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Thread: Ampeg SVT-3 Pro Repair help, R27 catches fire on power up

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    Ampeg SVT-3 Pro Repair help, R27 catches fire on power up

    Hello,
    So I am attempting to repair an SVT-3 pro. I think the input jack was broken and just wanted to check it to see if the connection was intermittent.
    Firstly I made a rather large mistake, from the speaker output I plugged into my fender amp 'power amp in' thinking it would pass through to the 8 ohm speaker. Turned it on and 1 second later R27 caught fire and R33 heated up and exploded. I turned it off and yeah realized how dumb that was, the amp says Effects loop over top the jack and power amp in underneath the jack. My assumption is that that impedance was too low or outputting a signal to the output stage of the svt-3. ???

    So, I replaced R27, R33, Q5, and Q6 because I think they were faulty. I turned the amp on again with nothing hooked up (input or output). And sure enough R27 went up in flames instantly. R33 survived but was warm to the touch, no other components felt hot that I could tell.
    Also I think C10 may have failed Open, is that possible? how important would that be to the circuit's functionality, is it for decoupling/noise? I don't have an immediate replacement for it.
    Other notes: D9, D8, D28, D29 did a diode check and they showed consistent voltages across each. (none failed short)
    Besides me being not so savvy with the music equipment would someone be able to give me some advice with finding the root cause? Maybe a next step?

    Thank you so much for any input. Everything helps.

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  2. #2
    Senior Member guitician's Avatar
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    So you were trying to run the Ampegs pre-out into the Fender amp? But, instead plugged the speaker out into the Fender? What signal was running the output of the Ampeg at the time?
    I'm guessing that you shorted the output devices and burned up those resistors in the process. You need to check the SoildState devices too.

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    Ampegs speaker outputs into the fender effects loop, power amp in.
    So you think I shorted the mosfets and any connected transistors? Is there a way to check the mosfets without taking them off of the board?
    SCM ELECTRONICS SVT3PRO SCH Service Manual free download, schematics, eeprom, repair info for electronics

    Thank you

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    Senior Member guitician's Avatar
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    The ground, or sleeve of the 1/4" plug doesn't seem to have a problem being shorted, so you maybe OK with the MOSfet's. but Q6 may have blown.

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    Ok, I'll replace that again.
    Do you know functionality of C10 in that part of the circuit?

    Thank you for your help.

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    Supporting Member Jazz P Bass's Avatar
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    My honest opinion is that you will not succeed in repairing this amp.

    You pulled a bone head move by plugging a speaker output, which can deliver amps of current, into a small signal power amp in jack.
    By the way, have you checked the Fender? (what model is it anyway?)

    You blew up a $1000 amp.
    As it is, it is most probably repairable by a very competent tech.
    You are only going to make it worse by messing with it.

    I am not trying to be a hardnose but this is not a simple amplifier.
    It will take a good understanding of each circuit function and a rack of test gear to repair this amp.

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    Yeah, I realize I made a huge mistake and am very disappointed with myself. Yeah, my logic was flawed and saw what I wanted to, I saw power amp in and thought well I have a power amp, impedance of the speaker is good, and disregarded effects loop label above it.
    I see it is possible to purchase the main circuit board for around $400.
    Could that possibly fix the problem or is it possible that it is damaged in the pre-amp stage as well?
    I do realize that this is not a simple amplifier, I would like to learn the functionality of each circuit, and attempt to find/fix the problem(s).

    I appreciate your honesty, I honestly want to fix this thing though not so much for the money but for the experience too.
    The Fender model is m-80 and works fine of course, much rather have blown that.
    Thank you for your input.

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    Senior Member guitician's Avatar
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    Pull the power amp board and see if just the pre-amp works by itself. You should have tested all the parts after the first thing blew, I don't think that you know enough to do a component level troubleshooting/testing to the power amp section. There may still need to be a setup/biasing procedure for the new power amp board which could require test equipment too.

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    So if I separate the boards I would have to input the proper voltage to pins on J5/J6 to power up that board, correct me if I am wrong.. I'll have to look at the schematic to see what pins/voltages.
    As for testing all the parts after the first thing blew, I checked what I thought was blown Q6 was toast and so were the resistors mentioned. Based on the part of the circuit that was pulling current I thought maybe replacing those would fix the problem. By test all the parts, you mean every single component on the board?

    If I had a working SVT-3, would it be safe to disconnect the jumper harnesses from the power amp board to the preamp board and then turn it on?

    Test equipment such as...

    Thank you

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  10. #10
    Supporting Member Jazz P Bass's Avatar
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    Where did you locate a power amp replacement board?

    Sometimes they arrive LESS the power transistors.

