11-01-2009 at 03:57 PM.

  6. #6
    Old Timer Tom Phillips's Avatar
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    Yea. Everybody thinks the other guy's job is easy.
    "All ya got to do is .... What's so hard about that."
    "You charged me how much to solder two joints?"
    Etc. etc. etc.

    Glad you got the SVT fixed.

    Regards,
    Tom

  7. #7
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    Hi, I got same problem. Poweramp shut down in 20 second as it were over heated, and it's not. Relay switch off and fun starts cooling. No sound at the output. Turning off the amp, waiting 2 minutes and turn it on again and same scenario. Amp looks perfect and during operation works well. Anybody can help with an idea??

    Thanks

    Lorenzo

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    Ampeg SVT

    Lorenz

    I can only advise what I did.
    Remove the main board & look for bad solder joints.
    Resolder the main components while your are in there.
    Look at the filter cap soldering & the transistor joints. If the joint looks shallow, that is a good indication that the board was not preheated to a high enough temperature at the wave solder process.
    You can tell sometimes if you see a recurring shallow solder joint.
    Sometimes it will be the parts that are attached to ground.
    Sometimes it will be one side of the board, or one side of the components.
    Happy Hunting.
    PS: you really should have started a new post.

  9. #9
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    Thank you Jazz, I start dismount the amp. It's strange it doen't seem to be a bad solder. Amp is really perfect and no sollecitation stop or start the problem. Now the problem is continuos. I think it could be a problem on the Breakoff Board, I'll try to change IC6 as it seem to controll relay on/off. I work in a repair shop and "hunting" for gost problem is my worst nightmare!!

    Next time I'll create a new post ;-) and tell you my solution

    Thanks again.

    Lorenzo

  10. #10
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    Im new here so i dont really know how these forums work, Any chance that someone here has a copy of the Power amp schematic for this amp(SVT4pro). Only thing i saw posted on here was preamp but i could be wrong, thanks

  11. #11
    Supporting Member Jazz P Bass's Avatar
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    Ampeg SVT4 Pro schematic

    On this Post go up seven posts.
    Enzo laid them all right out.

  12. #12
    Senior Member Enzo's Avatar
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    The five files I posted should be the complete set of schematics and the board layouts. I may have left off the page that shows two speaker jacks wired together. But that is all that is on the page - two jacks.

    Just click on each file to open it.
    Education is what you're left with after you have forgotten what you have learned.

  13. #13
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    sweet

    Nice i found it, thanks. I didnt realize the pdf was multiple pages before. Thats probably one of the best schematic/part diagrams ive seen

  14. #14
    Junior Member Englfireball82's Avatar
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    SVT 4 PRO Issue...

    Hi, everyone.

    I have a Ampeg SVT 4 PRO rack head that is having an issue of power amp A side isnt putting out any sound to the cabs neither through speakon or 1/4 inch outputs. Power amp B side seems to be working fine, however i need to use both sides to run to my cabs, Anyone have any ideas on what it could be? Thanks!

  15. #15
    Senior Member Enzo's Avatar
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    When you first turn the amp on, there should be a brief pause before the speakers click on. Each channel has an output relay to connnect the speakers. I woulod wager one of your relays is not clicking on. And the probably reason is that the channel has failed.

    But before we assume that, slide the MONO/BRIDGE switch back and forth a few times. ANy help? And since it is ch A not working, plug a spare cord from Preamp A out to Power AMp A in. ANy help?

    You can also just plug a guitar directly into Power AMp A in. Got sound?
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
    Junior Member Englfireball82's Avatar
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    I just tried everything u mentioned and still no sound from power amp A side, thanks for the reply....any other ideas?

  17. #17
    Senior Member Enzo's Avatar
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    You didn;t answer whether both relays are clicking on or only one.
    Education is what you're left with after you have forgotten what you have learned.

  18. #18
    Junior Member Englfireball82's Avatar
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    oh ok, well heres how i noticed what was going on....im using it through 2 2x15 mesa road ready cabs, and of course on the back of the svt there are 3 speakon outputs while looking at it from the back from left to right power amp B then bi amp then power amp A.....so i connected one speakon to one mesa cab from power amp b then the other speakon to the other cab from power amp a......and only the power amp b side was working...for each power amp there is a speakon connection and 2 1/4 connection outputs basically allowing you to run 3 different cabs from each power amp if you wanted to.....so hense theres no sound coming out of power amp A side cause i tried all 3 outputs on that side and no sound at all.....im still quite new to this amp so i dont know much about it.....if it was a relay that wasnt kicking on how would i be able to tell or find out? Thanks again.

  19. #19
    Senior Member pecorporation's Avatar
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    If the unit is set to "Bi-amp" and "Mono bridge", theamp will not operate correctly. The bridge output is not a third amp, it's a colmination of A and B.
    CAUTION!!! Solid state and tube amplifiers contain lethal voltages and can retain them even after unplugged. Refer service to qualified technician.

  20. #20
    Junior Member Englfireball82's Avatar
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    Thanks for the reply.....its not set to bi amp, im using it to run 2 cabs, as in one cable from power amp B to a cab, and another cable from power amp A to another cab....the bi amp output isnt being used. It is set to Stereo and not mono/bridge. Im not getting any output to the cab from power amp A side.....B side works just fine. Both cabs work great and the cables are great, ive tried swapping all cables and cabs around and just running on power amp A side alone, and still no sound from power amp A side at all.