    Edit: Not in this case though.
    They are already mounted to the heat sink.

    http://www.fullcompass.com/product/4...FY4XHwodeHgAgg

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    Same exact one you found. I should have linked that, sorry. I was a bit off on the price.

    Do you think it would be safe to swap in a new board like this? And what adjustments, if any, need to be made?

    Thank you

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  12. #12
    Supporting Member Jazz P Bass's Avatar
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    There are no 'safe' conditions until tested 'safe'.

    At the very least, the power supply voltages should be confirmed good.

    The new board may or may not be set for a proper low idle bias.

    I would hope that it was 'tested' at the factory.

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    So are you suggesting I take this route, and do some pre-functional checks like checking power supply? It sounds like I could do more, what else would I check? As far as biasing I have biased my tube amp before, would I be biasing the current flow through the tubes and/or power output stage?

    Looks like the driver tubes are not included, should I just put the old ones in?

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    I guess that you could replace the board and that may fix the amp. Or you could install it and the new board could blow up because of a problem with the power supply. Or ...

    This approach to amp repair bothers me. It's like saying , "My car won't start. I know, I'll replace the engine. That'll fix it."

    You can and should do whatever you want to do, as it is your amp, but it just goes against my basic thinking. Best of luck.

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  15. #15
    Supporting Member Jazz P Bass's Avatar
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    Way too many assumptions.

    My suggestion is to take the amp in it's present state to a qualified tech.

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    Thanks guys, I'll try to find someone, anyone know anyone in Michigan?

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    Quote Originally Posted by Barnaby Jones View Post
    Thanks guys, I'll try to find someone, anyone know anyone in Michigan?
    I don't know what Enzo's current situation is, but he is in Lansing. If his shop is still open, it would be worth the drive to have him fix it.

    If nothing else, he can suggest someone.

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  18. #18
    Lifetime Member Enzo's Avatar
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    Sorry to say, my shop is closed, and I am not taking on any more work. I am spending my evenings going through stuff, throwing things away or gathering them for sale.

    I left my schematics flash at the shop last night, so I can't look at my files right now. I'll see if there is something I can add to the discussion.

    Doesn't On Stage up in GR do work on these? Tell them I referred you to them. That may or may not mean anything to them.

    Music Equipment Repair, Grand Rapids, MI

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    If On Stage can't/won't/don't have time, I'm an hour north of Lansing right off US127 but I've got enough work right now it would be 3-4 weeks before I could do anything.

    Enzo, how are you going to sell what you have left? Ebay? Craig's List? Here?

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    He's like a new set of strings... he just needs to be stretched a bit.

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    Hello Enzo,
    Some time ago you posted two schematics a Peavey Classic 30 & a Peavey Delta blues 115-30.
    I was unable to download the Delta Blues 30 schematic.
    Any chance of a getting a copy of the DB 30 Amp schematic, parts list, etc.
    I would appreciate it very much.
    Thanks,
    Matt
    mtroy1369
    mtroy17@comcast.net

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    Old Timer J M Fahey's Avatar
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    It will be cheaper to send the amp to a competent Tech and being honest about what happened than ordering an expensive new board and still not fixing the amp.

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    Juan Manuel Fahey

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    Thank you Jazz P Bass for your help!!!!!

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by mtroy1369 View Post
    Thank you Jazz P Bass for your help!!!!!
    Uhmmm....
    All I did was use the Search Function up above.

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    you the man JPB!! Someone had to do it don't sell yourself short.

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    Lifetime Member Enzo's Avatar
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    And if you look, the ONLY difference between the Classic 30 and the Delta Blues is they added the little trem circuit. A couple parts to make the trem, and an optocoupler across the signal path.

    gui-tar, I will list some things here in the flea market, keep an eye there, some stuff will ebay, and some local Craigs Listings. I also will probably send some junk down to the annual Gear Fest down at Sweetwater next month. You are relatively local, and if interested, I have a bunch of projects I will not be completing. A number of dead SS combo amps - repair or use as parts, and a few power amps with dead channel (Crown, Mackie, others) Big stuff like that I won't be shipping anywhere.

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    Quote Originally Posted by gui_tarzan View Post
    If On Stage can't/won't/don't have time, I'm an hour north of Lansing right off US127 but I've got enough work right now it would be 3-4 weeks before I could do anything.
    Enzo/gui Tarzan- I'll check out those options definitely, thank you for the suggestions! Faster is better in this case, I will check out onstage and if they can't/won't/don't have time I'll get in contact with you guiTarzan.

    Quote Originally Posted by J M Fahey View Post
    It will be cheaper to send the amp to a competent Tech and being honest about what happened than ordering an expensive new board and still not fixing the amp.
    Thanks for your input, yeah that is the plan.

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