  21. #21
    Junior Member Englfireball82's Avatar
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    Im still new to the head....how could i tell if it was set to "bi-amp"? I see only the switch that says stereo or mono/bridge.....it is currently switched to stereo.

  22. #22
    Senior Member pecorporation's Avatar
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    I've seen a lot of those units with failing solder connections on the output amplifier board, mostly components like output FETS and large components like filter caps that are suseptable to heat and vibration stress. I usually reflow all these suseptable components, including all jacks, pots, free standing drive transistors, even the relays and the multitude of pc plug pins to ensure the amp will be reliable on the road. Visually inspect every solder connection closely on that board looking for a cracked solder ring around the component lead.
    CAUTION!!! Solid state and tube amplifiers contain lethal voltages and can retain them even after unplugged. Refer service to qualified technician.

  23. #23
    Junior Member Englfireball82's Avatar
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    Ok cool, so your next step would be to go into the head itself and browse the soldering connections...i usually install my own pickups so im familiar with soldering. Should it be quite easy to detect a broken solder when i get in there or would it be a pain to see? As i dont have a diagram of what it should look like. Thanks

  24. #24
    Senior Member Enzo's Avatar
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    SO lets label your stuff. We'll just call it all A and B. SO your setup is Output A through Cord A to speaker A, and then Output B through cord B to speaker B.

    Sound comes from speaker B.

    Unplug cord A from the amp and set it aside. Now plug cord B (still going to speaker B) into amp output A. Still no sound, or does sound now come out through speaker B?

    Just making sure there is no troubloe in the speaker and cord.


    I believe you when you say no sound comes out one side, but I still would really like to know if BOTH output relays click on or just the one on B channel. Usually you can hear it. A few seconds after turning on the power switch you hear a click inside. Actually there should be two clicks or at least a sort of "cli-click." Leave the speakers off or at least don't put any signal to the amp, so you can hear this. What I usually do is put my fingers on top of each relay, and I can feel it clicking into place inside it.

    This piece of information is a valuable clue as to what is happening inside. If it DOES click on, there is a very limited list of probably faults. If it does NOT click on, then the relay circuit has detected a problem in the power amp.
    Education is what you're left with after you have forgotten what you have learned.

  25. #25
    Junior Member Englfireball82's Avatar
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    exactly how you just said it....i have done that already....both cabs and speaker cables work great as ive ran both of them through power amp B cause its the only one working (and side B does run both cabs together)....and ive tried swapping them out and using them in power amp A and yes.....there is no sound at all....ive tried switching every button on and off thats pushable or switchable on the amp and have had someone else constantly playing a guitar through it while i was working on it just to see if there was any shorts or anything and still nothing from side A. I have also (just now) taken the amps top off to inspect for broken solders or loose wires and everything looks perfect inside it...no broken solders or loose wiring, and nothing touching metal. If there is a relay that isnt working i would have no idea what it looks like or what i should replace or look for in this particular amp. If i only had a circuit board diagram for the amp then i could see what the relays at least look like on the inside.

  26. #26
    Senior Member Enzo's Avatar
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    Did you read this whole thread? In post #3 of this thread, I posted the schematics AND board layouts for this amp.

    428XXp9.pdf is the file with the power amp board layout. On the furst page of it is the board layout. Upper right area there are four large round things - your main filter caps. Immediately to the left of those are two relatively large sorta rectangular things. The left end of each is taller than the right end with a step in the middle. They are numbered K101 and K201. THOSE are your speaker relays. They are plastic boxes on the circuit board.
    Education is what you're left with after you have forgotten what you have learned.

  27. #27
    Junior Member Englfireball82's Avatar
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    ahhh yes i remember seeing those and thought they may be the relays when i was in there. So in this case relay K101 would be for power amp A correct? How do you go about replacing it....does it just pop out of position by pulling it out (like replacing a power tube)? I wouldnt want to break anything. Thanks again.

  28. #28
    Senior Member Enzo's Avatar
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    One would desolder it from the circuit board and solder in a new one.

    BUT!!!

    I am not suggesting the relay is bad. It might be, but what I am more worried about is that it is not clicking on because something is wrong in the amp circuit. The relay action is merely a clue. Like a detective looking at a bed to see if it was slept in. The detective is not concerned that the bed is untidy, he is looking for what the bed tells him. There is a circuit for each relay. The circuit detects trouble in the amp, and when it finds trouble, it leaves the relay not energized.

    If the relay clicks on but there is still no sound, then we might suspect the relay after we measured the resistance of its contacts. But we could just as easily suspect the input stages of the power amp, possible bad IC. But if the relay does not click on, then we want to find out if the amp has failed to a DC voltage condition.
    Education is what you're left with after you have forgotten what you have learned.

  29. #29
    Junior Member Englfireball82's Avatar
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    ok yea....so it could be numerous things then. I will mostlikely have it taken to a shop and worked on. Thanks

  30. #30
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    Amepg SVT-4pro Side A no or very little sound output

    I have the same issue as some of the others. Taking it in to be checked. However, there is another thing you can do to test. Just plug your bass straight in to the amp A "in" on the back I did this and the volume was good and strong. If I plug in to the front input the sound is so low you can barley hear it but there is sound. I play out every weekend so it has been banged around and dropped. Maybe just a loose preamp tube. Side B works fine.

  31. #31
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    Enzo: Big Thanks Man from pbenjamins in Huntington Beach, CA ! (trying to get a buzz / tearing sound out or an SVT4-Pro even at low audio levels)

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